We have investigated the local Fe electronic changes in the square planar Fe(II) compound ${\mathrm{SrFeO}}_2$ by x-ray emission (XES) and x-ray absorption spectroscopy (XAS) through the pressure-induced spin transition up to 60 GPa. The analysis of Fe $K\beta$ XES confirms the transition under pressure from a high spin to intermediate spin state at $P_c=43$ GPa while XAS at the Fe K edge reveals the spectral signatures of the lattice compression under pressure. Simulations of the XAS spectra point to significant changes in Fe-$p$ states across $P_c$ and highlight the active role of the Sr-$p$ states at the transition.

We study photocurrent of exciton polaritons in inversion broken systems. We use an effective Hamiltonian and Green's function approaches to deduce the formula for polariton photocurrent. The obtained formula shows the polariton photocurrent is proportional to the polariton density and shows opposite signs for the upper and lower branches. Nonvanishing polariton photocurrent requires time-reversal symmetry breaking in addition to inversion symmetry breaking. We show that exciton states in transition-metal dichalcogenides such as ${\mathrm{MoS}}_2$ support polariton photocurrent once we break time-reversal symmetry by applying magnetic fields or using photons with circular polarization. We also perform a simulation based on a rate equation and study a time profile of polariton photocurrent after pulse excitation, which indicates that the photocurrent provides a useful nonlinear probe to study nonequilibrium dynamics of exciton polaritons.

The structural and electronic properties of ${\mathrm{KTaO}}_3$/${\mathrm{KZnF}}_3$ and ${\mathrm{KTaO}}_3$/${\mathrm{KNiF}}_3$ oxyfluoride superlattices are studied from first-principles density functional theory calculations. We highlight, that beyond a critical layer thickness, these systems exhibit an insulator to metal transition that gives rise to the appearance of two-dimensional electron and hole gas, confined both, due the band alignment, within the oxide layer. The origin of the insulator to metal transition is related to the polar discontinuity at the interfaces. The behavior is discussed in terms of a simple electrostatic model and compared to that of the prototypical ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ oxide system. The magnetic properties ${\mathrm{KTaO}}_3$/${\mathrm{KNiF}}_3$ superlattices are further discussed, revealing a sizable Rashba-type spin splitting at these interfaces, much larger than in similar oxide/oxide systems.

We present time-dependent density functional molecular dynamics (TDDFT-MD) simulations with an adiabatic approximation to the exchange-correlation potential for hyperthermal protons (${\mathrm{H}}^{+}$) with initial kinetic energies in the range of 2–50 eV impinging on the fcc-hollow site and the on-top site of an Al(111) surface. The surface is modeled by a finite-size cluster and the results are generalized to ${\mathrm{H}}^{+}$-Al surface scattering. From the simulation, neutralization distances are determined and the time development of the kinetic energy and the electronic excitation energy are derived. The results can be rationalized on the basis of the ground-state potential energy surface and the ${\mathrm{H}}^{+}$-Al(111) interaction potential. Furthermore, the difference in initial kinetic energy between ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^0$ projectiles required to yield identical exit velocities is derived. Notably, this difference changes sign within the studied range of kinetic energies. This is traced back to the neutralization distances and the difference between both the ground-state potential energy surface and the ${\mathrm{H}}^{+}$-Al(111) interaction potential at those distances.

The scattering of electrons on impurities with internal degrees of freedom is bound to produce the signatures of the scatterer's own dynamics and results in nontrivial electronic transport properties. Previous studies of polaronic impurities in low-dimensional structures, like molecular junctions and one-dimensional nanowire models, have shown that perturbative treatments cannot account for a complex energy dependence of the scattering cross section in such systems. Here we derive the exact solution of polaronic impurities shaping the electronic transport in bulk (3D) systems. In the model with a short-ranged electron-phonon interaction, we solve for and sum over all elastic and inelastic partial cross sections, abundant in resonant features. The temperature dependence of the charge mobility shows the power-law dependence, $\mu \left(T\right)\propto T^{-\nu }$, with $\nu$ being highly sensitive to impurity parameters. The latter may explain nonuniversal power-law exponents observed experimentally, e.g., in high-quality organic molecular semiconductors.

The large antiferromagnetic exchange coupling in the parent high-$T_{\mathrm{c}}$ cuprate superconductors is believed to play a crucial role in pairing the superconducting carriers. The recent observation of superconductivity in hole-doped infinite-layer (IL-) ${\mathrm{NdNiO}}_2$ brings to the fore the relevance of magnetic coupling in high-$T_{\mathrm{c}}$ superconductors, particularly because no magnetic ordering is observed in the undoped IL-${\mathrm{NdNiO}}_2$, unlike in parent copper oxides. Here, we investigate the electronic structure and the nature of magnetic exchange in IL-${\mathrm{NdNiO}}_2$ using state-of-the-art many-body quantum chemistry methods. From a systematic comparison of the electronic and magnetic properties with isostructural cuprate IL-${\mathrm{CaCuO}}_2$, we find that the on-site dynamical correlations are significantly stronger in IL-${\mathrm{NdNiO}}_2$ compared to the cuprate analog. These dynamical correlations play a critical role in the magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest-neighbor isotropic $J$ of 77 meV between the ${\mathrm{Ni}}^{1+}$ ions within the $ab$ plane. While we find many similarities in the electronic structure between the nickelate and the cuprate, the role of electronic correlations is profoundly different in the two. We further discuss the implications of our findings in understanding the origin of superconductivity in nickelates.

One striking property of the Landau level spectrum of a Weyl semimetal (WSM) is the existence of a chiral Landau level, in which the electrons propagate unidirectionally along the magnetic field. This linearly dispersive level influences the optical properties of WSMs. For example, it was recently shown that a complete optical valley polarization is achievable in a time-reversal symmetric Weyl semimetal placed in a magnetic field [S. Bertrand et al., Phys. Rev. B 100, 075107 (2019)]. This effect originates from inter-Landau level transitions involving the chiral Landau level and requires a tilt of the Weyl cones. In this paper we show how the magneto-optical Kerr effect (MOKE) is modified in a WSM with tilted Weyl cones in comparison with its behavior in a normal metal and how a valley polarization can be detected using MOKE. We study both the Faraday (longitudinal) and Voigt (transverse) configurations for light incident on a semi-infinite WSM surface with no Fermi arcs. We use a minimal model of a WSM with four tilted Weyl nodes related by mirror and time-reversal symmetry. In the Voigt configuration, a large peak of the Kerr angle occurs at the plasmon frequency. We show that the blueshift in frequency of this peak with increasing magnetic field is a signature of the chiral anomaly in the MOKE.

We present the results of our investigations of physical properties for the novel ${\mathrm{Ce}}_{1-x}{\mathrm{Pr}}_x\mathrm{Co}{\mathrm{Ge}}_3$ system performed with a number of experimental methods: magnetic susceptibility, specific heat, electrical resistivity, magnetoresistance, and thermoelectric power. Moreover, the electronic structure was studied by means of photoelectron spectroscopy measurements and first-principles calculations. All investigated compositions of the ${\mathrm{Ce}}_{1-x}{\mathrm{Pr}}_x\mathrm{Co}{\mathrm{Ge}}_3$ series crystallize in the tetragonal $\mathrm{Ba}\mathrm{Ni}{\mathrm{Sn}}_3$-type structure. The lattice parameters and unit cell volumes decrease with increasing Pr concentration. On the basis of the measurements taken, a preliminary magnetic phase diagram was created. Continuous suppression of the long-range magnetic ordering was observed with an increase of Pr concentration. The critical Pr concentration for magnetic moment ordering was determined from linear extrapolation of the ordering temperature versus $x$ to the lowest temperatures ($T=0$ K) and is equal to about 0.66. Based on the first-principles calculations, we show how the substitution of Pr for Ce affects the electronic structure and magnetic properties of the considered alloys. Within a single model, we take into account the magnetic ordering, fully relativistic effects, and Hubbard $U$ repulsion on Ce and Pr. The impact of Hubbard $U$ on the results of calculations is also discussed. We present the valence band analysis, Mulliken electronic population analysis, and calculated electronic specific heat coefficients. For $\mathrm{Ce}\mathrm{Co}{\mathrm{Ge}}_3$, it is found that the $++--$ configuration of magnetic moments on Ce is slightly more stable than the $+-+-$ one, and also that the calculated value of total magnetic moment on Ce (including spin and orbital parts) is in good agreement with the measurements.

Recently, uncovering the sources of the thermal Hall effect in insulators has become an important issue. In the case of ferromagnetic insulators, it is well known that the Dzyaloshinskii-Moriya (DM) interaction can induce a magnon thermal Hall effect. Specifically, the DM vector parallel to the magnetization direction induces complex magnon hopping amplitudes, so that magnons act as if they feel Lorentz force. However, the DM vector which is orthogonal to the magnetization direction has hitherto been neglected as a possible source of magnon thermal Hall effect. This is because they play no role in the linear spin wave theory, an often invoked approximation when computing the magnon thermal Hall effect. Here, we challenge this expectation by presenting a self-consistent Schwinger boson mean-field study of two-dimensional magnets with ferromagnetic Heisenberg interaction and in-plane DM interaction. We find that the relevant Schwinger boson mean-field Hamiltonian takes the form of a two-dimensional electron gas with Rashba spin-orbit interaction, which is known to show an anomalous Hall effect, spin Hall effect, and Rashba-Edelstein effect, whose thermal counterparts also appear in our system. Importantly, the thermal Hall effect can be induced when out-of-plane magnetic field is applied and persists even when the magnetic field is large, so that the spins are significantly polarized, and the linear spin wave theory is expected to be a reasonable approximation. Since the linear spin wave theory predicts a vanishing thermal Hall effect, our result implies that a linear spin wave is not a sufficient approximation and that magnon-magnon interaction must be taken into account to predict the correct thermal Hall conductivity.

We have probed the superconducting proximity effect through long high-quality monocrystalline Ag nanowires, by realizing Josephson junctions of different lengths, with different superconducting materials. Thanks to the high number of junctions probed, both the contact resistance and electron diffusion constant could be determined, enabling a comparison of the measured switching current to theoretical prediction, over the entire regime from short to long diffusive junctions. Although the length dependence of the switching current is as expected, the amplitude of the $R_N{I}_c$ product is smaller than predicted by theory. We also address the magnetic field dependence of the switching current. The quasi-Gaussian decay of the switching current with field expected in a long narrow junction is observed for all superconducting contacts we used except for aluminum. We present the striking nonmonotonous effect of field on the switching current of junctions with aluminum contacts when probed in a four-wire configuration and analyze it in terms of improved quasiparticle thermalization by a magnetic field.

The Maldacena-Qi model describes two copies of the Sachdev-Ye-Kitaev model coupled with an additional coupling and is dual to the Jackiw-Teitelboim gravity, which exhibits an eternal traversable wormhole in the low-temperature limit. In this work, we study an experimental consequence of the existence of the traversable wormhole by considering the tunneling spectroscopy for the Maldacena-Qi model. Making comparisons to the high-temperature black-hole phase where the bulk geometry is disconnected, we find that both the tunneling probability and the differential conductance in the low-temperature wormhole phase show nontrivial oscillation, which directly provides an unambiguous signature of the underlying $SL\left(2\right)$ symmetry of the bulk geometry. We also perform bulk calculations in both high- and low-temperature phases, which match the results from the boundary quantum theory.

A domain-wall configuration and a current-injection scheme are proposed which ensure the satisfaction of the boundary condition of the dynamic Walker wall driven by the spin-orbit torques (SOTs). The analytic solutions of the wall are obtained in the whole region of current by the traveling-wave method. In the steady regime, the initial velocity mainly depends on the Slonczewski SOT. Meanwhile, the final steady velocity largely relies on the fieldlike one. The critical behavior of velocity is closely linked to the ratio of the fieldlike SOT parameter $\beta$ to the Gilbert damping constant $\alpha$. Reducing the difference between $\beta$ and $\alpha$, the Walker breakdown is postponed and the Walker velocity is enhanced. Additionally, by adjusting an applied magnetic field, $\beta$ can be measured by halting the wall. In the periodic regime, the wall propagates back and forth with an average velocity increasing almost linearly for high currents.

Nonequilibrium conditions offer novel routes to superconductivity that are not available at equilibrium. For example, by engineering nonequilibrium electronic populations, pairing may develop between electrons in different energy bands. A concrete proposal has been made to photo-induce superconductivity in a semiconductor, where pairing occurs between electrons in the conduction and valence bands, even for repulsive interactions. Here, we calculate the superfluid density for such a nonequilibrium paired state, and find it to be positive for repulsive interactions and interband pairing. The positivity of the superfluid density implies the stability of the photo-induced superconducting state as well as the existence of the Meissner effect.

The unusual features of topological semimetals arise from its nontrivial band structure. The impact of strong electron correlations on the topological states remains largely unexplored in real materials. Here, we report the magnetotransport properties of YbCdSn single crystals. We found two fundamental experimental evidences of electron correlations through magnetic susceptibility and specific heat. The electron correlations in this compound lead to an intermediate valence state and enhance the effective mass of the charge carriers. This correlated state exhibits large nonsaturating magnetoresistance, low carrier density, magnetic field-induced metal-semiconductor-like crossover and a plateau in resistivity at low temperatures. This compound also shows a cusp-like magnetoconductivity at low magnetic field which indicates the presence of weak antilocalization effect. Our band structure calculations of ${\mathrm{Yb}}^{2+}$ state predict YbCdSn to be a topological nodal-line semimetal.

Among the large class of $A$-site ordered perovskites of stoichiometry $A{C}_3{B}_4{\mathrm{O}}_{12}$, the rare-earth $\left(R\right)$ ruthenates $R{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ $(R=\mathrm{La}, \mathrm{Pr}, \mathrm{Nd})$ are interesting compounds due both to $\mathrm{Ru}\text{−}4d$-derived electronic correlations and to unconventional crystal-electric-field effects of the $R$ ions. Here we report on detailed investigations of these compounds utilizing x-ray diffraction, neutron scattering, magnetic susceptibility, and electrical resistivity measurements as well as heat capacity and nuclear resonance experiments. A broad range of external parameters is scanned and depending on the specific technique, temperatures range from 100 mK to 730 K in external magnetic fields up to 14 T. In this work ${\mathrm{LaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ serves as reference compound with a nonmagnetic $A$ site, characterized in detail recently [S. Riegg et al., Phys. Rev. B 93, 115149 (2016)]. All compounds investigated reveal heavy-fermion behavior with a $T^2$ dependence of the low-temperature electrical resistivity and significantly enhanced Sommerfeld coefficients. Toward low temperatures, the compounds with $R=\mathrm{Pr}$ and Nd are dominated by the magnetic moments of the $R$ ions, which occupy crystallographic positions with point-group symmetry $T_h$. The crystal-electric-field effects are clearly visible especially in heat capacity and inelastic neutron scattering data from which the crystal-electric-field parameters are derived. The ground state of the ${\mathrm{Pr}}^{3+}$ ion is identified as a triplet $\left({\mathrm{\Gamma }}_4^{\left(1\right)}\right)$, whereas for ${\mathrm{Nd}}^{3+}$ it is a quartet $\left({\mathrm{\Gamma }}_{67}\right)$. Evidence for lowering of the $T_h$ symmetry is observed at the Pr site at temperatures below 10 K, suggesting the formation of orbital order. Moreover, the spin-lattice relaxation derived from ${}^{63}\mathrm{Cu}$ nuclear quadrupole resonance indicates characteristic temperatures close to 7 K and 350 mK, probably related to orbital and magnetic order, respectively.

Studies connected with the investigations of “non-Fermi liquid” (NFL) systems continue to attract interest in the condensed-matter physics community. Understanding the anomalous physical properties exhibited by such systems and its related electronic structures is one of the central research topics in this area. In this context, Ce-based and Ce-site diluted (with nonmagnetic ions) compounds provide a fertile playground. Here, we present a detailed study of nonlinear DC susceptibility and combined density-functional theory plus dynamical mean-field theory (DFT+DMFT) on ${\mathrm{Ce}}_{0.24}{\mathrm{La}}_{0.76}\mathrm{Ge}$. Theoretical investigation of $4f$ partial density of states, local susceptibility, and self-energy demonstrates the presence of NFL behavior which is associated with fluctuating local moments. Nonlinear DC susceptibility studies on this compound reveal that the transition from NFL state to the new phase is due to development of the biquadratic exchange coupling and it obeys the nonlinear susceptibility scaling. Under the application of magnetic fields, local moments interact spatially through conduction electrons resulting in magnetic fluctuations. Our studies point to the fact that the origin of the observed biquadratic exchange coupling is due to the spatial magnetic fluctuations.

Recent nuclear magnetic resonance experiments measuring the Knight shift in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ have challenged the widely accepted picture of chiral pairing in this superconductor. Here we study the implications of helical pairing on the superconducting state while comparing our results with the available experimental data on the upper critical field and Knight shift. We solve the Bogoliubov–de Gennes equation employing a realistic three-dimensional tight-binding model that captures the experimental Fermi surface very well. In agreement with experiments we find a Pauli limiting to the upper critical field and, at low temperatures and high fields, a second superconducting transition. These transitions, which form a superconducting subphase in the H-T phase diagram are first-order in nature and merge into a single second-order transition at a bicritical point $(T^{*},H^{*})$, for which we find (0.8 K, 2.4 T) with experiment reporting $(0.8 \mathrm{K},\sim 1.2 \mathrm{T})$ [Phys. Rev. B 93, 184513 (2016)]. Furthermore, we find a substantial drop in the Knight shift in agreement with recent experiments.

The spintronic properties of curved nanostructures derived from two-dimensional topological insulators (2DTIs) are explored theoretically with density functional theory (DFT)-based calculations and tight-binding models. We show that curved geometries make it possible to manipulate electron spins in ways that are not available for planar 2DTI devices. We predict that, unlike planar 2DTI devices, curved 2DTI-related nanostructures can function as highly effective two-terminal spin filters even in the absence of magnetic fields. We construct a generalization to curved geometries of our previous tight-binding model of the wide band-gap planar 2DTI bismuthene on SiC. The resulting model, applied to an ideal dome geometry with a free edge, is shown to exhibit quantum spin Hall physics, including spin-polarized edge states. The model predicts nearly perfect spin filtering by the dome for a particular two-terminal geometry in the absence of magnetic fields. Our DFT calculations predict a ${\mathrm{Bi}}_{105}{\mathrm{Si}}_{105}{\mathrm{H}}_{15}$ dome of bismuthene with adsorbed silicon and hydrogen atoms to be stable. Our tight-binding model, adjusted to match density of states given by DFT calculations, predicts that the ${\mathrm{Bi}}_{105}{\mathrm{Si}}_{105}{\mathrm{H}}_{15}$ dome should exhibit quantum spin Hall physics and very effective spin filtering in a two-terminal arrangement.

We report a ground state with strongly coupled magnetic and charge density wave orders mediated via orbital ordering in the layered compound ${\mathrm{TbTe}}_3$. In addition to the commensurate antiferromagnetic (AFM) and charge density wave (CDW) orders, new magnetic peaks are observed whose propagation vector equals the sum of the AFM and CDW propagation vectors, revealing an intricate and highly entwined relationship. This is especially interesting given that the magnetic and charge orders lie in different layers of the crystal structure where the highly localized magnetic moments of the ${\mathrm{Tb}}^{3+}$ ions are netted in the Tb-Te stacks, while the charge order is formed by the conduction electrons of the adjacent Te-Te layers. Our results, based on neutron diffraction and resonant x-ray scattering, reveal that the charge and magnetic subsystems mutually influence each other via the orbital ordering of ${\mathrm{Tb}}^{3+}$ ions.