The multiferroic behavior of rare-earth manganites is studied within a microscopic model including a symmetry-allowed magnetoelectric coupling between polarization and magnetization. The magnetic subsystem is described by a frustrated Heisenberg spin model, whereas the ferroelectric subsystem is characterized by an Ising model in a transverse field. Using Green's function method we find analytically the temperature and wave vector dependent elementary excitation of the magnetoelectric system, the polarization and the magnetization for different magnetoelectric coupling strengths. The system undergoes a magnetic transition at and a further reduction of the temperature leads to a ferroelectric transition at depending on the coupling strength. That coupling is also manifested as a kink in the magnetization and the elementary excitation at . Due to the magnetoelectric coupling the excitation energy exhibits a gap at zero wave vector which increases with increasing coupling. We show that the macroscopic magnetization can be slightly enhanced by an external electric field nearby . An external magnetic field leads to an increase of the polarization.

The reaction of Cu-clusters with a polar and a mixed terminated single crystalline ZnO-substrate upon thermal treatment in UHV is studied in comparison with Au-films. Scanning tunneling microcopy and spectroscopy in combination with photoemission experiments reveal the geometrical and chemical changes in the Cu-cluster system on ZnO(0001)-Zn and ZnO(100) upon annealing up to 770 K. On ZnO() the Cu-clusters show a roof like outline with Cu(111) side facets. The data points to a Cu(110) interface with the ZnO-substrate. The interface of the Cu-clusters and the ZnO-substrate was investigated by the controlled removal of clusters using STM-tip manipulation exposing the “footprints” of the clusters. On both investigated ZnO surfaces an entrenching of the Cu-clusters during annealing was found which partly explains the observed decrease of the amount of Cu visible above the ZnO-substrate level upon annealing. Even at higher annealing temperature the main body of the cluster surface is still pure copper. No large scale oxidation or brass formation was found. Scanning tunneling spectroscopy shows an increased density of occupied states at the cluster perimeter which is possibly relevant as an active site in catalysis.

The selective oxidation of aqueous ethanol solutions with air over two commercial 1.5 and 1 wt% Au/TiO₂ catalysts was investigated in six stirred mini-autoclaves operated in parallel. The catalysts were characterised by various techniques including elemental analysis, N₂ physisorption, X-ray diffraction and transmission electron microscopy (TEM). Temperature, pressure, ethanol concentration, catalyst concentration and reaction time were varied in the batch experiments to study the reaction kinetics. It was possible to confirm the generally accepted mechanism of primary alcohol oxidation, in which acetaldehyde is a primary product of the oxidation that quickly undergoes further transformation to acetic acid. In presence of both acetic acid and ethanol the formation of ethyl acetate takes place until equilibrium conditions are reached. The high yields of acetic acid can be rationalised by the inhibited total oxidation of acetic acid under the applied reaction conditions. Improper storage of the gold catalysts in air exposed to light was found to lead to an irreversible change of the performance, which cannot be restored by means of recalcination. Sintering and blocking of surface sites by deposits were ruled out as possible causes for the deactivation.

Structural, elastic, mechanical, electronic properties, and dispersion of optical functions of hexagonal ZnTiO₃ have been investigated from first-principles within density-functional theory (DFT) using the norm-conserving pseudopotentials method, within the generalized gradient approximation (GGA) to the exchange-correlation functional. The calculated structural parameters agree with the available experimental and theoretical results. The elastic constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Lamé's constants as well as mechanical stability have been studied, indicating that hexagonal ZnTiO₃ is mechanically stable, anisotropic, and ductile. The electronic structure and chemical bonding of hexagonal ZnTiO₃ have been analyzed according to the calculated energy band structure, density of states (DOS), and charge populations. Dispersion of optical functions is shown and analyzed, including the complex dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function, and optical conductivity.

The modified Landau-type free-energy expression and the Landau–Khalatnikov equation of motion are first used to study the influence of electrodes on the properties of polarization reversal in a ferroelectric thin film with surface transition layers. The combined influence of the electrodes and surface transition layers on the dynamic properties, including polarization-reversal distribution, hysteresis loops, and switching current of the ferroelectric thin films, is discussed in detail. The results show that the electrode materials can greatly affect the dynamic properties of a ferroelectric thin film.

The preferred location of boron in oxidized free-standing Si nanoparticles was investigated using a first-principles density functional approach. The nanoparticles were modeled by a silicon core about 1.5 nm in diameter surrounded by an outer shell of SiO₂ with a thickness of about 0.5 nm, and considered negatively charged. The calculated formation energies indicate that B is equally stable in the Si core and in the SiO₂ shell, showing preference for interface sites. This indicates that, in contrast with phosphorus, the ratio of the boron concentration in the silicon core to that of the silicon shell will not be improved over one upon thermal annealing.

A criterion for the growth of Stranski–Krastanov islands mediated by surfactants is obtained. It involves that surfactants decrease the island nucleation barrier. Thereby, at a low coverage (θ ≪ 1 mono layer) surfactants facilitate islands formation. At a high coverage (θ ≈ 1 mono layer) the islands growth might be still facilitated, but also hampered under special conditions. The latter is caused by the reduction of the ad-atom surface diffusion. We report on a non-monotonic dependence of Ge/Si(100) island array density on the Sb coverage and discuss it in the frame of the obtained criterion.

We have performed the first-principles calculations on the structural, electronic, and magnetic properties of zigzag GaN nanoribbon (ZGaNNR) with period vacancy located at different sites across the ribbon width. The results show that, the formation of the N-vacancy is easier than that of the Ga-vacancy at each equivalent geometrical site and both of them are endothermic. An inward relaxation of the three nearest Ga atoms around a N-vacancy occurs, while for the three nearest-neighbor N atoms around the Ga-vacancy, an outward relaxation occurs. Except for a typical nonedge N-vacancy, the N-, or Ga-vacancies at other sites induce magnetic moment and spin polarization implying such vacancy-defective ZGaNNRs can be useful in spintronics and nanomagnets. The magnetic moment of the N-vacancy is dependent on defect sites, while for the Ga-vacancy, it is less dependent on the defect sites. The net magnetic moment of the vacancy defective 8-ZGaNNR is mainly contributed by the atoms around a vacancy.

Magnetic and structural properties of (Co)_x(SiO₂)_1 − x nanocomposite systems have been investigated over a wide range of Co concentrations using small-angle neutron scattering in combination with magnetic, magneto-optic, and magneto-transport characterization. It was found that in the region of structural percolation , the characteristics of magnetically correlated clusters are several times larger than the structural size of Co granules. Starting from the percolation point, the spatial distribution of magnetic nanoclusters can be described by the surface fractal model. With further growth of cobalt concentration x ≥ 0.67 the formation of three-dimensional (3D) magnetic domains with typical sizes of 1.5 µm was observed.

Ionic liquids (ILs) offer outstanding possibilities as media for manufacturing nanoparticles. Synthesis conditions with high reaction and nucleation rates are achievable leading to the formation of extremely small particles. The IL itself can act as an electronic as well as a steric stabiliser preventing particle growth and particle aggregation. In addition, as highly structured liquids, ILs have a strong effect on the morphology of the particles formed. We have developed two synthesis techniques for the generation of metal nanoparticles that take advantage of the unique properties that ILs offer when compared to conventional volatile organic solvents (VOCs): microwave (MW) synthesis and physical vapour deposition (PVD). The ionic character and high polarisability of the IL renders it highly susceptible to energy uptake via MWs and extreme heating and reaction rates can be achieved. To make full use of the possibilities that ILs offer we have designed a set of reducing ILs which can be used as direct reaction partners for the generation of metal nanoparticles. The negligible vapour pressure of many ILs makes experiments under high vacuum possible and allows for the PVD of metals into ILs.

Physical vapour deposition (left) and microwave synthesis of metal nanoparticles in ILs.

The dependence of the optical magnetic property on metal film thickness and the origin of the magnetic property in thick metal (silver) film perforated with rectangular nanohole arrays were investigated using the finite-difference time-domain technique. A negative permeability was observed in the thickness range where two hybridized magnetic plasmon polariton resonances are present, and a big negative permeability was observed at certain thickness when the antisymmetric coupling between the electrical current density near the top and bottom film plane with the directions of electrical current density in the plane are present. Two types of antisymmetric coupling of electrical current density in the metal film were found to probably contribute to the permeability.

Auxetics are a modern class of material fabricated by altering the material microstructure. Unlike conventional materials, auxetics exhibit a negative Poisson's ratio when subjected to tensile loading. These materials have gained popularity within the research community because of their enhanced properties, such as density, stiffness, fracture toughness and dampening. This paper provides a critical oversight of the auxetic field with particular emphasis to the auxetic foams, due to their low price, easy availability and desirable mechanical properties. Key areas discussed include the fabrication method, the effects played by different parameters (temperature, heating time, cell shape and size and volumetric compression ratio), microstructural models, mechanical properties and potential applications.

In this Feature Article, Critchley et al. introduce the expanding field of auxetic materials and critically review the literature with a primary focus on auxetic foams. Particular attention is given to the fabrication methodologies, the influence played by the fabrication parameters, the microstructural models (both two- and three-dimensional), the mechanical properties and the potential applications of these materials.

We report a theoretical study of the electronic structure and optical properties of the B phase of niobium pentoxide (B-Nb₂O₅) and tantalum pentoxide (B-Ta₂O₅) by means of first-principle calculations. We have used density functional theory along with the revised Perdew–Burke–Ernzerhof (PBEsol) exchange-correlation functional and the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional. It has been found that these compounds are indirect wide-gap semiconductors, the calculated gaps for B-Nb₂O₅ (B-Ta₂O₅) are and  eV. We have also calculated the frequency-dependent and static dielectric tensor, the refraction index and the transmittance. The calculated average static dielectric constants of B-Nb₂O₅ (B-Ta₂O₅) are 33.7 (30.9), in good agreement with the available experimental data.

We present the quasiparticle energy and optical absorption spectrum results of 1D linear H, BN, C, and Au chains, which are the lower size limit of a periodic material, by using ab initio many-body approaches. Unprecedentedly large quasiparticle corrections and excitonic effects are revealed in these extreme systems compared with quasi-1D, 2D, and 3D periodic systems. The binding energy of the bound exciton is up to 3.55 eV in the semiconducting BN chain and 0.5 eV in the metallic C chain, the latter of which is the largest in a metallic system.

An overview of work with model systems designed to study metal–support interactions in heterogeneous catalysts is given. In these models, metal and support are both miniaturized by introduction as guests into a mesoporous host. The use of such models is demonstrated with AuTiO₂ clusters encaged in MCM-48, and CuZnO clusters encapsulated in siliceous mesopore systems and in carbon nanotubes. The models promise a better opportunity to track changes in the support component during catalyst activation and catalysis, including the action of poisons that may at first be trapped on the support surface. Challenges to be met are the stabilization of the mesoporous matrix during synthesis and catalysis, possible reactivity of the matrix surface towards any of the catalyst components, as well as clustering and segregation of the latter from the matrix. The challenges were encountered as pore damage during preparation of AuTiO₂/MCM-48 catalysts, as deactivating interactions of siliceous walls with zinc ions during deposition of zinc species from aqueous media, and as clustering of the Cu component during calcination and reduction. Among the conclusions drawn from the studies are the irrelevance of order at the AuTiO₂ interface (and, hence, of epitaxy and of crystal strain in gold) for high activity of Au/TiO₂ catalysts in CO oxidation. In the models for CuZnO methanol synthesis catalysts, two different types of CuZn interaction could be observed: a direct contact between Zn²⁺ and Cu(0) under strong reducing conditions, and the formation of alloy nanoparticles (nano-brass). A discussion of the relevance of these interactions for the methanol synthesis reaction is given.

Using first-principles calculations based on density functional theory (DFT) and nonequilibrium Green's function method, we systematically investigated the electronic and magnetic structures, local structural distortion, and electronic spin transport property of a zigzag silicon carbide nanoribbon (ZSiC NR) containing one isolated nonmagnetic impurity [boron (B) or nitrogen (N)] at the inequivalent substitutional sites. Our calculated results reveal that by controlling the doping position without an applied electronic field, greatly enriched electronic and magnetic properties, e.g., semiconducting, half-metallic, and metallic behaviors, as well as the ferrimagnetic–ferromagnetic conversion, can be achieved in both B- and N-doped ZSiC NRs. Interestingly, all the doped ZSiC NRs of B substituting Si atom and N substituting C atom have 100% spin transport polarization around the Fermi level and exhibit well spin filtering behaviors. It is found that except for at the edge C-site, the boron at other sites suppress the local magnetic moment at edge Si atoms and enhance the local magnetic moment at edge C atoms. On the contrary, except for at the edge Si-site and its nearest-neighbor Si-site, the nitrogen at other sites suppress the local magnetic moment at edge C atoms and enhance the local magnetic moment at edge Si atoms. It is also found that when impurity atom substitutes Si atom, visible local structural distortion occurs around the impurity atom, while the impurity atom substitutes C atom, only a minor local structural distortion occurs. Additionally, the local structural distortion around impurity atom becomes more pronounced, as impurity moves from the edge toward the center. These doping effects are discussed by using the band structures, projected density of states (DOS), electronegativity, as well as Mulliken charge analysis.

Tensile and compressive strains via epitaxial lift-off (ELO) techniques were applied on single-layer InAs/GaAs quantum dots (QDs). At low temperatures, due to the difference in thermal expansion coefficients of the ELO film and host substrate, the ELO QDs film bonded to Si and MgO substrates experienced tensile and compressive strain, respectively. At 13 K, we observed that the photoluminescence (PL) spectra of the ELO film bonded to MgO blueshifts by 10 meV while the ELO film bonded to Si redshifts by 8.5 meV with respect to the ground state of the as-grown sample. The estimated tensile and compressive strains at this temperature were determined by monitoring the valence-band splitting of the GaAs PL peak. The film bonded to Si has a light hole (lh) to heavy hole (hh) energy separation of 4.6 meV, resulting to values of strain,  = 6.049 × 10⁻⁴ and stress, X = 0.746 × 10⁻³ kbar or 74.6 MPa. On the other hand, the film bonded on MgO has an lh–hh energy separation of 3.7 meV, giving  = 4.8 × 10⁻⁴ and X = 0.24 × 10⁻³ kbar or 24 MPa. Furthermore, we also observed a reversal of the PL intensity peak between the ground and excited-state transition of the film bonded on silicon only. A plateau-like feature between the two peaks also emerged, indicating the presence of another optical transition, which is enhanced due to application of tensile strain. We associated this peak to the 1LO-phonon replica of the PL transition resulting from the excited state. Based on these observations, this reversal is most likely attributed to the reduction of the carrier-relaxation mechanism from excited states to the ground-state transition upon the application of tensile strain. Finally, the result of this study showed the efficacy of the ELO technique as an alternative way of introducing variable tensile and compressive strain in the InAs/GaAs QD's heterostructure.

Ab-initio density functional theory calculations are carried out to investigate the role of zirconium (Zr) impurity atoms during AlN(0001) surface growth. Adsorption and diffusion of Zr atoms on AlN(0001)-2 × 2 surface is examined and it is shown that Zr atoms preferentially adsorb at the T4 sites at low and high coverage (from 1/4 up to 1 monolayer). We found that the Zr adatom diffusion energy barrier between the T4 and H3 sites is around ∼0.4 eV, which is an indication of a significant Zr adatom diffusion on this surface. In addition, calculating the relative surface energy of several configurations and various Zr concentrations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the Zr and Al chemical potentials. Based on these results, we find that incorporation of Zr adatoms in the Al-substitutional site is energetically more favorable compared with the adsorption on the top layers. This effect leads to the formation of a non-reactive interfacial ZrN(111) layer on the AlN(0001) surface, which can offer a good interfacial combination between AlN substrate and other metal contacts, i.e. zirconium.

Defects can decrease the efficiency of scintillators by trapping electrons. Here, point defects in REAlO₃ and RE₃Al₅O₁₂ are predicted with pair potential simulations, where RE is yttrium or a trivalent rare earth cation. It was found that REAlO₃ shows a preference for Al₂O₃-excess whereas RE₃Al₅O₁₂ most readily exhibits RE₂O₃-excess. Also, lattice volume changes for the energetically favorable intrinsic mechanisms are relatively invariant as a function of RE cation size in RE₃Al₅O₁₂, but not in REAlO₃. However, in non-stoichiometric RE₃Al₅O₁₂, the energetically preferred disorder mechanism results in an increasing lattice expansion with increasing RE radius whereas, in non-stoichiometric perovskites, a relatively small, radius independent, lattice contraction is predicted. These results illustrate that defect behavior in REAlO₃ perovskites and RE₃Al₅O₁₂ garnets is quite disimilar.

In this research, the model of five basic structural units (tetrahedra) proposed by Verleur and Barker [Phys. Rev. 149, 715 (1966)] (VB-model) is completed by the statistical approach, which enables us to derive quantitative relations for the oscillator strengths of separate lines. The rule of proportionality of the oscillator strength sum to the content of the components for each dipole pair is developed as a criterion of random distribution of atoms in the lattice. Thus, a proper identification of observed lines was performed. It is shown that the optical-reflection spectra obtained experimentally by the VB-model authors for GaAsP solid solutions can be successfully explained using the VB-model, without the hypothesis of clustering of binary GaAs as well as GaP tetrahedra, which they introduced.

The magnetic response of electrons in a zigzag carbon nanotube (CN) threaded by an Aharonov Bohm flux ϕ is carefully analyzed by using a Hartree Fock approximation. The ground state energy as a function of the magnetic field, B, and the electron–electron repulsion strength, U, are obtained for N_e interacting electrons confined in short (∼100 nm) and long (∼1 µm) CNs. The spin and orbital magnetization responses to the magnetic field variations are discussed. The CN transport properties are investigated and periodic field dependent oscillations are predicted for both longitudinal ballistic and persistent currents. The behavior of persistent current is investigated as a function of U and a significant suppression in current amplitude is observed when the strength of the electron–electron repulsion increases.

Hydrogen titanate nanotube (HTNT) thin films were synthesized by hydrothermal method and then Co²⁺ and NH co-doped hydrogen titanate nanotube (Co, N-HTNT) thin films were prepared by ion-exchange method. The Co, N-HTNT thin films exhibit strong absorption in the visible light range compared with the HTNT and NH doped hydrogen titanate nanotube (N-HTNT) thin films. The first-principles calculations reveal that NH doping has no effect on the visible light absorption of HTNTs. The red shift of Co, N-HTNTs is only due to the mixture of the Co 3d and O 2p states in the top of the valence band, which results in the band gap narrowing. Relative to HTNTs and N-HTNTs, both the valence band maximum (VBM) and conduction band minimum (CBM) of Co, N-HTNTs shift to lower potential based on the valence band XPS spectra. Furthermore, the up-shift of the VBM is much larger than that of the conduction band, which can result in band gap reduction explaining the origin of the visible light absorption of Co, N-HTNTs.

We report the observation of a para-antiferromagnetic transition at ∼50 K in lithium iron phosphate, LiFePO₄ through DC magnetic susceptibility and Mössbauer spectroscopy. The ferrous ion Fe²⁺ (3d⁶, ⁵D) in LiFePO₄ exhibits relaxation effects with a relaxation frequency ∼1.076 × 10⁷ s⁻¹ at 300 K. The temperature dependence of the frequency suggests the origin of the relaxation is of the spin–lattice type. The quadrupole splitting at low temperatures indicates that the excited orbital states mix strongly to the orbital doublet ground state via spin–orbit coupling. Modified molecular field model analysis yields a saturation value for hyperfine field B_hf ∼ 125 kOe. The anomaly in susceptibility and Mössbauer parameters below 27 K may be ascribed to a contribution of orbital angular momentum. The high value of the asymmetry parameter η (∼0.8) of the electric field gradient obtained in the antiferromagnetic regime indicates a strongly distorted octahedral oxygen neighborhood for the ferrous sites.

We demonstrate in this work the effect of Rashba spin–orbit interaction (SOI) on multiphoton optical transitions of a quantum dot (QD) in the presence of a terahertz (THz) laser field and external static magnetic field. This combination is solved by accurate nonperturbative Floquet theory. Investigations are made for the optical response of intraband transitions between the various states of the conduction band with spin flipping. It is found that spin-flip transitions and corresponding multiphoton transitions are strongly affected by Rashba spin orbit coupling and THz laser fields. This aspect will enhance the ongoing efforts for making ultrasensitive optospintronics devices.

Relying on quasiparticle GW electronic structure calculations we present an approach to obtain an “optimal value” of the so called “mixing parameter,” α, in hybrid functional calculations. We show that when this “optimal” amount of exact exchange (we denote this as α_opt) is mixed with the semi-local PBE exchange-correlation functional, the net quasiparticle corrections to the PBE Kohn–Sham eigenvalues attain a minimum value. We employ this approach to perovskite oxide insulators SrTiO₃ and PbTiO₃ and compare that to the widely studied semiconductor compounds, Si and Ge. A system dependence of the optimal choice of the mixing parameter is revealed.

Adamantine ternary ordered-vacancy compounds (OVCs) of the AB₂X₄ family derive from the zincblende structure of binary AX compounds and share many properties with the chalcopyrite-type ternary ABX₂ compounds. AB₂X₄ mainly crystallize in the defect chalcopyrite (or thiogallate) structure but also in other vacancy-ordered phases, like the defect stannite (DS, or defect famatinite) and the pseudocubic (PC) structures, where vacancies occupy a fixed Wyckoff site in an ordered and stoichiometric fashion. Order–disorder phase transitions have been studied in several adamantine OVCs at high temperatures and/or pressures, where OVCs undergo a phase transition to a disordered zincblende (DZ) structure on increasing temperature and to a disordered rocksalt (DR) structure on increasing pressure. It has been suggested that these order–disorder transitions should undergo via intermediate phases of partial disorder between the fully ordered phases and the fully disordered phases. In this work we study the different possible intermediate phases of partial disorder and discuss the possibility to find them with the help of vibrational spectroscopies, like Raman scattering or infrared measurements.

We report the dynamical behaviors of a magnetic vortex driven by an out-of-plane spin-polarized current in a point-contact geometry using a micromagnetic simulation method. Polarity, chirality, and polarity plus chirality-switching diagrams are presented corresponding to different current states. Besides vortex core gyrotropic motion, polarity switching, chirality switching, and polarity plus chirality switching, we also observe non-switching and partial switching behaviors in some cases. Investigations of these switching behaviors find that polarity is switched by nucleation and annihilation of the vortex–antivortex pair for the polarity-switching only regime, and chirality reversal is realized by moving, nucleation, and annihilation of vortices in the disk for lower current density, but by propagating of spin waves for higher current density. Moreover, the reversal of chirality is always accompanied by polarity switching, and the accompanied polarity switching for the higher current density case is realized by the expansion and compression of the vortex core, not the traditional vortex–antivortex pair-mediated mechanism.

2-Propanol and oxygen were converted over titania and gold nanoparticles supported on titania to investigate the reactivity of the support, the influence of the metal and the role of metal–support interactions. The catalysts were characterized by N₂ physisorption and transmission electron microscopy. In addition to deriving the degrees of conversion and the yields as a function of temperature, temperature-programmed desorption and diffuse reflectance infrared spectroscopy were applied in fixed-bed reactors under continuous flow conditions. Over pure TiO₂ above 500 K the acid–base catalyzed dehydration yielding propene and water, the dehydrogenation to acetone and H₂, and the oxidative dehydrogenation to acetone and water were found to occur. The additional presence of Au nanoparticles induced the selective oxidation to acetone and H₂O at temperatures below 400 K, whereas the selective oxidation to acetone at higher temperatures above 500 K was also observed on pure TiO₂. Also the dehydration of 2-propanol to propene and H₂O and, to a minor extent, the total oxidation to CO₂ and H₂O were catalyzed by Au/TiO₂. Therefore, the Au/TiO₂ catalyst shows bifunctional properties in oxygen activation needed for the selective oxidation of 2-propanol. 2-propoxide species were detected by IR spectroscopy, which are identified as intermediate species in 2-propanol conversion, whereas strongly bound acetates and carbonates acted as catalyst poison for the selective low-temperature oxidation route, but not for the high-temperature route. Selective low-temperature oxidation is assumed to occur at the perimeter of the Au nanoparticles, which also enhance the high-temperature oxidation route on TiO₂ pointing to a Mars–van Krevelen mechanism based on an enhanced reducibility of TiO₂.

The adsorption of water on r-TiO₂(110) has been investigated with thermal desorption spectroscopy (TDS) and helium atom scattering. Conventional TDS using a mass spectrometer and He-TDS monitoring reflected He beam intensity consistently show the existence of a structurally well-defined monolayer as well as a highly ordered second layer of water and a disordered multilayer phase. He diffraction patterns recorded along the high symmetry [001], , and directions reveal a well-ordered superstructure with (1 × 1) symmetry, providing strong evidence for the absence of a partially dissociated monolayer on the perfect parts of the substrate. No changes in the diffraction patterns are observed after irradiation with UV-light.

Lineshape-fitting, using a model that takes into account of band tails, was performed on the photoluminescence spectra measured from InN samples at temperatures varying from 5 to 300 K. By analyzing how the fitted parameters varied with temperature, it was found that the previously observed blue and then red shift of emission peak position with increasing temperature is related to valence band tails. The initial blue-shift at low temperature is due to filling of valence band tails while the subsequent Varshni red-shift is due to lattice expansion. In a series of samples grown using increasing trimethylindium flow rates during metal organic chemical vapor deposition, the impurity potential that describes the width of the valence band tail increased from 18 to 26 meV, possibly due to more nitrogen vacancies being created in the material.

The isostructural metal-insulator transition in Cr-doped V₂O₃ is the textbook example of a Mott–Hubbard transition between a paramagnetic metal (PM) and a paramagnetic insulator. We review recent theoretical calculations as well as experimental findings which shed new light on this famous transition. In particular, the old paradigm of a doping-pressure equivalence does not hold, and there is a microscale phase separation for Cr-doped V₂O₃.

In this Feature Article, Hansmann et al. review recent calculations and experiments which shed a new light on the famous Mott–Hubbard transition in V₂O₃. Old paradigms, such as the pressure-doping equivalence need to be changed. The picture shows that, on the microscale, “metallic” Cr-doped V₂O₃ is actually phase separated into metallic islands (red) intermixed with insulating regions (blue).

The properties of the substitutional impurities of the second period of the periodic table incorporated into graphene were investigated. The Green's-functions framework and the tight-binding method without the effect of the lattice distortion were used. We discuss the possibility of the existence of well-defined resonant states in the valence band or the conduction band in the vicinity of the Dirac point. We conclude that in the vicinity of the Dirac point there was a possibility of a coincidence of the Fermi energy levels with impurity resonances (i.e., the Fermi level is pinned to the impurity resonance level.)

The internal friction and shear modulus for the particulate magnetoelectric composite (x)PbZr_0.53Ti_0.47O₃–(1 − x) Mn_0.4Zn_0.6Fe₂O₄ have been studied over a temperature range from room temperature to 670 K. In the vicinity of the ferroelectric phase transition temperature, we have observed the internal friction peak that is explained in terms of the fluctuation model. The “residual” internal friction near the ferroelectric Curie point has also been revealed. The “residual” internal friction is assumed to be due to the interaction of domain boundaries in PbZr_0.53Ti_0.47O₃ with point defects.

An electronic–thermal model of the switching effect in chalcogenide glassy semiconductors (CGSs) is investigated. The model fits the experimental current–voltage characteristics (CVC) of GeSbTe films in both the ohmic and exponential regions. Also, the model is in good agreement with the experimental dependences of threshold voltage and threshold current on temperature and thickness. The results indicate that multiphonon tunnel ionization (MTI) of negative-U centers and heating is a possible mechanism for the CVC nonlinearity and the switching effect in chalcogenides.

Nonlinear quenching of electron–hole pairs in the denser regions of ionization tracks created by γ-ray and high-energy electrons is a likely cause of the light yield non-proportionality of many inorganic scintillators. Therefore, kinetic Monte Carlo (KMC) simulations were carried out to investigate the scintillation properties of pure and thallium-doped CsI as a function of electron–hole pair density. The availability of recent experimental data on the excitation density dependence of the light yield of CsI following ultraviolet excitation allowed for an improved parameterization of the interactions between self-trapped excitons (STE) in the KMC model via dipole–dipole Förster transfer. The KMC simulations reveal that nonlinear quenching occurs very rapidly (within a few picoseconds) in the early stages of the scintillation process. In addition, the simulations predict that the concentration of thallium activators can affect the extent of nonlinear quenching as it has a direct influence on the STE density through STE dissociation and electron scavenging. This improved model will enable more realistic simulations of the non-proportional γ-ray and electron response of inorganic scintillators.

The paper presents a piezoelectric study, carried out at room temperature, for the Sr_1 − xBa_xBi₂Nb₂O₉ (x = 0, 15, 30, 50, 70, 85, 100 at%) ferroelectric (FE) ceramic system. The structural analysis has shown, for all the compositions, a pure orthorhombic phase with space group A2₁am. A large electromechanical anisotropy phenomenon has been observed between the electromechanical coupling factors for the thickness and the planar vibrations, which could be very interesting for high-frequency array transducers. The best values of piezoelectric parameters have been obtained for the Sr_0.70Ba_0.30Bi₂Nb₂O₉ composition. Our analysis of the X-ray diffraction patterns around the (200) and (020) planes, before and after the polarization process, has suggested a better polarization process for barium concentrations of 15 and 30 at%, which provides higher piezoelectric properties.

Covalent organic frameworks (COFs) are microporous crystalline organic frameworks with large specific surface areas. The area of COFs is rapidly developing in the direction of finding potential applications in fields like gas storage, photovoltaics, and catalysis. With respect to hydrogen storage, at first glance, COFs possess all the advantages of metal-organic frameworks (surface area, pore volume, rigidity of the structure). In addition, since the molecular frameworks of COFs are composed of light elements (C, Si, B, and O), these materials have exceptionally low densities. Due to this advantage, a lot of research activity (both theoretical and experimental) was reported in the recent literature on hydrogen-storage properties of COFs. Also, several strategies were suggested for enhancing H₂-storage capacities of COFs at cryogenic as well as room temperatures. In this feature article, we broadly discuss the scope of COFs as hydrogen storage media and also review the strategies suggested for enhanced room-temperature hydrogen-storage properties. Further, the concept of “spillover” is reviewed critically in metal@COFs and metal@MOFs systems.

This paper reviews our efforts to simulate methanol synthesis from CO and H₂ on defective ZnO surfaces using advanced molecular dynamics techniques. This apparently simple chemical reaction occurring on a seemingly well-defined surface appears to be astonishingly complex. First of all, the preferred oxidation state of F centers at the polar oxygen terminated surface is found to be dictated by the chemical composition and the thermodynamic properties of the gas phase in contact with . Secondly, reaction intermediates and pathways along the catalytic cycle taking place at or close to these defects are found to depend in a sensitive way on their oxidation state. Thirdly, it is seen that the gas phase close to the catalytic surface might be transiently involved in some of the reaction steps in a non-trivial manner. Last but not least, the scenario is found to be greatly enriched upon involving copper clusters on polar ZnO surfaces in view of utmost strong metal-support interactions (SMSIs), which are directly related to the polar nature of . Taken together, an unexpectedly rich picture is unveiled by the molecular dynamics approach to computational heterogeneous catalysis when applied to methanol synthesis on bare ZnO.

The strong influence of the support properties on the activity of gold catalysts has been observed in many publications. The most studied reaction in this respect seems to be CO-oxidation, for which gold catalysts have outstanding activity. However, since in most studies the support properties are also important in influencing the nature of the gold particles deposited on them by co-precipitation or deposition–precipitation, it is difficult to study the support effect alone. We have in a series of studies used colloidal impregnation of preformed gold particles approximately 3 nm in size on different supports in order to decouple the gold particle formation from the deposition process, in order to isolate the support effect. Even for such similarly prepared catalysts very strong differences between different supports were observed. The analysis of the data, also in the light of literature data, suggests that there is no unique factor explaining the high activity of gold catalysts, but rather a combination of effects, which act in different proportion for different catalysts.

Magnetic critical behavior of Mn₅Ge₃ ribbons was investigated by dc magnetization measurements. The critical exponents were obtained by a modified Arrott plot technique and the Kouvel–Fisher method. Finally, β = 0.372 ± 0.003 and γ = 1.05 ± 0.01 were determined. The exponent δ = 3.88 ± 0.02 obtained from the magnetization isotherm of T_C fulfills the Widom scaling relation δ = 1 + γ/β. Moreover, with these exponents, the magnetization curves collapse into two independent curves following a single equation around T_C. These results confirm the reliability of the obtained critical exponents. However, this set of exponents belongs to none of the classical theoretical models. This phenomenon is probably caused by the anisotropy of the Mn ions.

Measurements on transition energies near the optical absorption band edge for lithium niobate samples doped with indium at several concentrations are presented. Indirect and direct optical absorption processes were analyzed. It was found that the energies for these transitions are larger than those corresponding to undoped lithium niobate, suggesting that the energy band structure was modified by the presence of indium. For an indium concentration between 0 and 4 mol%, a value of energy of 3.8 eV was measured for indirect transition, and a range of 3.9–4.0 eV was determined for direct transition energy; whereas 3.6 and 3.8 eV were obtained, respectively, for the corresponding values of pure lithium niobate. In addition, Urbach and phonon energies were also calculated for the same indium-doped lithium niobate samples which values are lesser than those obtained for pure lithium niobate. Both behaviors remain unaltered regardless of whether the indium concentration is above or below the optical damage threshold value. Substitution of only lithium vacancies and antisite defects by indium atoms for any concentration of doping may explain the results.

In this contribution the development, definition and selected applications of a new force field (FF) for metal-organic frameworks MOF-FF is presented. MOF-FF is fully flexible and is parameterized in a systematic and consistent fashion from first principles reference data. It can be used for a variety of different MOF-families and in particular – due to the reparametrization of a variety of organic linkers – also to explore isoreticular series of systems. The history of the development, leading to the final definition of MOF-FF is reviewed along with the application of the previous incarnations of the FF. In addition, the parametrization approach is explained in a tutorial fashion. The currently parametrized set of inorganic building blocks is constantly extended.

Formate models of currently covered inorganic building blocks.

This study is concerned with a two-dimensional (2D) electron–hole (e–h) system in a strong perpendicular magnetic field with special attention devoted to the Rashba spin–orbit coupling (RSOC). The influence of this interaction on the chemical potential of the Bose–Einstein condensed magnetoexcitons and on the ground-state energy of the metallic-type electron–hole liquid (EHL) is investigated in the Hartree–Fock approximation (HFA). The magnetoexciton ground-state energy, and the energy of the single-particle elementary excitations were obtained. We demonstrated that chemical potential is a monotonic function versus the value of the filling factor with negative compressibility, which leads to instability of the Bose–Einstein condensate of magnetoexcitons. The energy per one e–h pair inside the electron–hole droplets (EHD) is found to be situated on the energy scale lower than the value of the chemical potential of the Bose–Einstein condensed magnetoexcitons with wave vector calculated in the HFA.

We report new photoluminescence (PL) data generated by pulsed optical excitation on the composites achieved by a mechanico-chemical reaction between ZnS in the cubic (c) and wurtzite (w) phases and polyaniline-emeraldine base (PANI-EB). Under continuous and pulsed optical excitation, powders of the (w)ZnS and (w)ZnS/PANI-EB composites display PL with different spectral compositions. Contrary to expectations, the (c)ZnS and the (c)ZnS/PANI-EB composite displayed weak PL and decay time in the range of nanoseconds. This results from the great involvement of the surface states in nanometric powders which creates efficient channels for the non-radiative recombination of the carriers.

PL spectra of (w)ZnS/PANI-EB under continous (a) and pulsed optical excitation (b).

The lattice thermal conductivity is known to depend on crystal quality, but the reduction in thermal conductivity due to specific defects is presently unclear. Molecular dynamics simulations were used to investigate the impact of microstructural defects on the thermal conductivity of gallium nitride. The conductivity of a finite crystal was reduced to (39 ± 4)% by a screw dislocation density of 2.0 × 10¹³ cm⁻² and to (51 ± 4)% by an edge dislocation of similar density, illustrating that the type of dislocation is important for thermal conductivity. The effect of stacking faults on thermal conductivity was also investigated.

Despite its enormous importance for heterogeneous catalysis, and in particular methanol synthesis, detailed information about the Cu/ZnO interface is still far from being complete. Here we present an overview of recent work carried out using different types of microscopical methods from which the complexity of the problem becomes apparent. In addition to results from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) data also obtained using scanning probe techniques, in particular scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are presented. Special attention is paid to the influence of elevated temperatures on the Cu/ZnO interface.

In this paper, a modified transfer matrix method is proposed to solve the transverse vibration bandgaps (BGs) in phononic crystal (PC) Euler beams on a Winkler foundation. An equivalent critical frequency is introduced to facilitate the calculation of the end frequency of the first BG. The band structures of a lead–steel PC Euler beam either on a Winkler foundation or not show that the existence of a foundation has a significant influence on the distribution of BGs. A new BG that starts at 0 Hz appears. Additional results demonstrate that a very narrow passband could exist between the first two BGs. This study is aimed at searching for large BGs, especially at low frequencies.

Band structures for the lead–steel PC Euler beam on a Winkler foundation (continuous line) or not (dash line). A new BG that starts at 0 Hz appears in the case of having a foundation.

Materials and structures with negative Poisson's ratio are characterized as auxetics. They are interesting for engineering applications due to their non-conventional behavior. However, most of the auxetic structures suffer with a disadvantage that they are difficult to be fabricated because of their complicated geometry structure. To overcome this problem, an auxetic structure made of tubes and corrugated sheets using conventional technology is proposed in this study. Its auxetic effect is investigated by both geometrical analysis and finite element method. For validating the analyses, a real auxetic structure is made with aluminum material and tested under compression condition. All the results obtained show that the proposed structure has very significant negative Poisson's ratio effect under compression. The study has provided a simple way to fabricate auxetic structure for using in a wide range of industrial fields at a low cost.

Spectral-kinetic characteristics of the undoped and Eu³⁺-doped alkali gadolinium phosphates of the type of MGdP₄O₁₂ (M = Li, Na) have been studied within the 4.2–300 K temperature range using time-resolved luminescence spectroscopy techniques. The photon cascade luminescence of Gd³⁺ ions in the undoped LiGdP₄O₁₂ phosphate has been observed. In the case of the Eu³⁺-doped MGdP₄O₁₂ phosphates, no quantum cutting process takes place due to the spectral overlap of the charge transfer band of Eu³⁺ with the bands arising from the ⁶G_J levels of Gd³⁺. The processes of energy migration along the Gd³⁺ sublattice and the Gd³⁺ → Eu³⁺ energy transfer have been studied. The phonon-assisted population of the Gd^{3+ 6}P_5/2,3/2 excited levels is suggested to be responsible for the increase in the probability of the energy migration along the Gd³⁺ sublattice via the ⁶P_J states with the subsequent energy transfer to the ⁵I_J levels of an Eu³⁺ ion.

A theoretical study on the kinematic and dynamic approximations of parametric X-ray radiation (PXR) is presented. The results obtained are examined and compared. The ratio of the absolute maxima of kinematic and dynamic PXR can be obtained. The results are closer at lower energies and their correlation depends on the angle of incidence. These theoretical results indicate that in some energy regions and angle of incidence it may be necessary to apply the dynamic approximation of PXR for greater accuracy.

In the literature, there is a striking controversy concerning the basic physical properties of UIr₂Zn₂₀ crystals that have been recently measured by various research groups. One may suppose that the particular samples are not free of defects and this is the likely reason for the discrepancy between the reported key experimental results. Here, positron annihilation spectroscopy serves as a very sensitive tool to probe lattice perfection with respect to the presence of open-volume defects. In the present work, we perform calculations of the electron density of states (DOS), positron distribution, and positron lifetime, τ, in the UIr₂Zn₂₀ perfect crystal of Fd-3m cubic structure (according to our best knowledge, for the first time in the literature). The results are compared with their counterparts for the crystals containing an Ir and Zn₁ monovacancy. Three sharp narrow peaks from 5f U electrons are observed in the DOS close to the Fermi energy, E_F, both for the perfect crystal as well as for the one containing vacancies. These peaks are located below, above and just at the Fermi level. Two peaks originate from f_5/2 and f_7/2 states, while the existence of the remaining one may be interpreted in terms of the dual character of uranium f electrons. The presence of vacancies considerably redistributes the positron charge in the unit cell and, in consequence, increases the value of the positron lifetime as compared with the perfect material.

The soft chemical route was used in the synthesis of undoped and 5% Mn doped ZnO nanocrystalline powders. XRD, TEM, TGA/DTA, FTIR, and superconducting quantum interference device techniques were used to study the structural, nano/microstructural, thermal decomposition and metastability aspects as a function of calcination temperatures (400–1100 °C) and magnetic properties. The evolution of the major wurtzite phase (ZnO) and minor non-stoichiometric nanocrystalline defect cubic spinel phase (ZnMnO_3–δ) at various temperatures is clearly seen. The magnetic hysteresis loop is observed at room temperature in the undoped and doped samples calcined at 400 °C. Interestingly, the hysteresis loop parameters (M_s, H_c) are found to enhance dramatically as soon as the concentration of the minor phase is large enough up to the calcination temperature 700 °C. In contrast, the magnetic hysteresis loop vanishes slowly for the sample calcined at 1000 °C, it disappears completely. The room temperature ferromagnetic behavior at 400 °C is understood in terms of intrinsic cationic/anionic defects, extrinsic defects associated with the various species chemisorbed on the surface of the nanoparticles of undoped and Mn doped ZnO. During thermal annealing a nanocrysatllline seconadary phase of non-stoichiometric defect cubic spinel ZnMnO_3–δ is formed, contributing to the enhancement of ferromagnetic behavior. All our experimental results are discussed in terms of model comparing various structural and localized electronic defects formed in the nanocrystalline powder.

Based on the invariant imbedding method and stationary phase approximation, we perform a theoretical investigation on the lateral shifts through an inhomogeneous medium, whose permittivity and permeability are sine functions of the thickness. We find the lateral shift is large near the angles of transmissivity resonances. Furthermore, using a numerical solution method, we discuss the dependence of lateral shift on the spatial period of permittivity, the angle of incidence, the slab's thickness and the signs of the permittivity and permeability, respectively. The results show that the angles of transmissivity resonance and the lateral shifts can be easily controlled by adjusting the parameters of the structure.

Bismuth was compressed hydrostatically and nonhydrostatically up to 55 GPa in a diamond anvil cell. The strength of bcc bismuth was determined by the radial X-ray diffraction (RXRD) technique: Y = −0.20(5) + 0.009(1)P, which is much smaller than that of normal pressure-transmitting media (PTM), such as NaCl, Ar, and He. Different PTM [silicone oil (SO), Ar, and He] were used to determine the hydrostatical equation of state of bcc bismuth. The determined results are consistent with each other when different PTM were used, even when no PTM was used. The typical bulk modulus and its pressure derivation of bcc bismuth are 42.7(6) GPa and 5.3(1), respectively. The differential strain introduced by nonhydrostatic stress was derived from Hooke's law and lattice strain theory (LST). The differential strain in bismuth is very small even when no PTM was used. Because of the simple, stable structure and small strength, bcc bismuth is an ideal candidate for an internal pressure standard.

The local structure of Mn-doped Li₂B₄O₇(001) was investigated using extended X-ray absorption fine structure (EXAFS) at the Mn K edge and electron paramagnetic resonance (EPR). The location of the Mn dopant in a lithium tetraborate crystal is consistent with occupation of a site with strong oxygen coordination. The MnO bond lengths are similar to those observed with Mn doping of the icosahedral based boron carbide where Mn is in a substitutional dopant in one of the cage sites. From EXAFS, the manganese does not appear to greatly alter the overall tetragonal form of lithium tetraborate, with the dopant most likely substituting for one of the two B sites and with placement of some of the Mn in a Li site still possible. The EPR spectra agree with the literature examined resolving multiple Mn species in the crystal lattice.

A double-cascade-type four-level system of semiconductor multiple quantum wells (MQWs) was constructed with biexcitons and excitons. The nonlinear optical properties for amplification, absorption, and dispersion of 2-weak fields in this scheme are investigated. It shows that the amplification, absorption, and dispersion responses of 2-weak fields can be achieved by appropriately adjusting the relative phase, the probe detuning, and the two control Rabi frequencies. The investigation is much more practical than its atomic counterpart because of its flexible design and the widely adjustable parameters. It may provide a new possibility in technological applications for the light amplifier and optical switch working on quantum coherence effects in MQW solid-state systems.

We present the metamorphosis in the effective-potential profile of layered heterostructures, for several III–V semiconductor binary compounds, when the band mixing of light and heavy holes increases. A root-locus-like procedure is directly applied to an eigenvalue quadratic problem obtained from a multichannel system of coupled modes, in the context of the multiband effective mass approximation. By letting grow valence-band mixing, it is shown that the standard fixed-height rectangular potential energy for the scatterer distribution is a reliable test-run input for heavy holes. On the contrary, this scheme is no longer valid for light holes and a mutable effective band offset profile has to be considered instead whenever the in-plane kinetic energy changes.

Metal oxides and metal nanoparticles dispersed on oxide substrates have gained increasing interest in surface science because of their widespread applications, especially in heterogeneous catalysis. In this review we summarize our recent vibrational spectroscopic studies on pure and metal-covered oxide surfaces (ZnO, TiO₂, Au/ZnO and Cu/ZnO) using a number of small molecules (H₂, CO, CO₂, NO and HCOOH) as probes and/or reactants. High-resolution electron energy loss spectroscopy (HREELS) turned out to be a powerful tool to investigate well-defined oxide and metal/oxide model systems. The application of a novel ultrahigh vacuum IR spectroscopy (UHV-FTIRS) apparatus allowed us to record high-quality IR data on oxide surfaces of both single crystals and polycrystalline powder particles. We will particularly focus on following important issues: (i) the interaction of hydrogen with ZnO; (ii) structure and reactivity of polar and nonpolar ZnO surfaces; (iii) the role of defects in surface chemistry of oxides; (iv) the origin of significant difference in photocatalytic activity between anatase and rutile TiO₂ and (v) the interaction between metal nanoparticles and oxide supports. We will demonstrate that the data from HREELS and UHV-FTIRS provide detailed insight into structural, electronic and chemical properties of the studied systems.

In order to investigate the influence of modified Keggin-polyoxometalates and supported gold particles on reactivity and reaction pathways in thermal 2-propanol oxidation, titanium-substituted, insoluble Cs⁺-salts of the Keggin-heteropolytungstate (H₃PW₁₂O₄₀) Cs₃PW₁₂O₄₀ 1, Cs₅PW₁₁TiO₄₀ · nH₂O 2, and supported gold composite Au/Cs₅PW₁₁TiO₄₀ · nH₂O 3 were synthesized and characterized as molecular model catalysts. The Ti-substituted polyoxometalate 2 did not show any observable activity in the gas phase oxidation of 2-propanol. The presence of gold nano-particles activates the supporting polyoxometalate 3 and enhances formation of different oxidation products (acetone, diisopropyl ether) depending on the presence of different types of active sites. The formation of the diisopropyl ether involves two adjacent 2-propanol molecules adsorbed on two different types of active sites, whereas the dehydrogenation reaction for the acetone formation involves the initial adsorption of 2-propanol on one type of active site.

Thermopower (TP), S of pure graphene in the Bloch–Grüneisen (BG) regime is investigated assuming electrons to be scattered by in-plane acoustic phonons and taking into account the temperature dependent effects arising from thermal averaging over carrier energy. Numerical calculations of electron mobility and TP show a signature of BG regime. The mobility is found to increase rapidly with decrease in temperature. The diffusion component S_d of TP is found to follow a non-linear temperature dependence showing changes in sign before becoming linear at very low temperatures; this is in contrast to the usual linear dependence reported in literature. At higher temperatures S_d, as does mobility, is found to merge with values calculated using equipartition approximation. The influence of phonon drag contribution S_g on the total TP, S, is also investigated. A fairly good agreement with existing low temperature data is obtained. Detailed analysis of TP data in pure samples will enable better understanding of the electron–phonon interaction in graphene.

We have presented new results on the EPR spectra and the ground state of the Cr³⁺ ion replacing Y³⁺ in YAl₃(BO₃)₄ and Eu³⁺ in EuAl₃(BO₃)₄ single crystals. The EPR studies performed at frequencies of about 9.4 and 37 GHz in a temperature range of 300–4.2 K have revealed the peculiarities and differences of EPR spectra of Cr³⁺ ion as well as determined the spin-Hamiltonian parameters (g-factors, D, and E fine-structure parameters and their signs) in both crystals. The value and sign of the D parameter in both crystals are determined by both the nearest ligands around the Cr³⁺ ion and the spin–orbit interaction. The interaction between Cr³⁺ ion and rare-earth ion weakly influences on the ground-state splitting, while the changes of g-factors are mainly caused by the Cr³⁺ and Eu³⁺ ions interactions. The EPR spectra and contributions determining the value of spin-Hamiltonian parameters were analyzed on the basis of the superposition model of the crystal field.

Bismuth vanadate Bi₄V₂O_11−y doped with Fe was studied by neutron diffraction. It was shown that Fe substitutes V only partially and the redundant iron forms an impurity hematite phase. In the sample with nominal content Bi₄(V_0.96Fe_0.04)₂O_11−y a new phase transition within the frame of the monoclinic structure with the appearing of inversion center was discovered in the temperature region 200–300 °C. This transition was also confirmed by DTA experiments. Slow stabilization of the crystal structure with stabilization times of the order of hours, accompanied by a decrease of the unit cell volume, was observed. At high level of Fe doping, in addition to the dominant tetragonal γ-phase some amount of the nanosized monoclinic α-phase was detected.

The internal transitions of Be acceptors confined in the center of GaAs/AlAs multiple quantum wells are investigated by Raman and photoluminescence (PL) spectra. A series of Be δ-doped GaAs/AlAs multiple quantum wells with doping at the well center and well widths ranging from 30 to 200 Å were grown on (100) GaAs substrates by molecular beam epitaxy. Both the Raman and PL spectra were measured at 4 and 20 K, respectively. The confined longitudinal optical (LO) modes, and transitions of Be acceptors from the 1S_3/2(Γ₆) ground state to 2S_3/2(Γ₆) first excited state were clearly observed in Raman spectra. Two-hole transitions of the acceptor-bound excitons from the 1S_3/2(Γ₆) ground state to the 2S_3/2(Γ₆) first excited state were also observed. It is found that the experimental results in the Raman spectra are close to those measured in the PL experiments.

The self-diffusion of a single Re adatom on the Re clusters with 967–8263 atoms is studied. The minimum-energy diffusion paths and the corresponding energy barriers for adatom diffusion on the Re hexahedral structure clusters surfaces are determined through a combination of the quenched molecular dynamics (MD) and nudged elastic bands (NEB) methods. Within the studied cluster size range, the ES barriers for an adatom across the step from a (0001) facet to a neighboring () facet are the same. This means that the ES barriers are independent of the size of the clusters, while, the ES barriers for an adatom across the step between two () facet have some dependence on the number of the shell (k). The ES barriers of the clusters with k = even number is lower than that of the clusters with k = odd number. For the former, the ES barriers decrease with increasing cluster size.

Raman spectra of single-walled carbon nanotubes (SWNTs) with diameter distribution of 0.6–1.6 nm have been studied under high pressure. We focused on the dependence of radial R-band frequency (associated with the radial breathing mode, RBM) on pressure, dω_RBM/dP, and studied the effect of experimental factors on the structural changes of nanotubes under pressure. We found that the nanotube diameter and the pressure transmission medium (PTM) strongly affect the dω_RBM/dP dependence of a nanotube in the experiments with PTMs, while the nanotube diameter and the intertube interactions between the tube walls play a dominant role on the upshift of RBMs for nanotubes without a PTM. These results are in line with the theoretical predictions that PTMs have a strong effect on the pressure-induced shift in the R-bands of SWNTs. In addition, we found that the collapse of nanotubes and their reversibility depend not only on diameter and pressure environment but also on the sample quality. This could explain that there have been different results reported by different groups even for nanotubes with the same diameter distribution.

Based on first-principles simulations, the electronic and magnetic properties of bare and hydrogenated CdS nanowires (NWs) are investigated. We find that surface relaxation plays an important role for the hydrogenated CdS NWs and therefore leads to drastic changes of electronic properties. The magnetic properties can be tuned by controlling passivation on surface sites with hydrogen. While hydrogenated NWs on surface S atoms are nonmagnetic, hydrogenation on surface Cd atoms, and especially a monolayer of H on the surface, results in half-metallic properties, due to the redistributions of surface electrons in the sulfur p orbital.

The lattice dynamics in the IV–VI compounds GeTe, SnTe and PbTe were studied by ¹²⁵Te and ¹¹⁹Sn nuclear inelastic scattering and the obtained partial density of phonon states were compared with published theoretical calculations. The phase purity and structure were characterized by high energy X-ray diffraction. The effect of the atomic arrangement, rhombohedral for GeTe and cubic for SnTe and PbTe, is visible in the density of phonon states. Vibrational properties are found to be in good agreement with available calculated data and the softer character of the NaCl-type structures in comparison with the rhombohedral GeTe is confirmed.

A physically valid parametrization of the crystal-field (CF) Hamiltonian H_CF has to reproduce not only the energy of Stark sublevels but also the multipolar structure of the central ion surroundings. The square of the second moment of electronic state CF splitting must be a superposition of squares of the second moments of the partial splittings produced separately by the individual multipoles: , where represents the modulus of a k-rank multipole of the central ion electronic eigenstate , whereas is the k-rank CF strength. The relationship between known from experiment and obtained from fitting procedure allows us to verify whether the parametrization is physically valid. It is demonstrated by several examples of CF splittings of electronic states of trivalent lanthanide ions in different crystal matrices. The approach is confined only to the states with good quantum numbers J. Nevertheless, by finding such states in any analyzed spectrum, the method can provide the actual multipole characteristics of H_CF, and plays the role of a preparametrization. In general, the stepwise fitting procedure can never lead to parametrizations with a correct multipolar structure due to the limitation of the initial eigenfunctions. In consequence, the pertinent crystal-field parameters (CFPs) are burdened with a methodical inherent fault. The loss of physical meaning of fitted CFPs, its source and consequences, constitute the main theme of the paper.

We report magnetic field dependence of both in-plane and out-of plane thermopower in Ca₃Co₄O_9+δ single crystal. Strong field dependence of both in-plane and out-of plane thermopower evinces giant spin entropy contribution to the thermopower in the thermoelectric material. These discovered effects indicate that spin entropy dominates the enhancement of thermopower in misfit-layered cobalt oxides materials. An obvious suppression of thermopower is stronger by an out-of-plane magnetic field than by an in-plane field.

Single-material rolled-up microtubes provide an opportunity to study the processes of strain relaxation and defect formation near the interface of strained-layer semiconductor heterostructures. We study theoretically tubes formed by a single strained Si layer grown beyond the critical thickness on a Ge substrate. The residual strain accumulated in the Si layer causes the layer to roll after it is released from the substrate by etching. Computer simulations are carried out using Keating's valence force field method, with uniform and non-uniform residual (initial) strain profiles and different strained layer widths. We show that the radius of the tubes is affected by a combination of these two factors and thus by the total elastic deformation present before rolling, a larger total deformation resulting in a smaller tube radius. However, uniform and non-uniform residual strain profiles can be easily distinguished by analyzing the final profiles of the azimuthal and the radial strain components.

Ag/Si(111) heterojunction having ∼25% lattice mismatch can be formed by placing four surface unit cells of Ag(111) on three surface unit cells of Si(111), thereby reducing the effective strain in the Ag film to ∼0.3% in this coincidence site epitaxial growth. We have carried out first-principles investigation of such Ag/Si(111) interface to establish the interplay between the structural and electronic properties. While the Ag overlayer affects the reconstruction of the Si(111) surface, we find that the geometrical relaxation of the Ag atoms is influenced by the sub-surface Si-layer and the concomitant lattice mismatch. The electronic density of states show some oscillations near the Fermi energy, which have been compared with the scanning tunnelling spectroscopic (STS) experimental data. The signature of metal-induced gap states (MIGS) for this metal–semiconductor heterojunction has been established from the evolution of localized gap states in the layer projected density of states. Our DFT estimated work function values for Ag(100), (110) and (111) have an excellent agreement with the available experimental results. Also, the p-type Schottky barrier height (SBH) of this rectifying Ag/Si contact has been calculated from the Kohn–Sham estimates of E_F − E_v modified by the interface induced dipole.

The elastic anisotropy and thermodynamic properties of the potential superhard carbon nitride phase C₂N₂(NH) have been investigated by using an ab initio plane-wave pseudopotential density theory method. The crystal parameters have been calculated at ambient as well as high pressure. The Young's modulus and shear modulus as a function of crystal orientations for C₂N₂(NH) have been systematically investigated. The Young's modulus is found to reach a maximum along the [100] direction. Using a set of total energy versus volume obtained with the first-principles calculations, the quasiharmonic Debye model is applied to the study of the thermal and vibrational effects. The dependence of Debye temperature, Grüneisen parameter, heat capacity, and expansion coefficient on the temperature and pressure are systematically explored in the whole pressure range from 0 to 60 GPa and temperature range from 0 to 2000 K.

The development of reliable interatomic potentials for large-scale molecular dynamics (MD) simulations of chemical processes at surfaces and interfaces is a formidable challenge because a wide range of atomic environments and very different types of bonding can be present. In recent years interatomic potentials based on artificial neural networks (NNs) have emerged offering an unbiased approach to the construction of potential energy surfaces (PESs) for systems that are difficult to describe by conventional potentials. Here, we review the basic properties of NN potentials and describe their construction for materials like metals and oxides. The accuracy and efficiency are demonstrated using copper and zinc oxide as benchmark systems. First results for a potential of the combined ternary CuZnO system aiming at the description of oxide-supported copper clusters are reported.

Model of a copper cluster at the ZnO() surface.

The magnetic compensation and magnetostriction properties in Fe-doped CoCr₂O₄ samples have been investigated. Structural and magnetic measurements imply that the doped Fe³⁺ ions initially occupy the B1 (Cr) sites when x < 0.1, and then mainly take the A (Co) sites. This behaviour results in a role conversion of magnetic contributors and a composition compensation between two competitively magnetic sublattices at x = 0.1. Temperature-dependent compensation has also been found in the samples with x = 0.1–0.22, with the compensation temperature in the range of 40–104 K. The Fe³⁺ doping also modulates the exchange interaction of the system and prevents the formation of long-range conical order of spins. The magnetoelectric transition temperature at 23 K in CoCr₂O₄ is shifted to lower temperature by increasing the dopants. The magnetostriction effect in this system has been observed for the first time. The strain has a maximum value of about 280 ppm at x = 0.4. The magnetostriction is consistent with the behaviour of the two magnetic compensations.

We present the results of calculations of SrTiO₃ and CaTiO₃ polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO₃ or CaO₃) of the SrTiO₃ and CaTiO₃ (111) surfaces. According to our calculations, atoms of the first surface layer relax inwards for Ti-, SrO₃-, and CaO₃-terminated (111) surfaces of both materials. The only exception is outward relaxation of the SrO₃-terminated SrTiO₃ (111) surface upper layer Sr atom. For both Ti-terminated SrTiO₃ and CaTiO₃ (111) surfaces our calculated second layer Sr and Ca metal atom inward relaxations are approximately four and two times larger than the upper layer Ti atom inward relaxation. Our calculated optical band gap for the SrO₃- and Ti-terminated SrTiO₃, as well as for CaO₃-terminated CaTiO₃ (111) surfaces, becomes smaller with respect to the bulk optical band gap. Our calculated surface energies for SrO₃- and CaO₃-terminated SrTiO₃ and CaTiO₃ (111) surfaces (6.30 and 5.86 eV) are considerably larger than the surface energies for Ti-terminated SrTiO₃ and CaTiO₃ (111) surfaces (4.99 and 4.18 eV). Our B3LYP calculations indicate a considerable increase of Ti–O chemical bond covalency near the SrTiO₃ and CaTiO₃ (111) surface (+0.098e and +0.094e) relative to the SrTiO₃ and CaTiO₃ bulk (+0.088e and +0.084e).

Details of the sophisticated scientific and technological infrastructures that supported the creation of European art are elucidated by Spike Bucklow on pp. 927–930. Recently, Stephen Elliott's group at Cambridge University has undertaken multivariate analysis on the combined results of different methods (FT Raman and fibre-optic reflectance spectroscopy) applied to model paint systems constructed following the traditional artists' materials and methods. The front cover shows a cross-section through a sample of paint (ca. 200 μm) taken from the now-faded purple trousers of the man in the sixteenthcentury portrait (insert). Ultraviolet fluorescence makes the layer structure visible. The top layer shows soot in varnish. This lies above a layer of synthetic alumina particles, which provide the substrate for a red dyestuff extracted from insects, together with discoloured particles of once-blue cobalt-rich glass, all distributed through polymerised linseed oil. The large black structures in a lower preparatory layer are charcoal and show the cell structure of the tree from which the pigment was sourced. (Inset image: Cornelis Ketel, A Giant Porter, 1580. Reproduced by kind permission of The Royal Collection, London.).

How realistic are models of disordered materials? Comparing and assessing the quality of atomistic configurations is a non-trivial in the absence of long-range periodicity. Cliffe and Goodwin (pp. 949–956) suggest two metrics for this task: namely, local geometric invariance and the degree of local symmetry. The cover image shows a configuration of a-Si where the atoms have been coloured according to the second of these criteria, with atoms in low symmetry environments shown in red and those in high symmetry environments in blue. These new metrics enable screening for structural simplicity in a way that does not rely on formal group theoretical language and outperforms established measures based on simple pairwise correlations.

High resolution imaging and spectroscopy significantly improves our understanding of surface structure and chemistry for complex materials. In the Feature Article by Markus Heyde et al. (pp. 895–921) low temperature ultrahigh vacuum noncontact atomic force and scanning tunneling microscopy has been employed to characterize surface structures ranging from periodic and ordered via defective towards amorphous networks. In the presented studies it has been shown how modern scanning probe microscopy techniques can complete and clarify the atomistic models, which have been derived so far mainly from diffraction experiments. The benefits of locally resolving complex surface structures are obvious. A direct atomic assignment from the obtained images is possible. Surface structures with increasing dimensionality from 0D to 2D are discussed. In this article three different sample systems are highlighted, namely, magnesia on Ag(001), alumina on NiAl(110) and silica on Ru(0001).

In the following, we demonstrate the atomic-scale analysis of oxide surfaces. Essential physical properties were extracted using noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM). The main focus has been put on the determination of surface structures. A review of the recent achievements towards atomic-scale resolution from highly crystalline to amorphous materials is provided. An overview of local probe microscopy and spectroscopy to get beyond the averaging character of diffraction methods is thereby summarized. In particular, surface defects of various dimensionality were investigated. Furthermore, acquisition of information on electronic properties is detailed. The presented material covers zero-dimensional (0D) point defects, one-dimensional (1D) line defects, and two-dimensional (2D) random networks, i.e., amorphous structures. First, we present spectroscopy data taken on thin MgO films grown on Ag(001). Distance- and bias-dependent nc-AFM and STM measurements were recorded on these films. The local work-function shift and electronic structure of color centers in the MgO surface were studied. In the next section, the structure determination of ultrathin alumina/NiAl(110) is shown. Atomically resolved nc-AFM reveals a detailed picture of various line defects in the film. Finally, we discuss the atomic structure of a recently discovered ultrathin vitreous silica film on Ru(0001). The atomic arrangement in the 2D random network, resembling the classical picture of Zachariasen, is analyzed in terms of the pair correlation function and ring-size distribution.

The determination of structure has always been in the focus of the scientific community. Still, diffraction methods are powerful tools in the field of surface science. However, they have limitations when it comes to the analysis of complex structures or materials without periodicity and order. This topic is clearly visible in the Feature Article by Markus Heyde et al. Here, the authors present how they have applied state of the art atomic force and scanning tunneling microscopy to verify oxide film structures ranging from zero-dimensional (0D) point defects, one-dimensional (1D) line defects to two-dimensional (2D) random networks, i.e. amorphous structures. The latter example has fully demonstrated the validity of Zachariasen's postulation and thereby unraveled for the first time the real-space structure of an amorphous solid in all of its details.

A basic understanding of the physical composition of historic works of art, such as Old Master paintings, is essential for their long-term preservation. Such a broad understanding has largely been obtained over the last fifty years. However, a more sophisticated understanding of paintings' complex structures can also provide evidence of their modes of production. Such research can offer insights into historic technologies, of which surviving paintings are some of the finest products. This paper is an overview that uses a few examples to illustrate the sophisticated scientific and technological infrastructures that supported the creation of European art up to the 17th century. It briefly indicates how knowledge about material behaviour was transmitted and then lost. It treats great paintings as engineered structures made to the most exacting specifications and suggests that the physical nature of artworks offers researchers significant challenges.

The 80 years since Zachariasen's famous paper, 20 years before Stephen Elliott was born, on the random network theory of glass structure have seen remarkable progress in our understanding of the structure of glassy materials through the construction of models and comparison with experiment. In the early days, models were hand-built with plastic units and had free boundary conditions. Today, very much larger computer models have periodic boundary conditions. We recount the progress that has been made in the last 80 years, and discuss the current agreement between models and experiments that remains imperfect. Stephen Elliott's work on medium range order forms an important part of the history of this subject.

The vibrational properties of glasses in the THz range differ very much from what is expected from Debye's elasticity theory: the density of states (DOS) deviates from Debye's ω² law [the “boson peak” (BP)], the sound velocity shows a negative dispersion in the BP frequency regime and there is a strong increase in the sound attenuation near the BP frequency. These anomalies are related to an anomalous temperature dependence of the specific heat and thermal conductivity in the 10 K regime. An overview of the heterogeneous-elasticity theory is given, by means of which all these anomalies can be explained and shown to arise from the structural disorder, leading to spatial fluctuations of the shear modulus. Further, a very general model-independent explanation of the BP-related anomalies, based solely on symmetry arguments, is given.

Phase-change materials have a wide range of optical and electrical applications due to a unique combination of structural, electronic and optical properties. The phase transition between the crystalline and amorphous phases is a central physical process in devices based on phase-change materials. In order to design and to develop future applications with novel functionalities we need to understand on the nanometre length scale and the (sub)nanosecond timescale the electrical, thermal and phase-transition behaviour of phase-change materials. At the nanoscale the phase-transformation in phase-change materials is governed by the chemical composition, atomic structure, and the input energy due to temperature (Joule heat), electric field and/or electronic excitations. Therefore, a successful phase-change model should capture all these elements and predict the phase information as an output. We discuss the theoretical models that have been used to study and predict the crystallization of phase-change materials that span the materials modelling spectrum from electronic/atomistic simulations (atomic behaviour) to microscopic and continuum models (bulk behaviour). Real applications and device design require the use of microscopic and continuum models due to large computational times of atomistic simulations. On the other hand the predictions provided by microscopic and continuum models depend on the chosen parameter set used to describe the phase transition. We stress the importance of ‘building bridges’ between atomistic and continuum modelling to design future phase-change devices with novel and enhanced functionalities.

We suggest two metrics for assessing the quality of atomistic configurations of disordered materials, both of which are based on quantifying the orientational distribution of neighbours around each atom in the configuration. The first metric is that of geometric invariance: i.e., the extent to which the neighbour arrangements are as similar as possible for different atoms, allowing for variations in frame of reference. The second metric concerns the degree of local symmetry. We propose that for a set of configurations with equivalent pair correlations, ranking highly those configurations with low geometric invariance but with high local symmetry selects for structural simplicity in a way that does not rely on formal group theoretical language (and hence long-range periodic order). We show that these metrics rank a range of SiO₂ and a-Si configurations in an intuitive manner, and are also significantly more sensitive to unphysical features of those configurations in a way that metrics based on pair correlations are not. We also report that implementation of the metrics within a reverse Monte Carlo (RMC) algorithm gives rise to an energy landscape that is too coarse (at least in this initial implementation) for amorphous structure ‘solution’.

Cliffe and Goodwin suggest two metrics for assessing the quality of configurations of disordered materials: namely, local geometric invariance and degree of local symmetry. These enable screening for structural simplicity in a way that does not rely on formal group theoretical language (and hence long-range periodic order). The authors show that various SiO₂ and a-Si configurations are ranked sensibly by this approach, which is found to be significantly more sensitive to unphysical features than pairwise correlations.

Hafnium oxide (HfO_x) is a high dielectric constant (k) oxide which has been identified as being suitable for use as the gate dielectric in thin film transistors (TFTs). Amorphous materials are preferred for a gate dielectric, but it has been an ongoing challenge to produce amorphous HfO_x while maintaining a high dielectric constant. A technique called high target utilization sputtering (HiTUS) is demonstrated to be capable of depositing high-k amorphous HfO_x thin films at room temperature. The plasma is generated in a remote chamber, allowing higher rate deposition of films with minimal ion damage. Compared to a conventional sputtering system, the HiTUS technique allows finer control of the thin film microstructure. Using a conventional reactive rf magnetron sputtering technique, monoclinic nanocrystalline HfO_x thin films have been deposited at a rate of ∼1.6 nm min⁻¹ at room temperature, with a resistivity of 10¹³ Ω cm, a breakdown strength of 3.5 MV cm⁻¹ and a dielectric constant of ∼18.2. By comparison, using the HiTUS process, amorphous HfO_x (x = 2.1) thin films which appear to have a cubic-like short-range order have been deposited at a high deposition rate of ∼25 nm min⁻¹ with a high resistivity of 10¹⁴ Ω cm, a breakdown strength of 3 MV cm⁻¹ and a high dielectric constant of ∼30. Two key conditions must be satisfied in the HiTUS system for high-k HfO_x to be produced. Firstly, the correct oxygen flow rate is required for a given sputtering rate from the metallic target. Secondly, there must be an absence of energetic oxygen ion bombardment to maintain an amorphous microstructure and a high flux of medium energy species emitted from the metallic sputtering target to induce a cubic-like short range order. This HfO_x is very attractive as a dielectric material for large-area electronic applications on flexible substrates.

A remote plasma sputtering process (high target utilization sputtering, HiTUS) has been used to deposit amorphous hafnium oxide with a very high dielectric constant (∼30). X-ray diffraction shows that this material has a microstructure in which the atoms have a cubic-like short-range order, whereas radio frequency (rf) magnetron sputtering produced a monoclinic polycrystalline microstructure. This is correlated to the difference in the energetics of remote plasma and rf magnetron sputtering processes.

Reduction of programming current is a major research goal in the development of phase-change random-access memory devices. The power-limiting step is the amorphization of the phase-change material (PCM), where a significant energy input is required to induce melting prior to amorphization. To address the challenge of reducing power consumption while retaining switching speed, a detailed understanding of the physics underpinning the amorphization process is required. As yet, little has been done to study the dynamics of the melt-quench process at the atomic level. In this article, we report a detailed study of the melting mechanism and kinetics, and the effect of quench rate on the amorphization process in the prototypical PCM Ge₂Sb₂Te₅, using ab initio molecular-dynamics simulations. We also study the evolution of the amorphous phase under low-temperature annealing, shedding light on the structural changes, which may occur after amorphization at device operating temperatures. Our results give microscopic insight into the amorphization of PCMs, and should inform future work to understand and resolve important issues in device engineering.

Effect of quench rate on the structure of Ge₂Sb₂Te₅: While quenching at −5 K ps⁻¹ leads to successful amorphization (left), quenching at a slower −1 K ps⁻¹ leads to crystallization as the temperature is lowered (right).

The structural and dynamical properties of densified sodium silicates are investigated using molecular dynamics (MD) simulations. From the analysis of the first sharp diffraction peak (FSDP) in the amorphous phase, it is shown that some of its characteristic parameters (position, width) in partial structure factors display minima in a certain pressure interval defining a window. The pressure window can be correlated with anomalies in transport properties (diffusion, viscosity) and their activation barriers. The count of topological constraints, sensitive to pressure and temperature, is also computed. In the pressure window, we find evidence that an adaptative behavior takes place as angular constraints soften and reduce the increasing network connectivity related strain induced by pressure. These findings display striking similarities with the Boolchand intermediate phase (IP) found in rigidity driven by composition. The present numerical results also suggest that structural signatures for the IP should be found from a detailed analysis of neutron structure factors involving the partials. Finally, on a more general ground, the present study links for the first time to the best of our knowledge characteristic features of the FSDP with transport properties in the liquid.

We present a photothermoelastic model for studying the photothermal effect in chalcogenide glasses. The temperature rise and thermoelastic expansion for a model material As₂S₃ glass are calculated. Our result indicates that significant expansion (e.g. >1 µm) on the free surface of glass can be induced with only moderate temperature rise (e.g. <30 K). This occurs because of the Poisson effect of the large compressive forces generated on the interface between the irradiated and unirradiated areas in the material. We argue that photoexpansion in chalcogenide glasses arise from the concurrent effect of photothermoelastic expansion and photofluidity. The temperature dependence of photoexpansion is also discussed. The present model provides an explanation for the generality of photoexpansion in these materials.

Photoluminescence in a chalcogenide glass system As–S and a related crystal As₂S₃ (orpiment) has been studied with respect to excitation spectra, temperature dependence, and sample varieties. The crystal exhibits the strongest luminescence, and in the glass system, luminescence is the most intense at the stoichiometric composition As₂S₃. Luminescence characteristics in the As–S glasses are substantially different under excitation of bandgap and subgap light, which seem to be governed, respectively, by spatially extended electronic relaxation and direct excitation to the gap state that produces weak absorption tails.

The phase change technology behind the current rewritable optical disks and the latest generation of electronic memories has provided clear commercial and technological advances for the field of data storage; by virtue of the many key attributes chalcogenide materials offer. In this work, germanium antimony (Ge–Sb) lateral nanowire phase change memory devices have been fabricated from thin films deposited by chemical vapor deposition (CVD). Deposition takes place at atmospheric pressure using metal chloride precursors at reaction temperatures between 750 and 875 °C. The fabricated devices have been characterized electrically demonstrating reversible phase change, while a lowering in power consumption in these memory cells is observed with scaling of the geometry of the nanowire cells. The results are investigated by electrothermal modeling to understand the temperature of the devices during operation. These prototype CVD-grown Ge–Sb lateral nanowire devices show promise for applications such as phase-change memory and optical, electronic, and plasmonic switching.

Artistic impression of Ge–Sb nanowire memory device.

Monitoring the silver photodiffusion in thin amorphous As₂S₃ film is addressed with a new experimental setup. A possible photo-diffusion mechanism of silver into the a-As₂S₃ thin film under green laser diode light (λ = 532 nm) irradiation is proposed. The proposed mechanism is based on a gradual filling of the structural voids existing in the network of the thin chalcogenide layer. This mechanism is supported by XRD measurements, optical absorption, and modeling data.

Although the steady-state secondary photoconductivity in amorphous semiconductors is fairly well understood, photocarrier transport with a subpicosecond time resolution (the THz region in the frequency domain) is still not clear. The frequency response of short-lived photocarriers in the THz region is discussed in a-Si:H here. It is shown that THz-photoconductivity is highly influenced by the mesoscopic inhomogeneity (10-nm scale) originating from potential fluctuations. This behavior is basically the same as observed in the radio-frequency (RF) range.

The topological transition from order to disorder in crystalline silicon was investigated by a computer simulation procedure. The gradually introduction of topological Wooten–Winer–Weaire defect states makes the crystal change in a more and more disordered assembly of atoms. The characterization of deformation energy around a single defect state is analyzed. The topological transition from graphene structure to an amorphous carbon layer, by introduction of a high number of Stone–Wales defect-type states was evidenced. The comparison of the disordered structure in tetrahedrally bonded semiconductors (silicon) and a two-dimensional network based on graphene structure was made.

The figure shown here illustrates the out-of-plane deformation of one of the imaginary-frequency normal modes in a flat amorphous graphene model. These imaginary modes have large components along the normal direction (relative to the longitudinal components). Such eigenvectors are localized on pentagons in the network (as color coded according to the bar adjacent), demonstrating their central role in inducing puckering from the flat sheet. The calculation was carried out with the local basis density functional code SIESTA. For further details see the article by Yuting Li and David A. Drabold on pp. 1012–1019.

Recently, the prospects for amorphous phases of graphene (α-g) have been explored computationally. Initial models were flat, and contained odd-member rings, while maintaining threefold coordination and sp² bonding. Upon relaxation, puckering occurs, and may be traced to the existence of pentagons, in analogy with the situation for fullerenes. In this work, we systematically explore the inherent structures with energy close to the flat starting structure. As expected, the planar symmetry can be broken in various ways, which we characterize for 800-atom model of α-g, always using local basis density functional techniques. The classical normal modes of various structural models are discussed, with an emphasis on imaginary modes indicating the evolution from flat to puckered. We also discuss very low energy conformational fluctuations akin to those seen previously in amorphous silicon, and reflect on the nature of the amorphous “ground state” within a network of fixed topology. For completeness, high energy modes were also computed, and are found to be associated with strained parts of the network.

After an introduction to the mid-infrared (IR) spectral region, the tremendous significance of mid-IR spectroscopic sensing is highlighted. The remarkable progress made towards mid-IR spectral in vitro mapping of tissue and cancer detection is reviewed, with emphasis on diagnosis of skin cancer. The status quo of chalcogenide glass mid-IR fibre optics and photonics for meeting opportunities for remote mid-IR sensing in general, and in in vivo cancer detection in particular, is assessed. Raman spectroscopy is a sister technique to mid-IR spectroscopy. The current success of Raman spectroscopy in medical diagnosis is appraised, with particular emphasis on Raman spectral imaging of tissue towards skin cancer diagnosis in vivo, based on a silica-glass fibreoptic sensor-head. The challenges to be met in chalcogenide glass science and technology towards facilitating analogous fibreoptic diagnostics based on mid-IR spectroscopy are addressed.

Heterogeneity is a factor inherent for many transport phenomena. It is a challenging theoretical task to deal with disorder and, in particular, to average the observables over different realisation of disorder. In the paper by S. N. Taraskin (pp. 1029–1043), a theoretical model for diffusion on a network of arbitrary topology with non-symmetric random transition rates is developed and solved analytically for the steady-state regime. It is demonstrated that disorder in transition rates for a random walker induces variable occupation probability for different nodes in the network depending on their location. The boundary nodes are shown to be occupied with higher probability than the central ones.

The general solution and its graphical interpretation for the stationary occupation probability of nodes for a random-walk on a connected network of arbitrary topology with random and non-symmetric transition rates are presented. Configurational averaging over the quenched random transition rates is performed for several simple networks: open chains, simple and general stars, two interacting stars. It is demonstrated that disorder in transition rates for a random walker induces variable occupation probability for different nodes in the network depending on their location. The boundary nodes are shown to be occupied with higher probability than the central ones.

The low-frequency optical dielectric response of glasses is discussed regarding its sensitivity to the presence of building blocks conforming their structure. The model suggested by Wemple and DiDomenico to describe the dispersion of the refractive index of solids is further developed in terms of the cation local bonding environment and its effect in the value of the dispersion energy. As₄₀S_60−xSe_x glass alloys have been studied under these considerations and quantitative estimates of the concentration of building blocks are reported. Results are consistent with Raman experiments.

For a group of charged particles obeying quantum mechanics (QM) and interacting with an electromagnetic field, the charge, and current density in a pure state of the system are expressed in terms of the many-body wave function. Using these as sources, the microscopic Maxwell equations can be written down for any given pure state of a many-body system. By employing semi-classical radiation theory (SCRT) with these sources, the microscopic Maxwell equations can be used to compute the strong radiation fields produced by interacting charged quantal particles. For a charged particle, three radiation fields involve only the vector potential A. This is another example demonstrating the observability of the vector potential. Five radiation fields are perpendicular to the canonical momentum of a single charged particle. For a group of charged particles, a new type of radiation field is predicted. This kind of radiation does not exist for a single charged particle. The macroscopic Maxwell equations are derived from the corresponding microscopic equations for a pure state by the Russakoff–Robinson procedure which requires only a spatial coarse graining. Because the sources of fields are also the responses of a system to an external field, one also has to give up the temporal coarse graining of the current density which is often supposed to be critical to the kinetic approach of conductivity.