This Special Issue is devoted to recent advances in the field of correlated electron materials, as studied within the Collaborative Research Center/Transregio 80. In such systems, unlike in ordinary metals or semiconductors, the Coulomb interaction plays an important and often a decisive role. As a consequence, correlated systems display a rich variety of quantum-mechanically entangled insulating, metallic, magnetic, or superconducting states. In eight reviews and twelve research articles, the routes from correlations to topology and from the bulk to surfaces and interfaces are explored. For further details see the Guest Editorial (article number 2200125) by Ulrich Eckern and Philipp Gegenwart.

The potential of the optical floating-zone technique for the growth of intermetallic compounds is demonstrated. Single crystals of cubic chiral magnets with intentional compositional gradient along the growth direction (Mn_1–xFe_xSi, Fe_1–xCo_xSi), single crystals with deliberate off-stoichiometry (Mn_1+xSi, $x=-0.01\text{\hspace{0.17em}\hspace{0.17em}to\hspace{0.17em}\hspace{0.17em}}0.04$; CrB_x, $x=1.95\text{\hspace{0.17em}\hspace{0.17em}to\hspace{0.17em}\hspace{0.17em}}2.10$), and single crystals of compounds with complex metallurgy or challenging growth conditions (ErB₂, MnB₂, VB₂) are prepared.

The radial anisotropy of measured and symmetrized 2D angular correlation of the positron annihilation radiation ($2\mathrm{D}$-ACAR) spectrum of LaB₆ (top) and the corresponding theoretical spectra (bottom) computed within the Density Functional Theory including electron–positron enhancement effects is discussed.

Herein, the band structure of the Weyl semimetal NbAs is presented. It is determined via angle-dependent quantum oscillations in the magnetization, torque, and resistivity combined with density functional theory calculations. New low-frequency branches are found. These most probably stem from tiny chiral Fermi-surface pockets, showing that the Fermi energy is very close to the Weyl nodes in this compound.

In light of the availability of upcoming high-brilliance neutron sources and novel optics, the history and development of in situ thin-film growth capabilities for polarized neutron reflectometry are reviewed. Previous challenges and the technological advances of the last years are discussed, with recent experimental results highlighting the unique novel research opportunities that the in situ technique provides.

A topological 2D s-wave superconductor possesses nontrivial spin textures only at zero temperature. It is investigated if signatures of phase transitions between topological ground states still exist at finite temperatures. In fact, changes in those spin textures can be identified through a sign change in the derivative of the magnetization with respect to temperature.

Honeycomb iridates are the most abundant family of Kitaev magnets. Their intriguing physics are reviewed from the perspective of Li₂IrO₃, its several polymorphs, and chemically substituted derivatives. The survey covers the diverse magnetic ground states of Kitaev materials, compares the efficiency of different tuning strategies, and highlights the acute effect of structural randomness.

The number of parameters required to accurately represent states undergoes global quantum quenches to time t ^* with different variational wavefunctions. The results for matrix product states are shown by the black line in the image. The colored lines represent the results for different neural-network wavefunctions. An exponential growth in time is observed for all neural-network wavefunctions considered.

Temperature-dependent Raman data along with theoretical simulations are presented for the Kagome ferromagnet Fe₃Sn₂. The phonon energies agree well with the simulations. The width of the softest A_1g mode varies strongly below 100 K and becomes nearly temperature-independent above when the Fe spins point along the c-axis, thus indicating enhanced spin–phonon coupling for spins parallel to the Fe plane.

The development of high-pressure equipment, including a setup for neutron depolarization measurements in diamond anvil cells (DACs) is reviewed. The progress is showcased in terms of high-pressure studies of the ferromagnetic order in SrRuO₃, the emergence of incipient superconductivity in CrB₂ under quasi-hydrostatic conditions, and resistivity measurements addressing the magneto-elastic coupling in CeCuAu₃.

Neutron spin-echo (NSE) spectroscopy is a powerful tool for the study of magnon lifetimes and magnetic critical dynamics. Herein, both typical experiments and the related technical innovation are discussed and an outlook on future developments is given.

Magnetic resonance is applied to characterize the electronic structure and collective magnetic excitations of the lacunar spinels GaV₄S₈ and GaV₄Se₈ showing cycloidal, Néel-type skyrmion lattice (SkL), and ferromagnetic phases. ⁷¹Ga nuclear magnetic resonance provides a local probe of the rhombohedral distortion and uniaxial magnetic anisotropy. Broadband electron spin resonance allows identifying clockwise, counterclockwise, and breathing modes of the SkLs.

Lacunar spinels (chemical formula $A{M}_4{X}_8$) form a populous family of narrow-gap semiconductors, which offer a fertile ground to explore correlation and quantum phenomena, such as transitions between Mott and spin-orbit insulator states, Jahn-Teller driven ferro/antiferroelectricity, and the magnetoelectric response of magnetic skyrmions. This review summarizes recent experimental progress in the field of optical, dielectric, and magnetoelectric properties of lacunar spinels.

When exciting correlated electrons with an electric-field pump pulse, their prethermal momentum distribution can be optimized through its two pump-frequency-dependent factors, shown in the figure: one which depends on the momentum k (shown on left) and one which depends on the interaction U (shown on right).

Wannier–Stark or “tilted-field” many-body localization provides an alternative route to engineer interacting localized systems without quenched disorder. The dynamics of entanglement in the density matrix of such a system coupled to a bath is numerically investigated. As an accessible entanglement proxy, the third Rényi negativity is computed. This proxy captures the characteristic logarithmic growth of interacting localized phases.

The design of perovskite transition metal oxide heterostructures with technologically interesting phases relies on the detailed understanding of interface reconstructions and their depth profiles. In this review, the focus lies on the complementary application of resonant X-ray and neutron reflectometry as nondestructive techniques that provide important contributions to the understanding of the underlying interactions in these quantum materials.

The half-Heusler C1_b structure is defined by the characteristic order of structural voids. Temperature-dependent neutron diffraction in the pseudobinary Ni_1+x MnSb phase diagram elucidates the entropy-driven transition to L2₁ order and uncovers the slow relaxation of thermal vacancy concentrations.

Electronic transport properties of 2D oxide heterostructures often deviate qualitatively from classical behavior at low temperatures and applied magnetic fields. Herein, such peculiarities observed in magnetotransport are studied both experimentally and theoretically and explained by the mutual influence of quantum interference, electron–electron interaction or multiband effects in the presence of spin–orbit coupling.

The transport properties of an interacting, spin-polarized heterostructure are theoretically studied. Resonances and bound states strongly affect the transmission, as is apparent from the spectral function. For specific model parameters, good spin filters are possible.

Recent advances in the field of transition metal oxide films and superlattices as thermoelectric materials are discussed combining advanced growth and characterization techniques with state-of-the-art first-principles simulations in the frameworks of density functional theory and Boltzmann transport theory. Focusing on delafossite films and perovskite and rocksalt superlattices, a selection of quantum-scale approaches to tune their thermoelectric performance is highlighted.

A growth technique for the chiral magnetic insulator Cu₂OSeO₃ hosting skyrmions is presented. In this technique, the solid transport agent SeCl₄ is used for simplifying the growth process. This method enables the growth of high-quality single crystals with relatively large sizes. Moreover, it allows growing enantiomeric pure single crystals.