Stimulating Oxide Heterostructures: A Review on Controlling SrTiO3‐Based Heterointerfaces with External Stimuli

Numerous of the greatest inventions in modern society, such as solar cells, display panels, and transistors, rely on a simple concept: An external stimulus is applied to a material and the response is then utilized. Oxides often exhibit a colorful palette of responses to external stimuli due to the close coupling between lattice, spin, orbital, and charge degrees of freedom. In particular, oxide heterostructures where oxide thin films are deposited on SrTiO3 have proven to be a fertile playground for material scientists, and a vast amount of interesting theoretical and experimental studies showcase the wide tunabilities of these heterostructures when subjected to external stimuli. Here, the authors review how the properties of SrTiO3‐based heterostructures can be changed by external stimuli using electric fields, magnetic fields, light, stress, particle bombardment, liquids, gases, and temperature. The application of a single stimulus or several stimuli combined often leads to unexpected changes in properties which can open up for designing new devices as well as expanding the boundaries of our understanding within fundamental science.

a low carrier density superconducting state, [1,2] a high electron mobility, [3,4] and ferromagnetism. [5] A big leap forward was made in 2004 where it was found that when depositing insulating LaAlO 3 (LAO) on insulating STO, a conducting interface was formed. [6] This spurred a large interest, and a wide range of STO-based heterostructures was soon synthesized in the wake of this discovery. [7][8][9][10][11] A common feature in conducting STO-based heterostructures is that the itinerant electrons are formed on the STO side of the interface, and the heterostructures thus share the interesting properties of bulk STO. In addition, various top films can be used to enhance the functionalities of the heterostructures beyond what is observed in bulk STO. For instance, the lattice symmetry breaking at the interface confines the itinerant electrons in the vicinity of the interface rather than being spread out throughout the bulk of STO, [12][13][14] which enables the field effect tuning of the interface properties.
The added top film, however, also introduces complexity, and contrary to the simpler case of bulk STO, where conductivity typically is formed by donor dopants such as Nb and oxygen vacancies, the origin of the itinerant electrons at the STO-based heterointerfaces continues to be discussed. The prevalent explanations rely on the formation of oxygen vacancies at the STO side of the interface [7,15,16] and a diverging potential in polar top films [17] that may spontaneously form defects on the top film surface. [18] The wide range of extraordinary properties at the STObased heterostructures stems from the interaction between the charge, orbital, spin, and lattice degrees of freedom, which may be used to create new electronic devices with properties differing from those known today. The close coupling between the various degrees of freedom also leads to significant changes in the electronic, magnetic, and structural properties when applying external stimuli. Due to this sensitivity, a vast amount of theoretical and experimental studies has investigated how various external stimuli can tune the properties of STO-based material systems. Here, we review the response of these systems to the following external stimuli: electric fields, magnetic fields, light, stress as well as exposure to gas, liquids, particle bombardment, and temperature changes (see schematic illustration in Figure 1). The review is aimed to provide a representative overview of the vast literature on applying external stimuli to STO-based heterostructures, rather than providing an exhaustive account of all studies related to application of external stimuli.

Electrostatic Potential
This section presents one of the principal means of tuning the physical properties of STO-based heterostructures and most functional materials: electrostatic stimulus.

Electrostatic Gating Geometries
The transport properties and interface conductivity of STObased heterostructures can be readily tuned by varying the deposition conditions during the growth of the top oxide thin film. [15,[19][20][21] With the notable exception of interface conductivity originating from oxygen vacancies in STO, [7,22] the conducting properties of STO-based heterostructures are generally very stable. However, it is possible to electrostatically modulate the interfacial properties of STO-based heterostructures by applying an external electric field using a gate. As electrostatic gating is the main operational principle of the electronic circuits available today, this tuning pathway is of great importance. Concerning STO-based heterostructures, electrostatic gating has been shown to allow modulation of the charge carrier density and a range of other properties such as the electron mobility, electron effective mass, Rashba spin-orbit www.advmatinterfaces.de coupling strength, superconductivity, ferromagnetism, tetragonal domain ordering, and quantum transport phenomena as described in the following subsections.
The electrostatic tuning of the STO-based heterointerface can be performed in a number of different gating configurations (see Figure 2). The most commonly used configuration is back-gating (Figure 2a) where the STO substrate serves as a gate dielectric. Global or local top-gating (Figure 2b,c) through the top film can be achieved either by LAO in the case of LAO/STO (or equivalent oxide overlayer) acting as gate dielectric or by inclusion of a conventional dielectric layer such as HfO 2 , SiO 2 , or Si 3 N 4 between the gate electrode and LAO. The conventional dielectric layer has also been replaced with a ferroelectric layer, providing an alternative way to top-gate the interface. [23][24][25] Moreover, several studies have gated the interface by means of an ionic liquid (Figure 2d) dispensed on top of the STO-based heterostructure. [26] The gating has even been shown possible by application of an electric field from the tip of a conducting atomic force microscope (c-AFM) (Figure 2e) whereupon nanosized conducting paths can be written and erased. [27,28] The different gating geometries differ substantially from each other particularly because the dielectrics separating the gate and the interface typically have very different thicknesses (a few nanometers for top layers and hundreds of micrometers for the STO substrate), but also because the dielectric constant of STO is generally much higher and is both highly temperature and electric field dependent. [29,30] The back-gate voltage applied across the typically 0.5 mm thick STO is therefore often in the range from 10 V to several hundreds of volts, whereas only a couple of volts are needed for the top-and ionic liquid gating setups.
In the following subsections, we will separately discuss the physical properties that have been shown tunable by electrostatic gating using the different gating setups shown in Figure 2. For convenience, we include a schematic drawing of the gating configuration and material system in the figures to easily discriminate between studies using, e.g., back-gates or ionic liquid gates. Note that this schematic drawing merely Adv. Mater. Interfaces 2019, 6,1900772  a) A global back-gate can homogenously exert an electric field through the STO substrate. b) A global top-gate can change the interface properties through the interspaced gate dielectric, which can, for instance, be LAO, HfO 2 , SiO 2 , or Si 3 N 4 . c) Local top-gates or side-gates can laterally constrict the interface properties by electrostatic pinching. d) A droplet of ionic liquid can be polarized to apply a very large electric field on the STO-based heterostructures without significant leakage currents at low temperature. e) By the usage of the tip of a c-AFM, nanoscale metastable gating can be performed to, e.g., write and erase conducting channels.
www.advmatinterfaces.de serves a rough guideline for the gating setup as the dimensions and device type (e.g. Hall-bar or van der Pauw square) may differ from what was used in the particular study.

Main Effects of Electrostatic Gating
A broad range of physical properties has been shown tunable by electrostatic gating in STO-based heterostructures. These include: 1) carrier density and band occupation, 2) electron mobility, 3) superconductivity, 4) spin-orbit coupling, 5) magnetism, 6) local microstructure and domain walls, and 7) quantum properties. Arrays of field effect devices relying on the electric field tuning of the above properties have also been fabricated. [31,32] Such field effect transistors based on oxide interfaces have also been reviewed by Kornblum. [33]

Gating of Carrier Density and Band Occupation
A change of the charge carrier density and band occupation can occur when applying an electric field from a gate to the STObased heterointerface. [34][35][36][37] Depending on the gating geometry, this electric field is often sufficiently large to induce changes in the charge carrier density of the same order of magnitude as the intrinsic as-grown value, i.e., n s ≈ 3-4 × 10 13 cm −2 . In continuation of the original discovery of interface conductivity between TiO 2 -terminated STO and LAO grown by pulsed laser deposition (PLD), Thiel et al. were the first to demonstrate electrostatic gating of the LAO/STO heterointerface. [38] In this seminal study, the authors found that a STO substrate with 3 unit cells (u.c.) LAO grown on top nominally displayed insulating transport properties. However, by applying an electric field from a back-gate (see Figure 2a), it was possible to induce a reproducible bipolar and nonvolatile insulator-to-metal transition both at low temperatures as well as at 300 K (see Figure 3b). This large change in sheet resistance was explained to occur due to a moderate modulation of the band structure. As LAO/STO heterostructures with 3 u.c. LAO are expected to be on the verge of undergoing an insulator-to-metal transition due to the polar discontinuity at the interface, [17] an externally applied electric field could thus release mobile charge carriers on the order of n s ≈ 3-4 × 10 12 cm −2 . A similar accumulation and depletion of carriers was later observed in a range of other STO-based heterostructures including amorphous-LAO/STO [39,40] and γ-Al 2 O 3 /STO (GAO/STO). [41,42] The conductivity in STO-based heterostructures originates from the population of Ti 3d t 2g orbitals, which are energy split into nondegenerate d xy subbands and degenerate d xz /d yz subbands due to the inversion symmetry breaking and electrostatic confinement at the interface. [35] Due to the different coupling orientations, the relative t 2g subband occupation dictates the overall conductivity anisotropy of the electronic system as well as more subtle interactions such as the overall Rashba coefficient discussed below. By studying the low-temperature response of magnetoresistance (MR) and Hall resistance upon application of an electric field from a back-gate, Joshua et al. found that a certain critical carrier density exists where the electronic system undergoes a Lifshitz transition from one band to two types of bands. [43] This critical carrier density of n c ≈ 1.7 × 10 13 cm −2 (see Figure 4b) corresponds to a specific dopant level in STO where the higher energy subbands d xz /d yz become populated, and the STO quantum well transitions from displaying formal one-band nature to two-band characteristics (see Figure 4c), as discussed in more details in Section 3. The transition dictates the manifestation of several important physical properties of STO-based heterostructures such as the superconductivity, magnetism, and spin-to-charge interconversion processes as described in the following.
The LAO/STO heterostructure has also been studied extensively by global top-gating (see Figure 2b). For instance, Hosoda et al. was the first to study top-gating of LAO/ STO where a tuning of the carrier density in the range 0.9-2.5 × 10 13 cm −2 was shown possible at low temperature for a top-gate potential between −1.0 and +1.0 V. [44] Later the same year, Eerkes et al. reported a tuning of the carrier density at low temperature between 1.6 × 10 13 and 2.2 × 10 13 cm −2 for a topgate potential of −0.3 and +0.5 V, respectively. [45] A wide range of carrier densities between 2.0 × 10 13 and 6.0 × 10 13 cm −2 at 2 K have later been demonstrated by Smink et al. [46] using a top-gate (see Figure 5). First, the authors concluded that the aforementioned critical density for the Lifshitz transition is not a universal quantity for all STO-based heterostructures as otherwise first inferred. Here, the critical density for the The sheet resistance (R s ) of a LAO/STO heterostructure with three unit cells LAO is changed by more than four orders of magnitude at 300 K upon cycling of the backgate potential (V g ). b,c) Reproduced with permission. [38] Copyright 2006, American Association for the Advancement of Science.

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Lifshitz transition in the top-gated LAO/STO was found to be 2.9 × 10 13 cm −2 . [47] Second, it was found that above the Lifshitz transition, the carrier density of lowest laying d xy -subbands reduces upon application of larger positive top-gate voltages due to the change in quantum well confining potential (see Figure 5).
In semiconductor heterostructures, the ability to spatially pattern 2D electron gases (2DEGs) by local top-gating (see Figure 2c) has been an important tool for realizing mesoscopic quantum devices. Key examples are the observation of quantized conductance in lateral point-contact devices and Coulomb blockade in single and double quantum dots. [48] In the case of STO heterostructures, this functionality can be combined with the unique properties offered by these materials to realize exotic hybrid devices such as gate-tunable Josephson junctions or negative-U quantum dots. [49,50] For STO-based heterostructures, several studies have investigated the application of local top-gating and, quasi-equivalently, side-gating. Goswami   . a) Schematic illustration of the gating configuration in the present study. b) Carrier densities (n) deduced from the Hall coefficient at B = 0 T (red) and B = 14 T (blue) as a function of back-gate voltage (V G ) at 4.2 K with a deviation occurring at a certain critical carrier density (n C ). c) This critical carrier density is assumed to occur due to a Lifshitz transition in the band structure between single-band (d xy ) occupation to two-band occupation (d xy and d xz /d yz ). Reproduced with permission. [43] Copyright 2012, Springer Nature Publishing AG. Figure 5. a) Schematic illustration of the gating configuration in the present study. b) Subband carrier densities extracted from two-band fits to the Hall resistance as a function of top-gate voltage (V TG ) at 2 K. c) Comparison between experimentally observed subband densities and theoretically calculated values shows a reduction of the lowest laying subband density (d xy ) above the Lifshitz transition. d) The corresponding band structures originating from the calculations referred to in (c). Reproduced with permission. [46] Copyright 2017, American Physical Society.

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top-gates thus leaving a narrow conducting channel with a width that could be controlled with the top-gate voltage both at 4.2 and 300 K (see Figure 6). [51] A similar depletion and lateral constriction of STO-based heterointerface conductivity have subsequently been undertaken by other studies. [52][53][54][55] However, issues with top-gating often can arise due to electrical breakdown through defects in the gate dielectric whereupon leakage currents shunt the gate and interface electronic system. This shunting prevents further tuning of the interface charge carrier density and other physical properties. Consequently, another top-gating method have widely been employed for STO-based heterostructures where an ideally insulating gate composed of ionic liquid is dispensed on top of the oxide heterostructure (see Figure 2d). While in its liquid state, the ionic species in the dispensed gate are free to rearrange and segregate into an effective dipole layer, which exerts a substantial electric field on the STO-based heterointerface below. Upon cooling of the sample, the ionic species are frozen into place and thus prevent current flow to occur while still maintaining the imposed large electric field. Ionic liquid gating of LAO/STO using this electrical double layer approach was studied by Zeng et al. where a modulation of the charge carrier density was observed to be possible up to 3.0 × 10 13 cm −2 at 2-3 K (see Figure 7). [26] In other studies, the induced carrier density was found to be one to two orders of magnitude larger. [56,57] By combining the ionic liquid setup with a back-gate in a dual-electrostatic configuration, it was shown possible by Lin et al. to obtain a larger modulation of the charge carrier density between 5.0 × 10 12 and 5.0 × 10 13 cm −2 at 180 K (see Figure 8). [26] By scanning a positively charged tip of a c-AFM (see Figure 2e) at room temperature across the surface of heterostructures just below the metal/insulator transition (with 3 u.c. LAO grown on STO), the surface can be protonated and locally induce a metastable interface conductivity. [27,59] Vice versa, passing a negatively charged c-AFM tip will remove this metastable interface conductivity. In this way, one can write and erase conducting channels at the LAO/STO interface with nanoscale lateral dimensions. This metastable gating method (also known as charge writing), has led to remarkable discoveries stemming from the behavior of correlated electrons, e.g., electron pairing without superconductivity [50] and testifies to the rich phase diagram of STO. Concerning charge carrier modulation by gating, it was early demonstrated by Cen et al. that the written conducting channels likewise can act as gateelectrodes themselves with lateral transistor behavior possible at low temperatures (see Figure 9). [28] We have so far only described STO-based heterointerfaces where the charge carriers are electrons. However, following the thin-film stacking sequence previously investigated, [60] the STO/ LAO/STO heterostructure was proven to exhibit a 2D hole gas by Lee et al. [61,62] Here, the top STO/LAO interface hosts a hole gas whereas the bottom LAO/STO interface simultaneously hosts an electron gas. The low-temperature back-gating of this dual electron-hole system was soon after studied by Singh et al. (see Figure 10). [63] For this parallel plate capacitor system with Adv. Mater. Interfaces 2019, 6, 1900772 Figure 6. a) Schematic illustration of the gating configuration in the present study. b) By the usage of local top-gates, it is possible both at 4.2 and 300 K to deplete the conducting regions below the gates and subsequently continuously reduce the width of the remaining conducting channel between the gates. Reproduced with permission. [51] Copyright 2015, American Chemical Society. b) The ionic liquid gating allows a large tuning of the charge carrier density in LAO/STO heterostructures at 2-3 K, however, with a need to exceed the freezing point of the liquid between each gate voltage applied for thawing the ionic liquid. Reproduced with permission. [26] Copyright 2016, American Chemical Society.
www.advmatinterfaces.de three capacitor plates composed of the back-gate, electron gas, and hole gas, the authors found an increase of both the electrons and holes when applying a larger positive voltage on the back-gate.

Gating of Electron Mobility
Modulations of the charge carrier mobility can likewise be induced using gating. When the quantum well in STO is subjected to the application of an external electric field, aside from changing the occupation of the well, the confining potential changes as well. This change can consequently affect the average scattering time of electrons, which in turn can alter their corresponding carrier mobility value. By gating through the STO substrate with a back-gate, it was found by Bell et al. that this modulation can induce much larger changes in the electron mobility than carrier density at 2 K (see Figure 11). [64] Moreover, the dependence of electron mobility with back-gate voltage was found to correlate positively with the carrier density modulation, i.e., with positive voltages applied resulting in larger electron mobilities as well as more charge carriers and vice versa. It is worth noting that this trend is opposite to the typical reciprocal relationship between electron mobility and charge carrier density in as-grown STO-based heterointerfaces, [65,66] which may be explained by a stronger scattering at the interface due, e.g., broken lattice symmetry, STO vicinal steps, or preferential defect formation at the interface. [67,68] Smink et al. studied the effect on low-temperature electron mobility in different bands by top-gating of STO/SrCuO 2 / LAO/STO (see Figure 12). [46] In this study, the authors found that although the overall mobility is reduced upon increasing Adv. Mater. Interfaces 2019, 6,1900772  b) The combined usage of a back-gate and ionic liquid allows charge carrier density changes approaching one order of magnitude at 180 K. Reproduced with permission. [58] Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA. Figure 9. a) Schematic illustration of the gating configuration in the present study. b) Using the positively charged tip of a c-AFM, it is possible at 300 K to induce metastable conducting paths in the nominally insulating LAO/STO heterostructures. c,d) These written nanocircuits can themselves act as lateral gates displaying field effect transistor behavior at 2 K as measured by the current/voltage characteristics between the source and drain (I D vs V SD ) upon application of different gate voltages (V GD ). Reproduced with permission. [28] Copyright 2009, American Association for the Advancement of Science. www.advmatinterfaces.de the applied voltage from the top-gate, the electron mobility of the individual subbands is expected to change much less as the band occupation is varied.
Applying electrostatic gating by an ionic liquid was, on the other hand, demonstrated by Zeng et al. and it was found to have a profound effect on the low-temperature electron mobility in LAO/STO heterostructures (see Figure 13) [26] leading to electron mobilities approaching 20 000 cm 2 V −1 s −1 . This high mobility allowed the observation of Shubnikov-de Haas oscillations (see later in this section and Section 3).
The low-temperature back-gate dependence of carrier mobility in the previously mentioned dual electron-hole system in STO/LAO/STO heterostructures was also investigated by Singh et al. (see Figure 14). [63] The authors of this study elucidated that while electrons behave similar to when being hosted at the simple electron gas LAO/STO heterointerface, holes might display a nonmonotonic dependence with back-gate voltage assuming a maximal value for certain gate voltages. This trend, however, needs to be further investigated by sampling with finer resolved gate-voltage values.

Gating of Superconductivity
A unique property of STO-based heterostructures is the gate tunability of the superconducting phase.
Superconductivity in bulk STO appears at very low carrier concentrations making it the most dilute superconductor found in nature. [69] Interestingly, because of its peculiar domeshaped phase diagram for superconductivity, STO partly motivated the search for high-temperature superconductors. [70] The critical transition temperature for STO shows a dome-shaped dependence on the carrier density with a maximum value of T c ≈ 400 mK for n ∼ 10 20 cm −3 . [71] The dome-shaped dependence of T c as well as the emergence of superconductivity at low carrier densities allows for inducing a normal conductor-superconductor phase transition at a fixed temperature using gating.
After the discovery of interface superconductivity in STObased heterostructures, [72] its back-gating behavior was studied by Caviglia et al. where a dome-shaped dependence with carrier density was also found (see Figure 15). [73] Subsequently, several groups have found a similar behavior of the superconducting phase in other STO-based heterostructures, e.g., amorphous-LAO/STO. [74] On the other hand, Lin et al. argued by considering the bulk SrTiO 3−x that the interface and bulk superconducting phases are behaving differently. [75] In order to investigate the origin of the superconducting phase, multiple groups have used electrostatic gating as a handle to aid in its understanding. Singh et al., for instance, concluded based on resonant microwave transport and backgating experiments that the emergence of superconductivity was linked to the carrier population of higher energy d xz /d yz subbands in STO (see Figure 16). [76] This indicates that the system transitions from initially being an array of weakly coupled Josephson junctions to a homogenous superconductor  b) The dual electron-hole system in STO/LAO/STO heterostructures can readily be gated at 2 K by the electric field from a back-gate with largest modulation of the hole density. c) Back-gating at 2 K of the ordinary 2DEG in LAO/STO heterostructures shows similar characteristics as previously observed for this system. Reproduced with permission. [63] Copyright 2018, American Physical Society. Figure 11. a) Schematic illustration of the gating configuration in the present study. b) Here, the authors found a primary modulation of the electron mobility over the carrier density when the electric field was applied from a back-gate at 2 K. The carrier density and mobility are extracted using the Hall coefficient at 2 and 8 T. The heavy lines are changes in the carrier density expected from the capacitance. Reproduced with permission. [64] Copyright 2009, American Physical Society.
www.advmatinterfaces.de as the carrier density are increased by back-gating. Thus, the maximal critical temperature is expected to occur above the Lifshitz transition in the phase diagram where d xz /d yz subbands start to become populated. The authors ascribe this maximal critical temperature to a competition between electron pairing and phase coherence.
Ultimately, the relationship between the emergence of superconductivity and carrier density as well as electron mobility is still not fully understood. As the gate modulation is expected to change the quantum well confinement and the effective carrier density, it is consequently difficult to disentangle the effects of gate-tuned carrier densities and electron mobility on superconductivity. To this vein, Smink et al. studied the effects of performing top-and back-gating (separately or simultaneously) in LAO/STO heterostructures on superconductivity (see Figure 17). [77] In this study, the authors found that the electrostatic effects on the critical temperature with electric fields applied from top-and back-gates where not identical. Thus, by studying the combined effects of applying top-and back-gate voltages simultaneously and independently, it was shown possible to carefully tune the emergence of the superconducting phase. The authors ascribed the maximal critical temperature to occur when the second d xy subband becomes depleted above the Lifshitz transition. Here, it is worth recalling from Section 2a that the carrier density of d xy subbands was found to be reduced above the Lifshitz transition.   [46] Copyright 2017, American Physical Society. Figure 13. a) Schematic illustration of the gating configuration in the present study. b) While gating with ionic liquid at 2-3 K, it is possible to enhance the electron mobility up to values approaching 20 000 cm 2 V −1 s −1 . Reproduced with permission. [26] Copyright 2016, American Chemical Society. b) The hole and electron mobility in STO/LAO/STO heterostructures appears to respond unequally to the influence of electric field from a back-gate at 2 K with the hole mobility seemingly displaying a maximal value at a certain critical gate voltage. c) The electron mobility of individual subbands in LAO/STO heterostructures show similar response to back-gate voltage at 2 K as previously observed for this system. Reproduced with permission. [63] Copyright 2018, American Physical Society.

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Finally, a number of studies have investigated the effects of laterally constricting the superconductivity by top-gating. For instance, it was demonstrated by Monteiro et al. that the aforementioned Josephson junctions can be formed at STO-based heterointerfaces (see Figure 18). [52] Fluctuations of the critical current through a single Josephson junction was demonstrated in this study. Moreover, the gate-tunable Josephson junction was shown to display superconducting quantum interference oscillations of the critical current through the junction as a function of the externally applied magnetic field.

Gating of Spin-Orbit Coupling
One of the defining features of charge carriers residing at STObased heterointerfaces is a significant gate-tunable Rashba spin-orbit coupling. In the pioneering works of Caviglia et al. and Ben Shalom et al., the low-temperature spin-orbit coupling of LAO/STO was studied from two different angles: magnetotransport and superconductivity (see Figure 19). [78,79] In both of these works, it was inferred that it is possible to tune the spin-orbit coupling by an order of magnitude in the investigated range of back-gate voltages. Both studies supported a nonmonotonic dependence of the spin-orbit coupling with the largest values found to coincide with maximal critical temperature for superconductivity.
However, from the back-gate geometry employed in these two studies, it is not trivial to disentangle the effects of charge carrier density, electron mobility, subband occupation, and quantum well confinement to the resulting spin-orbit coupling. As described in the previous subsections, it is clear that each of these critical parameters (responsible for the spin-orbit coupling) responds dissimilarly upon application of an electric  b) The superconducting transition of LAO/STO heterointerfaces can be tuned by the back-gate potential (V g ) with the critical transition temperature (T BKT ) tunable from 300 mK and below. Reproduced with permission. [73] Copyright 2008, Nature Research. Figure 16. a) Schematic illustration of the gating configuration in the present study. b) Subband carrier densities extracted from two-band fits as a function of top-gate potential (V G ) mapped with the superconducting phase. c) Superfluid density n s 2D calculated from superconducting critical current compared with carrier density of the high effective mass subband (d xz /d yz ). Reproduced under the terms of the CC-BY license. [76] Copyright 2018, Springer Nature. www.advmatinterfaces.de field as well as unequally to different gating geometries. The dependence of spin-orbit coupling on applied top-gate voltage was studied by Hurand et al. [80] In this study, the Rashba spinorbit coupling was found to linearly depend on the applied topgate voltage, with larger spin-orbit splitting as the gate voltage was increased (See Figure 20). Curiously, this dependence did not show much correlation with the position of the superconducting phase as was the case in the previous two studies by Caviglia et al. and Ben Shalom et al. [78,79] In a later study, Niu et al. studied the low-temperature ionic liquid gating of γ-Al 2 O 3 /STO (GAO/STO) where a nonmonotonic dependence on the spin-orbit coupling likewise was found (see Figure 21). [81] This study will be revisited later in Section 3.
One principal consequence of the magnitude of the spin-orbit coupling is to be able to dictate the efficiency for spinto-charge interconversion. Spin-to-charge conversion was demonstrated in NiFe/LAO/STO by Lesne et al. [82] It was found that the electric field from a back-gate at 15 K not only controls the magnitude of generated charge current but also the sign. This sign change of the effective Rashba coefficient was explained by the two types of relevant t 2g subbands in STO (d xy and d xz /d yz ) having opposite Rashba coefficient signs. In this way, the relative population of each of the two types would correspond to an effective overall Rashba coefficient that in turn would dictate the net spin-to-charge conversion (see also Section 3).

Gating of Magnetism
Magnetic order and exchange coupling strength in STO-based heterostructures have been shown to depend on electrostatic gating. This was investigated at low temperatures using anisotropic MR and anomalous Hall effect of back-gated LAO/ STO by Joshua et al. [83] In this study, it was found that the interface system displays a large field anisotropy and spin polarization above the Lifshitz transition originating from an exchange coupling between localized d xy magnetic moments and the itinerant electrons. The relative carrier population of the d xy versus d xz /d yz plays a significant role in determining the resulting magnetic properties of the interface. Here, a ferromagnetic coupling between localized d xy magnetic moments and d xz /d yz itinerant electrons was proposed to compete with an antiferromagnetic coupling between the magnetic moments and d xy itinerant electrons. The magnetic state at room temperature proposed to originate from the localized d xy electrons was furthermore modulated by Bi et al. [84] Using magnetic force microscopy, the authors found it possible to induce and observe in-plane ferromagnetic phases. In another seminal study of magnetic interactions at STO-based conducting interfaces, Stornaiuolo et al. studied the effect of inserting a ferromagnetic spacer layer (EuTiO 3 ) between LAO and STO. [85] Below the ferromagnetic transition temperature of EuTiO 3 , an exchange coupling was found between the localized moments of Eu atoms and the above-mentioned electrons in Ti 3d orbitals. This resulting magnetic state was found to be gate tunable as observed from the anomalous Hall effect, with a sharp transition around a few tens of volts (see Figure 22). The co-existence of a superconducting Adv. Mater. Interfaces 2019, 6, 1900772 Figure 17. a) Schematic illustration of the gating configuration in the present study. By individually b) top-gating and c) back-gating LAO/STO, the response on critical transition temperature for superconductivity is found to be dissimilar due to the different subband tuning in the top-and backgate configurations. Reproduced with permission. [77] Copyright 2018, American Physical Society.   Reproduced with permission. [81] Copyright 2018, American Chemical Society. c) The temperature dependence of the anomalous Hall effect gated well into the exchange coupling regime. Reproduced with permission. [85] Copyright 2015, Springer Nature. Figure 19. a) Schematic illustration of the gating configuration in the present study. b) The electric field from a back-gate is found to allow tuning of the inelastic scattering time (τ i , red), spin-orbit scattering time (τ so , blue), and spin relaxation time (open circles). c) Electric field tuning of the Rashba spin splitting (Δ, red) and the Rashba coupling constant (α, gray). Reproduced with permission. [78] Copyright 2010, American Physical Society.

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phase was also found to be present in this heterostructure despite the presence of the EuTiO 3 layer.

Gating of Local Microstructure and Domain Walls
One physical property that has been shown possible to modulate is the local tetragonal microstructure and domain walls at temperatures below 105 K. A couple of studies have demonstrated that the tetragonal domains forming in STO below the cubic-to-tetragonal structural phase transition at 105 K can be controlled and indeed moved by the application of an electric field from a back-gate. By probing the electrostatic landscape of LAO/STO interface by a scanning field effect transistor, Honig et al. found lateral tetragonal domain motion with applied back-gate voltage at 2 K (see Figure 23). [86] At the same time Kalisky et al., who studied back-gated LAO/STO with a scanning superconducting quantum interference device (SQUID), found no discernible movement of such tetragonal domains under similar experimental conditions. [87] The gate control of domain structure hence remains under-investigated and should be the subject of further investigations.

Gating of Quantum Transport Phenomena
The ability to gate the electron mobility in STO-based heterostructures opens up for the observation of quantum transport phenomena that require long mean free paths to be detectable. For example, using back-gate stimulation of delta-doped STO and La 7/8 Sr 1/8 MnO 3 buffered STO-heterointerfaces, the quantum Hall effect has been demonstrated for the first time in complex oxides. [88][89][90] The manifestation of the quantum Hall effect in these two systems will be covered later in Section 3. Likewise, gating by back-gate and ionic liquid have allowed observation of Shubnikov-de Haas oscillations in LAO/ STO. [26,91,92] Moreover, tuning of the spin-orbit coupling and the superconducting phase have allowed the observation of electron pairing behavior without the presence of superconductivity. [50,54] Finally, by lateral gating with c-AFM written conducting paths in LAO/STO, ballistic transport of single electrons as well as electron pairs across length scales approaching 20 µm have been demonstrated. [93]

Future Prospects of Electrostatic Gating
STO possesses a rich landscape of physical properties that can be electrostatically modulated simultaneously by application of the electric field from a gate. Due to this intertwined nature of the physical properties, the key to unlocking the future potential of STO-based devices with novel functionalities may lie in disentangling the degrees of freedom, e.g., by double gating, applications of magnetic fields, structural confinement in nanowires, or even quantum dots just to name a few. For example, a study by Chen et al. exploited the individual control of topand back-gating to study the superconducting phase of LAO/ STO. [94] The growth of STO thin films with high mobility and crystalline quality [95] further expands the possible gating geometries as it enables the design of symmetric quantum wells gated in close proximity by heavily doped epitaxial layers. Finally, unlocking and engineering the domain walls upon electrostatic gating of STO holds the prospect of many exciting discoveries to come. [96]

Magnetic Field
In this section, we review the effect of a magnetic field on STObased heterostructures with a focus on the magnetoresistance (MR), Hall effect, magnetic proximity effect, and spin/charge interconversion.

Magnetoresistance
MR is the resistivity change occurring in materials subjected to the application of a magnetic field. When applying the magnetic field perpendicular to the sample surface, a positive MR is commonly observed, whereas magnetic fields applied parallel to the surface may lead to a negative MR. At low temperatures quantum corrections to the conductivity such as weak localization (WL) and weak anti-localization (WAL) can occur in the low magnetic field regime while Shubnikov-de Haas oscillations can appear at high magnetic fields.

Ordinary MR
Electrons moving under the presence of a perpendicular magnetic field will deflect from their original trajectory  is demonstrated to be possible with the application of an electric field from a back-gate at low-temperatures. Reproduced with permission. [86] Copyright 2013, Springer Nature.

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due to the Lorenz force, leading to a positive MR as shown in Figure 24a. Such a positive ordinary MR can often be observed in metals. In a metallic system with carriers occupying only a single band, the ordinary MR shows a parabolic field dependence where the MR is proportional to (µB) 2 with µ being the carrier mobility. For 2DEGs at STO-based oxide interfaces, a bell-like MR has been widely observed when the carrier density is higher than the critical carrier density mentioned in Section 2 above which both d xy and d xz / d yz bands start to become occupied (the Lifshitz transition). In this case, the MR is parabolic at low field, but at a critical field it deviates from the parabola and turns into a belllike shape. [43,46,97] The corresponding Hall resistance shows a nonlinearity at the same critical field as described in the following subsection on the Hall effect. The bell-shaped MR originates from both d xy and d xz /d yz electrons, when the Fermi level is higher than the bottom of the d xz /d yz band, which is well explained by a two band model. [43] In some cases such as Figure 24a, the MR deviates from its quadratic dependence at low fields by turning into a linear, nonsaturating positive MR, which has been attributed to an inhomogenous conductivity. [98,99] When applying a magnetic field parallel to the interface, the MR is often reported to be negative and dependent on the orientation between the current and magnetic field, as shown in Figure 24b,c. [98] Both the negative and anisotropic in-plane MR is often associated with the presence of magnetism. [83,98,[100][101][102] In this vein, the negative MR occurs as the magnetic field aligns the magnetization of magnetic domains and thus results in a lowering of the scattering and the resulting resistance, whereas the in-plane anisotropy in the MR may stem from the easy axes of the magnetization. An anisotropic MR with twofold and sixfold symmetries has also been observed for the (110)-and (111)-oriented STO when rotating the inplane magnetic field from 0° to 360° with respect to the current direction. [103]

WAL and WL
Disorder at the interface becomes very important at low temperature and may originate from various sources such as oxygen/cation vacancies, cation intermixing, and tetragonal domain walls. Disorder can modify the wave behavior of electrons from extended states to localized ones, resulting in a peak in the MR at low magnetic fields. The peak is a signature of WL, which originates from the path of an electron undergoing several elastic scattering events which may interfere positively with the same path traversed in the opposite direction. [104] This increases the probability of localization and hence increases the resistance. Breaking of the inversion symmetry at the interface may lead to the Rashba spin-orbit coupling, which can result in the negative interference that characterizes the WAL and produces a lowering of the resistance. [105] When magnetic fields are applied, both the positive and negative interferences are destroyed by the extra phase picked up by the Aharonov-Bohm effect. This results in a peak (dip) in the MR around B = 0 T for WL (WAL).
WL and WAL have frequently been observed in STO and STO-based heterostructures as also reviewed by Gariglio et al. [105] The WAL provides a convenient handle to study the spin-orbit coupling, which has been shown to be tunable using electrostatic gating, [78,81] as discussed in the previous Section 2. This is well in line with first principles density functional theory (DFT) calculations that have predicted that the Rashba spinorbit coupling reaches a maximum due to the band hybridization at the critical point where d xy and d xz /d yz bands cross. [106][107][108]

Shubnikov-de Haas Oscillations
Shubnikov-de Haas oscillations are formed due to a magnetic field-induced change in the band structure and provide an accessible avenue to probe the Fermi surface at the conducting Adv. Mater. Interfaces 2019, 6, 1900772 Figure 24. MR of LAO/STO under application of: a) an out-of-plane magnetic field, b) an in-plane magnetic field parallel to the charge current direction, and c) an in-plane magnetic field perpendicular to the charge current direction. Reproduced with permission. [98] Copyright 2011, Springer Nature.

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interfaces. [34,109] Superimposed on the MR background, Shubnikov-de Haas oscillations can be observed in samples with high mobility (≥ 2000 cm 2 V −1 s −1 ) at low temperatures (<5 K) and high magnetic fields (a few T), where the requirements of ω c τ > 1, ℏω c > k B T are fulfilled. [91] Here, ω c and τ are the cyclotron frequency and scattering time, respectively. Many methods have been employed to achieve a high mobility in oxides as reviewed Trier et al. [66] For example, using surface treatments, Xie et al. achieved a mobility higher than 20 000 cm 2 V −1 s −1 for the LAO/STO system. [110] Huijben et al. incorporated a thin layer of SrCuO 2 between the surface of a LAO/STO heterostructure and a capping layer of STO. This enhanced the oxygen exchange and a mobility up to about 50 000 cm 2 V −1 s −1 was achieved due to a reduction of interfacial impurity scattering from oxygen vacancies. [47] Chen et al. found a very high mobility of ≈1 40 000 cm 2 V −1 s −1 at the spinel/perovskite heterointerface of GAO/STO. [111] Shubnikov-de Haas oscillations were apparent in these high-mobility 2DEG systems, as shown in Figure 25a for the case of GAO/STO.
Information about the Fermi surface, carrier density, effective mass, and mobility can be deduced from Shubnikov-de Haas oscillations. The angular dependence of Shubnikov-de Haas oscillations also gives information on the confinement of the heterointerfacial conducting subbands. This is the case since only the out-of-plane component of the magnetic field leads to Shubnikov-de Haas oscillations for the confined electrons. After optimizing the growth conditions of LAO/STO to achieve a mobility of 6600 cm 2 V −1 s −1 , Caviglia et al. observed that the frequency of the Shubnikov-de Haas oscillations only depended on the perpendicular magnetic field component, which is consistent with 2D confinement of the electron gas at the interface (see Figure 25b). [91] A common observation in all the studies of Shubnikov-de Haas oscillations at STO-based heterostructures is that the carrier density deduced from the oscillation frequency (n SdH ) consistently was found smaller than the one obtained from the Hall measurements (n Hall ). [109,112,113] Some mechanisms have been proposed to account for this apparent disagreement. A portion of the carriers measured using the Hall effect may suffer from extensive scattering leading to a mobility that is too low to meet the condition needed for observing Shubnikov-de Haas oscillations. [114] Another possible explanation for the discrepancy relies on nontrivial degeneracies in oxide 2DEG systems. For example, the spin, valley, and magnetic breakdown orbits may lead to twofold, threefold, and fourfold degeneracies, respectively. [112,115] In addition, the presence of multiple subbands with different carrier densities in the quantum well may also account for the difference between n Hall and n SdH . [88,115] The presence of complicated degeneracy or subbands makes it difficult to derive the accurate carrier density from Shubnikovde Haas oscillations.

Hall Effect
When applying a magnetic field perpendicular to the heterointerface, different contributions to the measured Hall effect can occur such as the normal Hall effect, anomalous Hall effect, [116][117][118] quantum Hall effect, [88,90,115] and spin Hall effect. [119] The following gives a short description of the different types of Hall effects that have been observed in STObased heterointerfaces.

Normal Hall Effect
In a simple conductor with the conductivity occurring in a single band, the Hall resistance (R xy ) varies linearly with magnetic field. For STO-based heterostructures, the Hall resistance typically deviates from linearity at temperatures below 105 K. This deviation arises from the two subband occupation of the t 2g orbitals. By carrier depletion under the application of an electric field from a gate (see Section 2), the Hall resistance can revert between nonlinear and linear (see   [111] Copyright 2015, Nature Research. b-c) Reproduced with permission. [91] Copyright 2013, American Physical Society.

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crossing the Lifshitz transition determining if the conducting electrons occupy a single subband (d xy ) or two types of subbands (d xy and d xz /d yz ). [43]

Anomalous Hall Effect
The anomalous Hall effect arises from a coupling between the itinerant electrons and magnetic moments, as observed in several oxide heterostructures. [83,97,101,117] As shown in Figure 27a, a signature of the anomalous Hall effect is a downward curvature that bends the curve clockwise, in contrast to the aforementioned counter-clockwise bending with the nonlinear Hall effect stemming from conduction in two n-type subbands. Subtracting the contribution of the ordinary Hall effect determined from the low-field slope of R xy versus B makes the anomalous Hall effect clearer, as displayed in Figure 27b. In contrast to the case in Figure 27, the anomalous Hall effect typically only gives a small contribution to the total Hall effect due to the weak magnetization and/or a weak coupling between itinerant moments and the magnetization. [83,97,117] In these cases, the anomalous Hall effect can be hard to deduce directly from the raw data, whereas dR xy /dB provides a better way of identifying this contribution. [97,117] In order to make the effects of the magnetic state more pronounced, one of the common strategies used is to introduce ferromagnetism in LAO/STO using the magnetic proximity effect from magnetic dopants as discussed in Section 3.3.

Quantum Hall Effect
By improving the electron mobility of the 2DEG in STO-based heterostructures, observation of the quantum Hall effect is possible. This has been observed in the modulation-doped   . Reproduced with permission. [43] Copyright 2012, Springer Nature.

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amorphous-LAO/STO heterostructure, which exhibits both high electron mobility exceeding 10 000 cm 2 V −1 s −1 and low carrier density on the order of ≈10 12 cm −2 at temperatures below 1 K. [88] As shown in Figure 28a, the magnetic field dependence of R xx shows quantum oscillations at a temperature of 30 mK and a gate voltage of 6 V. Concomitantly, R xy exhibits a steplike behavior as a function of B where the minima in dR xy / dB coincide with the minima in R xx . Surprisingly, R xy −1 does not show steps with integer values of e 2 /h as always observed in conventional semiconductor quantum wells comprised of a single band. Moreover, as shown in Figure 28b, the value of R xy −1 for the same plateau varies as a function of V G , in contrast to the case of conventional semiconductors where it remains constant. The plateaus, however, appear regularly spaced for all V G with either ΔR xy −1 ∼ 10 ± 2 e 2 /h or ΔR xy −1 ∼ 20 ± 2 e 2 /h for B > 6 T and B < 6 T, respectively. Trier et al. conclude that when the carrier density is below the Lifshitz point, the interface 2DEG is comprised of a single quantum well with multiple parallel conducting channels, here 10, as displayed in Figure 28c. These channels have a similar effective mass and Hall mobility, whereas their individual carrier densities differ. While the mean carrier density approximately was concluded to dictate the frequency of Shubnikov-de Haas oscillations, the total carrier density from these channels corresponds to that measured from the Hall effect n Hall . This provided a possible explanation for the commonly observed difference between n Hall and n SdH . [88]

Spin Hall Effect
In a system with a strong spin-orbit interaction, a longitudinal charge current can give rise to transverse spin current via the spin Hall effect. Jin et al. demonstrated a sizeable spin/ charge interconversion through the spin Hall effect in LAO/ STO. [119] As shown in Figure 29a, an injected charge current through two opposing Hall bar probes can, via the direct spin Hall effect, induce a transverse spin current. If the polarization of the spins is not lost before they reach a neighboring probe pair, the spins can become reconverted into a charge current through the inverse spin Hall effect and induce a detectable nonlocal voltage. The spin diffusion induced by the spin Hall effect through the bridging channel in the Hall bar device can be confirmed by the signature of spin precession and the dependence on probe spacing. The nonlocal voltage as a function of in-plane magnetic field produced a Hanle curve as shown in Figure 29b. Here, the spin Hall effect induces the spin current along the bridging channel with its polarization perpendicular to the plane. Thus, the in-plane magnetic field causes the Larmor precession of spins. The Hanle curves shown in Figure 29b displayed a narrower width for the longer channel because the transit time for the carrier spin to process was increased. The amplitude of the nonlocal resistance signal was also shown to decay exponentially with the channel length (see Figure 29c). [119]

Magnetic Proximity Effect
The possible 2D magnetic ground state at LAO/STO has attracted great interest, although it remains elusive whether the ground state of the LAO/STO system is ferromagnetic. One of the common strategies to introduce ferromagnetism in LAO/STO is through the magnetic proximity effect where magnetic dopants are used to induce or enhance the magnetic state in their vicinity. [120] Anomalous Hall effect provides strong  www.advmatinterfaces.de evidence of coupling between itinerant electrons and magnetic moments, as shown in Figure 30. As also described in Section 2, Stornaiuolo et al. reported a gate-controlled spinpolarized 2DEG by inserting a few unit cells of ferromagnetic EuTiO 3 at the LAO/STO interface. In this system, the exchange interaction between the magnetic moments of Eu-4f and Ti-3d electrons is dominant. [85] Zhang et al. observed a similar anomalous Hall effect at a La 7/8 Sr 1/8 MnO 3 -buffered LAO/STO interface. In this case, Mn ions provides a magnetic scattering center with the local magnetic moment on Mn 2+ or Mn 3+ . However, these spin-polarized electrons have been detected often in the high carrier density range where both the d xy and d xz /d yz electrons are populated. [120] Based on the ferromagnetism of LMO thin films on the STO, [121] Gan et al. later detected the anomalous Hall effect in oxide interfaces of LaAl 0.7 Mn 0.3 O 3 / STO where only the d xy band are populated. [97] Since superconductivity has been reported widely at the LAO/STO interface (see Section 2), the magnetic proximity effect not only provides an effective way to induce/create ferromagnetic states but also opens a door to explore the coexistence of superconductivity and magnetism. In a conventional physical picture, superconductivity and ferromagnetism are mutually exclusive phenomena, but a possible coexistence has been reported both in Stornaiuolo's and Gan's studies [85,97] as well as in regular LAO/STO heterostructures without magnetic proximity effects. [122,123] The former study suggested that the ferromagnetism and superconductivity possibly occur in different bands that are spatially separated from each other. However, this suggestion was challenged by Gan et al. who observed the spin-polarized and superconductive d xy electrons in a single device with only d xy subbands occupied.

Spin/Charge Interconversion by use of Ferromagnets
The giant tunable Rashba spin-orbit coupling, high mobility, and long momentum relaxation time at the oxide interfaces provide a promising combination for applications in spintronics. [124][125][126] Using a three-terminal device, Reyren et al. first reported injection of a spin current from a ferromagnetic Co layer into the LAO/STO 2DEG through Hanle experiments. They attributed this injection to take place through defects in LAO. [127] However, Swartz et al. argued that the signal measured by the three-terminal geometry should be attributed to spurious effects such as spin traps rather than the spin accumulation. [128] The four-terminal nonlocal configuration gives opportunities for interpreting the spin-related effects unambiguously, as displayed in Figure 31a.
Besides transport experiments, spin pumping in ferromagnetic resonance experiments provides an effective avenue to inject spin currents (Figure 31b). In 2D systems, Rashba spinorbit coupling induced by inversion symmetry breaking allows charge-spin interconversion by the direct and inverse Edelstein effects. Lesne   www.advmatinterfaces.de the 2DEG at the interface of LAO/STO by performing spin pumping ferromagnetic resonance experiments. Herein, a 3D spin current was converted into 2D charge current due to the inverse Edelstein effect and the spin-to-charge conversion efficiency was unprecedented high with λ IEE = 6.4 nm at T = 7 K where the inverse Edelstein effect length, λ IEE , indicates the efficiency of the spin-to-charge conversion. [82] Using the same method, Chauleau et al. achieved temperature-dependent spin-to-charge conversion, the λ IEE was 1 nm at 77 K and decreased upon increasing temperature. [130] Beyond the scope of LAO/STO, Zhang et al. injected a spin current into the conducting STO surface, but the spin-to-charge efficiency was low with λ IEE = 0.23 nm at room temperature, which might be due to the surface roughness of Ar + irradiated STO samples. [131] Later, Han et al. achieved successful spin-to-charge conversion in LAO/STO 2DEG systems with different LAO thickness ranging from 3 to 40 u.c. There is a striking discrepancy between their results and previous reports as high efficiency spin-to-charge conversion only occurred at room temperature in their samples. Surprisingly, when the thickness of LAO reached 40 u.c., spin current was still injected into 2DEG at the interface. [132] This indicates that spin current transport in LAO occurs via a mechanism, [127] such as defect hopping, which requires further exploring. It is proposed that the giant Rashba spin splitting at STO surface [133] and at the interface of GAO/STO with extremely high mobility [111] could yield a new record of conversion efficiency with a value of λ IEE beyond 100 nm. [125] Moreover, the tunability of the Rashba spin-orbit   [85] Copyright 2015, Springer Nature. b) Reproduced with permission. [120] Copyright 2017, American Physical Society. c) Reproduced with permission. [97] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.  [125] Copyright 2018, Springer Nature. c) Reproduced with permission. [129] Copyright 2018, American Chemical Society.

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Han et al. had for instance already enhanced and controlled the conversion efficiency by applying gate bias. [82,129] Spin-orbit coupling-related spin-orbit torque can affect the magnetization effectively, showing potential in low power consumption, high density, and fast speed magnetic random access memories. Highly efficient charge-to-spin conversion is the key factor for the spin-orbit torque technique. Direct chargeto-spin conversion via the Edelstein effect remains scarce. Charge-to-spin conversion in oxide interface was first demonstrated by Wang et al. via spin torque ferromagnetic resonance (Figure 31c). The conversion efficiency was estimated to be 6.3 and inelastic tunneling via localized states in LAO was attributed to the spin transmission process. [129]

Perspective
The interfaces of transition metal oxides exhibit a tunable spinorbit coupling, which may find use within quantum computations and multifunctional spintronic devices. [134] STO-based 2DEG systems can achieve gate-tunable spin-to-charge interconversion with a high efficiency as well as being a good spin conductor with long-range spin transport. Prediction for STO with its large Rashba splitting and for the interface of GAO/ STO with the high mobility suggests that a new record for the spin-to-charge conversion, λ IEE exceeding 100 nm could be achieved, which exceeds that of LAO/STO by an order of magnitude. [125] Since the performance of spintronic device is limited by the efficiency of spin-charge interconversion, pursuing the high efficiency in these two STO-based systems is of great significance and value. However, this efficiency may be highly reduced by the capping layer such as LAO and GAO, which impede the spin transferred from the magnetic layer to the interface. The mechanism for the transmission of the spin current to the interface is still unclear and needs further investigations. More efforts should be given to understanding the parasitic effects and spurious signals possible by spinpumping in ferromagnetic resonance experiments. Interestingly, a spin current was recently shown possible to be injected into the ferromagnetic 2DEG via the spin Seebeck effect in EuO/KTaO 3 , [135] providing inspiration for an effective spin-tocharge conversion pathway.

Light/Matter Interaction
Among the large spectrum of attractive properties and emergent phenomena at oxide heterointerfaces, some of the most fascinating properties are found upon light illumination. Light illumination can generate electron-hole pairs, which not only can contribute to the conductivity but also affect the diffusion of oxygen vacancies and thereby significantly modify the transport properties. [136,137] For example, tunable persistent photoconductivity (PPC) has been reported in STO-based heterointerfaces [138,139] and combining light illumination with a gate bias, an enhancement of the resistance gatability has been demonstrated. [40,140,141] In this section, we review the optical tunability and the underlying physical mechanisms. The application of light may offer new insights toward alloxide optoelectronics devices. [136]

Persistent Photoconductivity
PPC is an effect where the conductivity is increased upon light illumination and remains increased after the light is turned off. This enhanced conductivity may persist for days after turning off the light, [139,142] which offers new opportunity toward the photoelectronic applications, such as radiation detectors. [139,143] In conventional semiconductors, such as AlGaAs/GaN or Si membranes, PPC has already been reported. [144] However, their practical application is limited by the low magnitude of the PPC or the low operation temperature. [145,146] A large PPC was observed at room temperature in the LAO/STO heterostructure, when illuminated either by a UV lamp of 395 nm or with visible light, as shown in Figure 32. [139] The photo-induced conductance is increased by five orders of magnitude, much higher than the one observed in conventional semiconductor heterostructures. [139,145,146] Generally, the PPC is attributed to a spatial separation of the photoexcited electron-hole pair in LAO/STO heterostructures. [139,147] Due to the existence of this induced photoconductivity, special care should be taken before performing transport measurements, such as storing the sample in dark for more than 24 h. [142] Similar illumination experiments were carried out by Guduru et al. at the LAO/STO interface with a LAO layer thickness of 10 nm, as shown in Figure 33a. Here, they illuminated the interface with different photon energies ranging from 1.44 to 3.65 eV. [148] Upon illumination, the resistance decreased gradually with increasing photon energy, however, with a relatively small decrease observed when the photon energy was smaller than the band gap of STO at approximately 3.4 eV. The authors attributed these small changes to the reduced absorption of light by in-gap (IG) states in STO. However, when the photon energy exceeded the band gap, the PPC was enhanced by more than 50%. Using Hall measurements without or with light, the authors found a transition from a linear Hall resistance to a nonlinear Hall resistance (Figure 34b). This suggests that additional parallel conducting channel was created by the illumination. [148] Adv. Mater. Interfaces 2019, 6, 1900772   Figure 32. Normalized photo-induced conductance at STO or the interface of LAO/STO kept in dark or illuminated with a 395 nm UV lamp (pink dots) or visible light (blue dots). Reproduced with permission. [139] Copyright 2012, American Chemical Society. www.advmatinterfaces.de

Modulation of the PPC Effect
The conductivity, carrier density, mobility, and even the electronic structure of STO-based heterointerfaces are found to be very sensitive to the growth condition and in particular to the oxygen pressure and the growth temperature. [142] Likewise, the magnitude and relaxation dynamics of the PPC effect also depend crucially on the growth parameters, doping, and surface treatment as will be discussed in the following.

The Effects of Doping
Tailoring the properties of heterointerfaces, such as the ground state and metal-insulator transitions, remains challenging, but substitution or doping may provide an effective way of tuning oxide interfaces. [97] In conventional semiconductors, such as GaAs, a large enhancement is achieved in the relaxation process of the PPC effect by Cr doping. [149] Motivated by this, Dogra et al. substituted Al with Cr during growth of LAO in LAO/ STO heterostructures. Here, electron correlations are expected to be increased due to the 3d character of Cr which led to an enhanced photoresponse and decay time. [150] In addition, Rastogi et al. have investigated the effect of δ-doping on the change in the photoconductivity in the LAO/STO interface using an insulating LaMnO 3 mono layer. The authors found that photoresponse of the δ-doped LAO/STO interface shifts toward a lower photon energy and has a large photoresponse while the pristine LAO/STO heterointerface is sensitive to near-ultraviolet radiation and has a relatively small change in resistance upon illumination. [151] Yan et al. investigated the transport properties of LaAl 1−x Ni x O 3 /STO under light irradiation after substituting Al with Ni. The relative photoinduced resistance changes were improved from 800% to 6600% upon increasing the dopant level of Ni from x = 0 to 0.05. They attributed this change to the band bending and the larger ionic size of Ni compared to Al, which may cause defects that can hinder the recombination of electrons and holes. [152]

The Effects of Surface Treatment
Although the conducting interface in STO-based heterostructures is capped by an oxide thin film, the interaction with surface adsorbed species on the oxide top-layer can very heavily influence the interface 2DEG (see Section 6). [153,154] In this vein, Xie et al. have showed that surface states can be changed by polar liquids leading to a prominent impact on the transport properties of the heterointerfaces. [153] Brown et al. achieved a giant reversible switching of the conductivity at the interface of LAO and STO by means of immersing the sample in deionized water and exposing it to light as shown in Figure 34a. [155] By applying water to the surface, the interface turned insulating. The conducting states could be recovered by illumination using a broadband light. The reversible switching induced four orders of magnitude change in the resistance and was attributed to the protonation and deprotonation of the LAO layer. Tuning of the interfacial conductivity was also carried out using surface absorbents by depositing Pd nanoparticles with a size of 2 nm on the surface of LAO. [156] Compared to the bare LAO/STO samples, as displayed in Figure 34b, a large optical switching with a photoconductivity on/off ratio of 750% under UV light irradiation was observed. The catalytic effect of Pd nanoparticle, as commonly used in the light sensing and gas sensing applications, [157,158] was suggested as the origin of the enlarged photoresponse.

Combined Effect of Gate Bias and Light Illumination
Electrostatic gating and light illumination are two common and independent ways used to tune the carrier density, electronic    [155] Copyright 2010, The Authors, published by Nature Research. b) Reproduced with permission. [156] Copyright 2018, American Chemical Society. www.advmatinterfaces.de structure, and ground state of oxide heterointerfaces. Gate bias provides an effective way to accumulate or deplete carriers (see Section 2), while light illumination can generate extra carriers from promotion of itinerant carriers from the valence band or trapped states, for instance, the IG states of STO. Lei et al. reported a significantly enlarged gate bias dependence when it is coupled to light illumination. [40] As sketched in Figure 35a, they applied electrostatic gating and light illumination simultaneously at the epitaxial LAO/STO interface as well as the amorphous-LAO/STO interface. [7] Without illumination, as displayed in Figure 35b,c, the authors found two responses when applying a negative bias: a slight rapid jump in resistance due to the capacitive depletion of carriers and a subsequent slow incremental increase attributed to a redistribution of oxygen vacancies. Once applying light illumination, the slow process presumably caused by of oxygen vacancies was accelerated dramatically and the interface resistance was turned "OFF" by a 200-fold increase in the resistance as seen in Figure 35b. The combined effect of light illumination and application of a negative bias results in a decrease in the carrier density, rather than an accumulation as expected from light illumination alone. [40] Further investigation have revealed that this cooperative effect on the metallic heterointerface is temperature sensitive: [159] this effect was relatively weak at the intermediate regime of temperature (50 K ≤ T ≤ 200 K) but strong at temperatures of T < 50 K and T > 200 K. In contrast to the long recovery by electrostatic gating, an instantaneous transition to the metallic ground state was achieved by the illumination with visible light as reported by Safeen et al. [160] Encouraged by the tunability of this joint effect where gate bias is combined with light illumination, Cheng et al. demonstrated that the quantum states could also be tuned via a gating effect by light at the STO-based interfaces, as shown in Figure 36. [140] The transition from WL to WAL implies that the Rashba spin-orbit coupling is tunable by the light illumination with different wavelengths (470 and 940 nm). [140] In contrast to the conventional electrostatic gating at low temperatures (see Section 2), this optical manipulation is nonvolatile and the tunability of the spin-orbit coupling persists after turning off the light. The effect of the light could, however, be removed by warming up the sample to room temperature and then cooling it down to 1.5 K. [140] This advantage shows promi sing application for the optical nonvolatile devices. Zhang et al. have expanded the application of optical gating to other oxide heterointerfaces such as KTaO 3 -based 2DEGs. [141] In their KTaO 3based samples, the Fermi energy was tuned from 13 to 488 meV via optical gating leading to a control of the electronic structure and the Lifshitz transition, which echoes previous theoretical predictions and experimental findings in STO-based 2DEGs. [81,161]

Suppression of Kondo Effect
A resistance upturn for T < 20 K is often observed in STO-based systems and it is frequently attributed to the Kondo effect. The Kondo effect arises as the itinerant electrons interact with the magnetic moments, such as localized carriers associated with oxygen vacancies. [58,81,162] Jin et al. first reported that the Kondo effect at the LAO/STO interface could be suppressed under the illumination of ultraviolet light. [163] Later, in LaAl 1−x Ni x O 3 / STO, it was found that the Kondo effect could be enhanced by increasing the Ni level from x = 0 to 0.05 as the dopants provided localized spin centers, but after light irradiation this effect was also reduced. [152] Similarly, the Kondo effect was also found to be suppressed under the visible light of 650 nm in amorphous-LAO/STO. [164] It was proposed that the coherence between the localized spin centers was minimized by the light Adv. Mater. Interfaces 2019, 6, 1900772   Figure 36. Transition from WL to WAL by optical manipulation with different wavelength (470 and 940 nm). For each wavelength, the LAO/ STO sample has been subjected to one illumination step followed by a measurement of the magnetoconductance (Δσ). This has been repeated, giving rise to different curves with the darkest blue being the initial curve and the darkest red being the curve after around 25 times illumination. Reproduced with permission. [140] Copyright 2017, American Chemical Society. Figure 35. a) The schematic set-up for the joint effect of gate bias and light illumination. b) Sheet resistance tuned by the combination of gate bias and light. c) Zoom-in on the sheet resistance evolution when applying negative bias in the absence of light illumination. Reproduced with permission. [40] Copyright 2014, Springer Nature.

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irradiation thus suppressing the Kondo effect. Concomitant with this suppression of the Kondo effect by light illumination, the mobility at the interface was enhanced from ≈50 cm 2 V −1 s −1 without light irradiation to 125 cm 2 V −1 s −1 while the carrier density was unchanged at a temperature of 2 K. [164]

Photon-Induced Oxygen Vacancies
In contrast to using the light illumination to probe the interface properties, additional effects such as formation of oxygen vacancies is often experienced when irradiating with synchrotron radiation. [165][166][167][168] A comprehensive investigation on photoinduced oxygen vacancies was carried out by Gabel et al. [165] Figure 37a shows how irradiating LAO/STO with UV-light from a synchrotron under controlled molecular oxygen dosing causes the emergence and disappearance of delocalized electrons probed using photoelectron spectroscopy. [165] Recording LAO/STO samples at the Ti L edge in resonant photoemission spectra, two peaks dominate: the IG peak from localized carriers at ≈1.3 eV binding energy induced by oxygen vacancies and the quasi-particle (QP) peak at Fermi level leading to the metallic behavior at the interface. [147] The strength and weight of the IG peaks increased with the exposure of X-ray, indicating that more oxygen vacancies are induced under irradiation (see Figure 37b). This increase in the oxygen vacancy level could be countered by supplying oxygen using a flow of O 2 to the sample during the irradiation of the synchrotron beamline. A dynamic equilibrium between the photon-induced oxygen vacancies and the supplement of oxygen could be achieved by controlling the oxygen dosing, and therefore the intrinsic properties of the heterointerfaces could be further explored by synchrotron irradiation upon minimizing the concentration of oxygen vacancies.
Chikina et al. wrote metallic conducting areas between STO and silicon using X-ray irradiation. [166] Here, light was used both to induce oxygen vacancies and conductivity in STO by transfer of oxygen from STO to Si in addition to real-time monitoring this process using photoemission. This photolithography-like approach for patterning stable metallic channels can be of interest for future oxide-based electronic devices if it can be implemented using more accessible equipment.

Perspective
The tunable photoresponsive properties of the STO-based conducting interfaces offer additional insights toward photoelectric applications. For examples, oxygen vacancy creation or acceleration of their movement by illumination provides not only an effective way to probe the on-demand properties in STO-based 2DEG systems, but also an easy way to "write" conducting channels, fabricate 2DEG devices or enhance switching speeds in memristors via light radiation. In addition, the large tunability of the conductivity at the interface when simultaneously applying light and a gate bias provides the possibility to explore the photocontrolled Rashba spin-orbit coupling and related quantum phases, as well as all-oxide spintronic devices. Furthermore, since magnetism has been observed and can be tuned by electrostatic gating in STO-based heterostructures, [84,120] the question remains whether magnetism can also be conveniently induced and tailored by light. Antiferromagnetism induced using light irradiation has already been predicted in STO-based systems. [169] The antiferromagnetic order was proposed to be formed by the Pomeranchuk-type instability. However, optically induced magnetism in STO-based heterostructures remains undemonstrated.

Stress and Strain
With the ongoing scaling of semiconductor devices, the semiconductor industry is facing several critical challenges. New device structures, new materials, and strain engineering are investigated in order to improve the performance of the devices. [170] The predominant focus of the industry is on biaxially stressed devices, where strain arises from stretching in two distinct directions simultaneously. Attention has also been given to uniaxial stress due to the possibility of obtaining large mobility enhancements and small shifts in transistor threshold voltages. [171] Implementation of strain as a performance enhancing element has been phenomenally successful and effective since the introduction of the 90 nm technology, and uniaxial stress was successfully integrated into MOSFET processes to improve device performance. [172] In oxides, the lattice is closely coupled to the charge, orbital and spin degrees of freedom, and strain in this class of materials therefore typically has profound influence on the electrical, optical, and magnetic properties. Bulk oxide materials can often only sustain low nonhydrostatic strain lower than 1% before fracture occurs. However, advances in growth of thin films, freestanding oxide membranes, and elemental substitutions present pathways to apply large, nonhydrostatic stresses.

Pathways to Apply Stress and Measure Strain
There are different pathways to apply stress to strain materials as illustrated in Figure 38.

Epitaxial Strain
Epitaxial strain occurs in thin films grown epitaxially on substrates with a lattice mismatch between the film and substrate.
Adv. Mater. Interfaces 2019, 6, 1900772   Figure 37. a) The schematic experiment set-up for synchrotron radiation and the oxygen dosing. b) The evolution of the IG state and QP peak under irradiation and oxygen dosing. Reproduced with permission. [165] Copyright 2013, American Physical Society.

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This typically leads to a biaxial strain where the thin film is elongated (compressed) in the plane of the film and compressed (elongated) perpendicular to the plane. By using appropriate substrates, the value of the strain can be varied and large biaxial strain of several percent can typically be achieved. [173] Biaxial strain was used, e.g., to increase the transition temperature in high-Tc superconductors and ferroelectric materials [174] as well as the superconducting transition temperature in STO. [175] Biaxial strain is, however, often accommodated by misfit dislocations that can partially relax the strain and cause structural and physical inhomogeneity in the films. To circumvent this problem, fully coherent, strained, extremely thin epitaxial films can be grown. By growing such ultra-thin layers on atomically smooth substrates, misfit dislocations can be avoided so that the intrinsic effect of the strain can be extracted, providing a route to achieve novel functionalities. A striking example on the interaction between a thin film and a substrate is the single monolayer of the superconductor FeSe grown on a STO substrate which leads to a dramatic enhancement of the superconducting transition temperature compared with the bulk. [176][177][178]

Mechanical Strain
Mechanical strain occurs when external mechanical stresses are applied to a sample, for instance, using a piezoelement. Typical mechanical strain devices used include 1) hydrostatic devices where isotropic stress is applied using a pressure transmitting medium, 2) a three-point bending device where the sample is fixed at the ends and bend by pressing at the center, and 3) uniaxial strain devices where the sample is elongated or compressed in one direction by pulling or pushing along one of the sample dimensions. Fracture often occurs at small tensile strain values, however, applying mechanical stress benefits from the possibility to dynamically tune the strain in a continuous manner. In addition, a complete study can be performed on a single sample, thereby preventing sample-to-sample variability as well as the influence of varying substrates to achieve the appropriate strain. Alternative ways to apply mechanical strain have also been demonstrated such as immobilizing the sample onto a magnetostrictive template that exerted uniaxial stress when a magnetic field was applied. [179] This was used to dynamically tune the 2DEG transport properties at the LAO/STO heterostructure. [179] The uniaxial strain resulted in breaking the symmetry in the plane of the interface, which induced splitting of the electronic band structure and anisotropic conductivities.

Chemical Strain
Chemical strain can occur when modifying the lattice chemically by varying the defect level, inserting/removing elements or substituting constituent elements with elements of different sizes. This can be efficiently done in most oxides by controlling, e.g., the density of oxygen vacancies, which tend to exert tensile strain to the lattice. This was, for instance, the case for PrVO 3 films grown on STO where the presence of oxygen vacancies resulted in straining the lattice by a few percent, leading to significant changes in the magnetic properties. [180] Although large strain values can be obtained in this way, the chemical strain pathway suffers from the risk of introducing side effects not stemming from the strain itself.

Combination of Strain Pathways
Combination of strain pathways may result in interesting opportunities where the drawbacks of the individual methods may be circumvented. For instance, using epitaxial and mechanical stress combined, one can explore a large strain window where the epitaxial strain is used as a large strain offset upon which the dynamical strain from the mechanical stress can operate. The combination of chemical strain and epitaxial strain was reported to result in a dynamic strain state of STO. Here, when STO was grown on top of Si, and as the top Si layer was oxidized to SiO 2 by post-annealing, the lattice parameters of STO changed. [181]

Measurements of Strain
In order to be able to measure the strain, different techniques have been suggested including measurements of strain in thin films by X-ray diffraction, [182] tensile testing of thin freestanding films, [183] a capacitive strain gauge located underneath the sample, [184,185] and in situ measurement of displacement in the order of nanometer during micro-tensile test for thin films by using CCD camera as a sensing device. [186]

Strain and Its Influence on the Atomic Structure
Perovskite oxides, ABO 3 , consists of corner-sharing BO 6 octahedra. The distortions of these BO 6 octahedra often determine the functionalities of the perovskites. However, a versatile control of the octahedral distortions is challenging as they do not respond directly to electric or magnetic fields. The control of octahedral connectivity in perovskite oxide heterostructures and their relation to strain have been discussed by Rondinelli et al. [187] Figure 39 shows DFT calculations on how biaxial strain influences the structure of perovskite LAO and LaNiO 3 and drastically changes the octahedral rotation as well as the metal-oxygen bond angle and bond length. The structure of STO has been also reported to be highly responsive to stress. For instance, epitaxial strain on STO films may result in a transition to a ferroelectric phase at room temperature [188] and an enhancement of the superconducting transition temperature [175]  www.advmatinterfaces.de whereas strain gradients can induce a polarization by the flexoelectric effect. [189] Hydrostatic pressure has furthermore been observed to induce a cubic-to-tetragonal phase transition in STO even above room temperature as depicted in Figure 40. [190] In the case of the 2DEG in STO-based heterostructures, the conductivity occurs in the Ti 3d orbitals. [191] A small change of strain can affect the occupation and overlap of the Ti 3d orbital and therefore the 2DEG carrier concentration and mobility. In addition, the octahedral may also undergo additional distortions as mentioned above that can severely affect the resulting properties.

Strain-Induced Changes in the Electronic Structure
A change in the atomic structure due to strain can also highly influence the electronic structure via changes in bond angles, bond lengths, and lattice symmetries. The influence of the strain on the band structure of STO or STO-based heterostructures was studied computationally [179,[192][193][194] as well as experimentally using angle-resolved photoemission spectroscopy using a STO single crystal bent in a three-point bender. [192] As shown in Figure 41, the degeneracy of the d xz , d yz , and d xz bands in bulk STO is lifted when the lattice symmetry is lowered using uniaxial strain.
It was also demonstrated using resonant soft X-ray linear dichroism that the out-of-plane d xz /d yz bands have a lower energy state than the in-plane d xy bands in the GAO/STO heterostructure (see Figure 42). [173,195] This is in sharp contrast to the heterostructure where LAO is deposited on the (001) surface of STO and the in-plane d xy state is the lowest energy subband. In the study by Cao et al., GAO/STO was deposited epitaxially on NdGaO 3 and TbScO 3 substrates, which caused a 1.16% compressive and 1.29% tensile strain to the heterostructure, respectively. [173] The splitting between the in-plane and out-of-plane bands were found to be highly tunable with compressive strain leading to an enlargement of the splitting while practically degenerate bands were observed with tensile strain.

Tuning Transport Properties with Strain
Control over the carrier density and mobility of the 2DEG is essential for applications and may be achieved by both epitaxial, chemical, and mechanical strain. Using single-crystal substrates to produce interfaces with controlled levels of biaxial strain, it was found that the carrier density and critical LAO thickness to achieve conductivity in LAO/STO was controllable   [190] Copyright 2007, American Physical Society. Figure 41. Band structure of STO in absence (red, dashed lines) and presence (blue, solid lines) of strain calculated using k · p theory. The left panel shows the cut at the Fermi energy level whereas the right panel shows a momentum dispersive curve for k y = 0. Reproduced with permission. [192] Copyright 2013, American Physical Society.
(see Figure 43). [196] Tensile strain prevented formation of a 2DEG at the LAO/STO interface, whereas compressive strain retained the 2DEG albeit at a lower carrier concentration and an increased critical thickness.
Chen et al. have also shown that it is possible to induce electrons in the CaZrO 3 /STO heterostructure using a strain-induced polarization. [197] Here, the epitaxial strain transforms CaZrO 3 from being nonpolar to polar, which in turn makes it energetically favorable for electrons to transfer from CaZrO 3 to STO and form a conducting interface. The resulting electron mobility in this heterostructure exceeded 60 000 cm 2 V −1 s −1 at 2 K. [197] Mechanical strain was also found to be highly effective in tuning especially the electron mobility and the carrier density. [179,[198][199][200] When mechanically straining La-doped STO by approximately 0.3% using a three-point bending device, the electron mobility at low temperatures was found to increase by 300% as shown in Figure 44. [198] The enhanced mobility of the electron-doped STO films under compression was suggested to occur as strain breaks the threefold t 2g band degeneracy. The population of electrons in these bands then reduces the average effective mass and suppresses interband scattering. [192] The LAO/STO interface and field effect transistors made hereof were shown to exhibit an increase in the carrier density, decrease of the mobility, and change in the transistor behavior upon application of hydrostatic pressure. [199,200] A new approach for mobility enhancement was proposed by applying chemical strain using defect engineering. [201] Here, by means of a unique crystal engineering approach it was possible to alter the strain in Nb-doped STO films by deliberately introducing Sr vacancy clusters into the film. Films produced using this method resulted in an enhanced electron mobility exceeding 53 000 cm 2 V −1 s −1 .
Application of local mechanical stress was studied using various scanning probes. It was demonstrated that the tetragonal domain walls in STO could be tuned by applying a slight pressure of the order of 10 7 Pa on a LAO/STO heterostructure using the tip of a scanning SQUID. At temperatures below 40 K, the current at the LAO/STO interface is modulated by the tetragonal domain walls, [87] and applying a local stress to the domain walls was found to significantly alter the interface current distribution. [202] The reconfiguration of the current distribution was suggestive of a pressure-sensitive polar state at the domain walls between the nonpolar tetragonal domains. The possibility to apply local strain to affect the domain wall properties opens up for the possibility of domain walls engineering.
A large change in the local conducting properties was reported for the voltage-free tuning of LAO/STO interface conductivity at room temperature. [203] Here, local electrostatic gating could be used to place the interface in a low-or highresistive state, and following this, stress exerted by the tip of a scanning probe microscope could significantly lower the resistance of the high-resistive state (see Figure 45).  [196] Copyright 2011, National Academy of Sciences.

Tuning Magnetic Properties with Strain
Despite only being the main focus of a few studies, the magnetic state of STO-based heterostructures has been shown to be highly sensitive to strain. A long-range magnetic order in LAO/ STO, GAO/STO, and bare STO surfaces has been observed using a scanning SQUID. [101] The magnetic order was manifested as striped modulations in the magnetic field escaping the sample. The magnetic state can be tuned by applying local external forces using the tip of the SQUID, which turned the weak modulating stripes into sharp and strong stripy modulations (see Figure 46). When the contact was removed, the magnetic signal turned back to its initial state. As the striped modulations were oriented in the same directions as the ferroelastic domain walls of STO, the inhomogeneity was believed to originate from the tetragonal domain structure of STO and are thus present in both STO and the STO-based heterostructures. As the magnetic state couples directly to high-mobility electrons, this is particularly interesting for strain-dependent spintronic studies.
In a prior study, a strain dependence of the magnetic state was observed in LAO/STO using a similar scanning SQUID. [204] Here, the magnetic state was found to be resolution-limited ferromagnetic patches rather than a long-range magnetic order (see Figure 47). The magnetization of the ferromagnetic patches was found to change direction and magnitude upon touching with the tip of the SQUID. Unlike the ordered magnetic stripes, the final state remained stable when the contact was removed.
A few theoretical studies address the effect of strain on the magnetic properties in STO using DFT. [205,206] In a study performed by Zhang et al., it was calculated that oxygen , and when applying different forces mechanically to the high-resistive state. Reproduced with permission. [203] Copyright 2015, American Chemical Society. Figure 44. a) A three-point bending device where compressive strain is applied on the sample surface and tensile strain is applied on the backside of the sample. b) The enhancement of the electron mobility in a compressively strained La-doped STO film grown with MBE. Reproduced with permission. [198] Copyright 2011, American Institute of Physics. . ΔΦ denotes the peak-to-peak amplitude of the magnetic stripes. Reproduced with permission. [101] Copyright 2018, Springer Nature. Figure 47. Ferromagnetic patches observed in LAO/STO using scanning SQUID as a function of scan number where in each scan the tip of the SQUID is in contact with the sample surface (upper panel) or hovering above the surface in noncontact mode (lower panel). The signal is the magnetic flux (measured in mφ 0 ) in the scanning SQUID pick-up loop, which reflects the magnetic field produced by the sample. Reproduced with permission. [204] Copyright 2012, American Chemical Society. www.advmatinterfaces.de vacancies in STO lead to a nonzero magnetization in absence of an applied stress, consistent with experimental results. [5] The magnetization diminished as a positive hydrostatic pressure of 30 GPa was applied, however, a high-spin state was predicted when applying a negative hydrostatic pressure of −10 GPa (see Figure 48). In agreement with the local strain experiments, the strain could therefore be used to switch the magnetization on and off.

Perspective
One of the challenges for attaining epitaxial strain in the lab is the limited number of substrates available. Therefore, only a discrete number of different strain values can be achieved. Introducing strain by epitaxial growth is therefore only a viable option if substrates with the required mismatches are available and, equally important, if variations in substrate and epitaxial film quality (impurities, surface defects, crystal miscut angles, etc.) can be avoided as these can destroy the desired properties entirely. These drawbacks are to a large extent avoided when using mechanical strain, however, the fragility of oxides often severely limits the achievable strain values.
An epitaxial STO template layer grown on Si by molecular beam epitaxy (MBE) or PLD seems not only to be able to bridge oxides and semiconductors, [207,208] but as thin Si substrates are flexible, it also provides a pathway for achieving large mechanical strain in STO. In another work, a general method to create freestanding oxide heterostructure membranes was reported. [209] This was done using a sacrificial layer of Sr 3 Al 2 O 6 thin film which acts as a template for epitaxial perovskite growth and was subsequently dissolved by water to form freestanding oxide membranes. It was shown that it was possible to transfer millimeter-size single crystalline La 0.7 Sr 0.3 MnO 3 /STO superlattices onto Si. The physical properties of the layers were preserved, or even enhanced, in terms of the Curie temperature and residual resistivity. [209] Using such membranes, one can envision that a large strain may be achievable either by placing the membranes on a flexible substrate or performing strain studies on a freestanding membrane.
The tetragonal domain walls of STO has attracted more attention recently as they may be used to control the 2DEG properties at the nanoscale. This is particularly interesting as the domain walls have different magnetic, [101] polar, [202] and current-carrying [87] properties. Moreover, as the domain walls separate tetragonal domains with the unit cell elongated in the x-, y-, or z-direction, strain is expected to couple directly to the domain landscape with the potential of designing writable, erasable, and movable nanoelectronics.
Strain may not only be used as a tuning knob for already grown STO-based heterostructures. Defects and in particular oxygen vacancies play an important role in the resulting properties of the heterostructures. These defects exert a positive or negative stress to the surrounding lattice, and their formation energy will thus be dependent on the stress applied to the sample. [205] During a typical growth of STO-based heterostructures, it should therefore be possible to control the defect level by applying mechanical stress to the sample. If the defects are kinetically trapped after the growth, releasing the strain may thus lead to strain-induced permanent tuning of the properties.

Other Factors: Adsorbates, Particle Bombardment, and Temperature
The previous sections serve as an overview of the major pathways used to tune the properties of STO-based heterostructures via external stimuli. However, these pathways are far from encompassing all studies where external stimuli have been shown to change interface properties of STO-based heterostructures. In this section, we discuss how the interface properties can be changed by additional factors such as 1) interaction between the oxide surface and liquid/gaseous adsorbates, 2) bombardment of high-energy particles, and 3) temperature cycles at elevated temperature or across lowtemperature structural phase transitions. This paves the way for producing nanosized electronic circuits, tuning the interface properties with real-time monitoring during growth, and using the STO-based heterostructures for molecular sensing applications.

Liquid/Gaseous Adsorbates
In this section, we will cover the literature where the explicit effect of adsorbates was studied on the interface conductivity in STO-based heterostructures.

Controlling the Interface Conductivity Using Surface Adsorbates
Due to dangling bonds of apical atoms at the LAO surface in LAO/STO heterostructures, the surface is highly susceptible for adsorption or even chemical binding with molecules present Figure 48. The pressure dependence of the magnetic moments in a 2 × 2 × 2 unit cell large supercell of oxygen deficient STO. Reproduced with permission. [205] Copyright 2015, Royal Society of Chemistry.
www.advmatinterfaces.de on the surface. As the heterointerface often is near the oxide surface, there consequently exists an intimate relationship between the nature of the adsorbed surface molecules and the interface conductive state. The relation has been explicitly investigated by several studies. Dai et al. showed that it is possible to control the local oxygen surface content by oxygen plasma bombardment (see Figure 49). [210] It was found that areas exposed to the oxygen plasma were rendered insulating thus providing a convenient pathway to pattern the interface conductivity by protecting selective areas with resist. Specifically, it was found that exposure to oxygen plasma left the LAO surface highly hydroxylated that due to the chemical binding of the hydroxyl group displayed a good degree of stability at room temperature and ambient conditions. [210] Dai et al. later found that by scanning the LAO surface using the tip of a c-AFM biased with a negative voltage at room temperature, the interface conductivity could be locally erased (see Figure 50 and Section 2). [211] Upon dispersing a polar solvent such as isopropanol on the sample surface, the interface metallic state could be recovered.
By probing the Ti valance state using in situ X-ray photoelectron spectroscopy (XPS), it was possible for Scheiderer et al. to pinpoint the linkage between the density of interface charge carriers and the presence of surface-adsorbed atomic hydrogen from molecular hydrogen as well as water. [212] The authors demonstrated reversible cycling of this n-type doping with surface-adsorbed atomic hydrogen (see Figure 51).
Several additional studies have investigated the effect on the LAO/STO conductivity when exposing the LAO surface to water droplets, [213] water moisture, [214,215] and acids. [155] Brown et al. showed that whereas the change in the resistance upon immersion was poorly correlated with the permittivity of the solvent, a good correlation was found with the strength of the acids (see Figure 52). [155] Here, weak acids with a high proton affinity were found to drive the interface conductivity toward an insulating state. This was attributed to a lowering of the proton concentration on the sample surface thereby removing the attractive forces confining the electrons to the interface. This led to an increase in the resistance in analogy to removing a positive top-gate potential.

Reversible Metal-to-Insulator Transition Using a Combination of Adsorbates and Additional Stimulus
A few studies have investigated the effects of combining additional external stimuli with the presence/absence of surfaceadsorbed species. It was found by Brown et al. that an insulating LAO/STO heterostructures prepared by deprotonation from surface-adsorbed water could be rendered conductive by irradiation with photons at room temperature (see Figure 53). [155] This insulator-to-metal transition may share similarities with the previous discussed effect of light irradiation and application of electric fields from, e.g., a back-gate (see Section 2), however, this needs to be elucidated further.

Origin of the Adsorbate Effect
As it can be seen from the above-mentioned studies, underpinning the mechanism of how the adsorbed species directly affects the interface electron doping level is not trivial. Nonetheless, by systematically studying the influence of surface adsorbed water with almost the entire family of existing STO-based conducting heterointerfaces, Zhang et al. identified surface oxygen vacancies as playing the central role. [216] The authors argued that water chemistry at surface oxygen vacancies serves as a common mechanism for supplying electrons to the interface Figure 49. Left panel: Electrostatic force microscopy phase image of a LAO/STO device exposed to the oxygen plasma with dark orange corresponding to regions with interface conductivity. Right panel: Schematic illustration of hydroxyl termination at the LAO surface and extent of the 2DEG after exposure to oxygen plasma. Reproduced with permission. [210] Copyright 2016, American Chemical Society. in stable (solid circles) and metastable (dashed circles) states. Middle panel: Interface conductance measured between two contacts as a function of time with the c-AFM scanning and polar solvent dispersion indicated at "1" and "2," respectively. Right panel: Schematic illustration of the working principle behind the polar adsorbate interface doping. Reproduced with permission. [211] Copyright 2017, American Chemical Society.

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for all the investigated heterostructures. The formation energy of these surface oxygen vacancies in the particular oxide thin film would dictate the concentration of chemically bound water molecules (i.e., hydroxyl groups) as well as the robustness of interface conductivity when the samples were exposed to oxygen annealing (see Figure 54). This study provides an auxiliary explanation to the origin of interface conductivity in STO-based heterostructures, complementing the existing explanations based on polarity-induced spontaneous oxygen vacancy formation at the top surface [18] and adsorbate-induced changes in the conductivity. [155,212,215]

Particle Bombardment
Particles with high kinetic energies impacting the surface of STO or STO-based heterostructures can lead to intentional or unintentional incorporation of elements, formation of defects, or activation of chemical processes. Particle bombardment may occur in situ during the deposition of thin films or ex situ after the sample synthesis by an external energetical particle source.

Effect of Particle Bombardment During PLD Growth
Heterostructures are often synthesized using PLD of thin films on a single crystalline STO substrate. In this deposition process, the laser impacts a target that ablates into a plasma comprised of particles traveling with kinetic energies on the order of tens of electron volts. The high kinetic energies enable the epitaxial growth of crystalline films on STO even at room temperature, [217] but it may also cause defects as well as interdiffusion of particles between the top film and STO. Sambri et al. investigated the effect of depositing LAO at room Figure 53. The resistance of LAO/STO as a function of sample conditions with the sample being exposed to either photons (light) or surface adsorbates (water). Reproduced under the terms of a Creative Commons Attribution 4.0 International License. [155] Copyright 2016, The Authors, published by Springer Nature. Figure 54. a) Rough ranking of conductivity stability at different STObased heterointerfaces when subjugated to oxygen annealing at temperatures between 300 and 600 °C. b) The estimated energy difference (E drive ) between shallow surface levels at the top film surface and the Fermi energy of STO, plotted as a function of robustness to oxygen annealing. Reproduced with permission. [216] Copyright 2018, American Physical Society.  Reproduced with permission. [212] Copyright 2015, American Physical Society.
www.advmatinterfaces.de temperature on STO in an atmosphere of argon or oxygen with varying pressures. [218] The room temperature deposition of LAO results in an amorphous phase, and the primary driver for obtaining conducting interfaces is the transfer of oxygen from STO to the oxygen-deficient LAO film. Increasing the oxygen partial pressure in the deposition increases the interaction between the plasma and the background oxygen gas, resulting in both a lowering of the kinetic energy and an increased oxidation of the plasma plume species. At low oxygen deposition pressures, the interfaces were therefore conducting, but at pressures of approximately 10 −3 mbar or above, the interfaces were insulating (see Figure 55). Replacing the oxygen gas with argon, however, resulted in approximately the same lowering of the kinetic energy, but without a significant plume oxidation. In this case, depositing in an argon pressure of 10 −3 mbar produced conducting interfaces, signifying the importance of the oxidation state of the plasma plume. A metal-to-insulator transition occurred at an argon pressure of 10 −1 mbar, and by measuring the kinetic energy it was proposed that a minimum kinetic energy on the order of 1 eV was needed to kinetically activate the transfer of oxygen from STO to LAO at room temperature. In this picture, the particle bombardment was necessary to activate the redox reaction causing the formation of oxygen vacancies and conductivity at the interface.
Several studies also investigate the effect of the particle bombardment along with other stimuli occurring during the PLD. [137,212,[219][220][221][222] This can be done by measuring the conductivity of the STO-based heterostructure in real-time during the deposition. This sheds light on the origin of conductivity in heterostructures with top films deposited on STO at room temperature. It was well-established that conductivity was observed ex situ only above a critical top layer thickness for LAO/STO and GAO/STO, but not at the interface of La 7/8 Sr 1/8 MnO 3 /STO independent of the top layer thickness. [7,223] It was inferred that oxygen transfer from STO to the top film was paramount for the conductivity. Measuring the conductivity during the deposition, however, revealed that conductivity was established after the first few laser pulses, much earlier than the critical thickness (see Figure 56). Using oxygen dosing in the deposition chamber, the discrepancy between the results obtained in situ and ex situ was investigated. The discrepancy was attributed to annihilation of oxygen vacancies in STO in the case where deposited material on STO was insufficient to protect the oxygen vacancies from molecular oxygen in the atmosphere. Surprisingly, deposition of La 7/8 Sr 1/8 MnO 3 was also found to induce conductivity initially due to the formation of oxygen vacancies caused by the bombardment of the plasma particles. Thus, the resulting conductivity was found to be a competition between several mechanisms: particle bombardment, oxygen transfer across the interface, oxygen annealing, and (to a small extent) UVradiation from the plasma plume.

Effect of Ex Situ Particle Bombardment
Following the removal of the STO-based heterostructure from the PLD growth chamber, ex situ particle bombardment has likewise been demonstrated to constitute a convenient pathway for altering the interfacial state.
It is well-established that bulk insulating STO may be rendered 3D metallic conductive if subjugated to bombardment by ionic species under vacuum even at room temperature. [224] This transformation from insulator to metal is driven by the prevalent formation of oxygen vacancies throughout the STO lattice that correspondingly n-type dope the system. By carefully controlling the energy of incoming ionic species from an Ar + source, Aurino et al. found that originally metallic LAO/ STO heterostructures could be rendered insulating in a narrow window of irradiation times (see Figure 57). [225] This method represents an effective pathway for patterning the interface conductivity by covering selective areas of the LAO/STO heterostructures with, e.g., polymer resists during the Ar + irradiation.
The mechanism for this persistent metal-to-insulator transformation of the LAO/STO heterointerface was further investigated by Aurino et al. [226] Here, it was concluded that the Ar + irradiation leaves a progressively growing layer of amorphous-LAO on top of the LAO/STO heterointerface (see Figure 58). When the thickness of crystalline LAO this way is reduced below the critical thickness for interface conductivity of 4 u.c., the metal-to-insulator transition occurs. However, it was found in the study that the process indeed is reversible by high-temperature annealing in oxygen whereupon the amorphous-LAO recrystallizes thus rendering the interface metallic again.
The effects of Ar + ion bombardment is not limited to microstructure changes as etching of the top layer readily can occur as well. Ridier et al. studied the effects of Ar + cluster ion bombardment. [227] The authors of this study uncovered the dynamical effects occurring during the Ar + ion bombardment and found that despite the entire LAO top-layer being removed during the bombardment, the STO below remained conductive in the near-surface region following ion exposure. Moreover, the authors elucidated that the lateral effects of the ion exposure extend up to several millimeters away from the irradiated area [227] (see Figure 59). Finally, the authors show that high-temperature annealing in oxygen to a large extent could reverse the effects to interface conductivity of Ar + cluster ion bombardment.

Figure 57.
Resistance as a function of irradiation time for LAO/STO heterostructures at various Ar + beam acceleration voltages. Reproduced with permission. [225] Copyright 2013, American Institute of Physics. www.advmatinterfaces.de

Temperature-Induced Changes
The temperature has a dramatic effect on the properties at STObased interfaces and in particularly on the conductivity and electron mobility, which have both been reported to increase by up to four orders of magnitude when cooling down from room temperature to 2 K. [20,111] In most cases, the properties change reversibly as the temperature is varied. Noticeable exceptions include the cases where the temperature is elevated with respect to room temperature to activate oxygen diffusion in the lattice or when crossing the 105 K phase transition of STO between a cubic and tetragonal crystal structure. [228]

Annealing at Elevated Temperatures
Annealing in the presence of molecular oxygen has frequently been used to minimize the amount of oxygen vacancies in oxides as well as to test the influence of such vacancies, however, with the risk of inducing cation movement and cation vacancies. [230] In an elaborate study, Gunkel et al. measured the conductivity of a range of STO-based heterostructures with varying top films at 950 K while varying the oxygen partial pressure. [231] These high temperature equilibrium conductance measurements were used to determine whether the interface conductivity originated from thermally unstable oxygen vacancies or had annealing resistant contribution, as expected if the conductivity arises from the polarity of the top film. In this way, the interface conductivity in amorphous-LAO/STO and GAO/ STO was attributed to oxygen vacancies, whereas the conductivity in crystalline-LAO/STO, NdGaO 3 /STO, and (La,Sr)(Al,Ta) O 3 /STO scaled with the top film polarity and was attributed to stem from the polarity. This suggested that the GAO film is overall nonpolar possibly due to an inhomogenous distribution of aluminum vacancy. [232] For the heterostructures where the conductivity is formed by oxygen vacancies, the conductivity was found to weakly degrade over time at ambient conditions. [229] The degradation (δR s /δt, see Figure 60) could be minimized by storing the samples in oxygen-deficient environment or lowering the oxygen diffusion through the top film by increasing the layer thickness or utilizing a material with a high oxygen diffusion barrier such as GAO. [22,229] The case of GAO/STO with a GAO film thicker than 1-2 nm is particularly interesting, as the conductivity here is stable at room temperature on the time scale of years at ambient conditions, but by elevating the temperature to 200-300 °C the conductivity could be tuned to a desired level. [22] This is depicted in Figure 61 where GAO was deposited by PLD at an oxygen partial pressure of 10 −5 mbar resulting in an initial sheet carrier density of around 2 × 10 14 cm −2 . Annealing in pure oxygen at ≈200 °C resulted in a controlled decrease of the carrier density and eventually a metal-to-insulator transition. This change in the resistance was used to enable conducting AFM writing of nanocircuits as discussed in Section 2. Writing of nanocircuits was initially unsuccessful using an as-deposited GAO/STO heterostructure with a low resistance. However, after raising the resistance of the same sample to an insulating state using annealing on a hot plate in ambient conditions, conducting AFM writing was possible. [22] Annealing also has a drastic effect on the low temperature properties. A curiously example was reported in the GAO/STO heterostructure: [168] whereas the room temperature conductivity, electron mobility, and carrier density of GAO/STO remained stable during 6 months of sample storage in a vacuum desiccator, the low temperature properties were observed to change (see Figure 62). After storage, both the sheet conductance and electron mobility at 2 K increased by roughly a factor of four. This dynamical mobility enhancement was proposed to originate from a redistribution of oxygen vacancies that minimizes the number of collisions between itinerant electrons and oxygen vacancy scattering sites.

Tetragonal Domain Formation
Upon cooling from room temperature, STO exhibits a structural phase transition from a cubic to a tetragonal lattice at around 105 K. [233,234] This leads to formation of tetragonal domains separated by domain walls with well-defined orientations. The magnetic and electronic properties can vary significantly when crossing such domain walls, and the properties, in particularly of small devices, may be highly affected by the density and orientation of the domain walls. [235] If the domain walls can be controlled, it offers a mechanism for creating writable, erasable, and movable nanoelectronics from the domain walls themselves. The particular configuration of the tetragonal domains is highly dependent on the thermal history, and whether the sample is heated/cooled across the 105 K transition or not: temperature changes below the 105 K transition will keep the configuration of tetragonal domains frozen, whereas cycling events across 105 K will thaw the domains that upon recooling will organize themselves in a new configuration. This was reported to result in both a dramatic change of charge current flow within STO [87] as well as the magnetic landscape. [101] The charge current flow may change so dramatically that it leads to a change in the four-terminal resistance of more than a factor of four in patterned van der Pauw squares of size ≈200 µm × 200 µm (see Figure 63). [235] Consistent with this, the striped features in the magnetic landscape was also reported to move or become undetectable after a thermal cycling above 105 K. [101] Similar to the current stripes, these magnetic stripes were found to exist below 40 K, however, only when the 105 K phase transition was exceeded, a change in the stripe pattern was detected (see Figure 64).

Perspective
Several interesting perspectives exist for the external stimuli discussed in this section. The changes in the properties of the interface conductivity in STO-based heterostructures can in general be used for sensing the external stimuli applied to the sample. This is particularly the case of the influence of surface adsorbates, which in principle may be envisioned to design gas detectors. By varying the top film material, one may get different responses to different gaseous species, which could lead to selectivity in the detection.
The real-time monitoring of the conductivity in STObased heterostructures offers the potential for tailoring the conductivity on-the-fly by in situ switching from one target to another or by varying the deposition conditions. [219] Combined with other in situ characterization techniques such as reflective high-energy electron diffraction and XPS, this is a powerful Figure 64. The magnetic flux detected by scanning SQUID in STO and GAO/STO heterostructures shows motion of ferroelastic domain walls when thermally cycling across the tetragonal-to-cubic transition in STO. Reproduced with permission. [101] Copyright 2018, Springer Nature.  . Reproduced with permission. [235] Copyright 2016, American Chemical Society. Figure 62. GAO/STO spinel/perovskite heterostructures can show lowtemperature mobility enhancements when stored around 6 months in a vacuum desiccator. Reproduced with permission. [168] Copyright 2017, American Physical Society.
www.advmatinterfaces.de method for achieving the desired properties of heterostructures. In particular, one can tune the properties in real-time by varying, e.g., the kinetic energy for the impacting plasma species, alter the reducing or oxidizing properties of the capping layers, and modulate the crystallinity of the top film. The in situ conductivity measurements can also be combined with the annealing in order to make an even more versatile tuning of the properties.

Conclusion and Perspectives
The increasing demand for new materials with combined new and targeted functionalities realizable in, e.g., oxides, [236] requires the control of microstructure and architecture at the nanoscale. This has been done for many years using nano-engineering. Another way which has the potential to make a significant impact in a multitude of diverse areas within materials science is modifying the materials by applying external stimuli such as light, temperature, magnetic fields, electric fields, and stress.
Functional perovskite oxides available as thin-films, such as STO, offer a close interaction between the lattice, spin, charge, and orbital degree of freedom and the possible to confine the conductivity in two, one, or zero dimensions. Traditionally, the search for new functional bulk materials involves also the influence of additional single external stimulus where the materials properties can be modified within a particular parameter space dictated by the stimulus. This offers additional external control over properties and nanostructure of materials enabling the production of multifunctional devices with unique properties, ranging from "writing" a conducting channel (Section 2) and fabricating 2DEG devices via X-ray radiation (Section 4), spintronic devices where magnetism is induced by light (Section 4), selective detection of gases (Section 6), and controlling the level of defects in real-time during growth (Section 6).
Within the research framework of surfaces and interfaces of oxides, the question remains on how a material will respond when not only a single stimulus is applied at the time, but rather multiple stimuli are applied. This research topic is to a large extent still unexplored both experimentally and theoretically. Navigating this multi-dimensional space in search of functional materials with improved properties is an enormous task without an additional guidance. Therefore, computational materials science is one of the key tools for understanding the responds of materials to multiple stimuli. Adding multiple external stimuli to surfaces and interfaces where broken symmetry, reduced dimensionality, atomic relaxations, and intermixing of atoms occur at the interface can substantially affect the magnetic and transport properties of these materials. This provides another dimensionality to the richness of stable or metastable states that oxide surfaces and interfaces may display depending upon their environment.
We believe that a theory-driven intelligent approach is imperative for efficiently identifying and understanding the role of single and even multiple stimuli interaction. In addition to understanding the microscopic origins of the coupling mechanisms, the other most immediate task is to understand the role of micro-or nanostructure when such a coupling occurs.
Finally, compared to response from a single stimulus material, applying multi-stimuli is a fascinating way to achieve finer modulations of the materials through a larger parameter space. Inspired by these, the next challenge is to transfer the findings of single and multiple stimuli into applications to open up new technologies and markets opportunities.