We present a formalism based on nonequilibrium dynamical mean-field theory (DMFT) which allows to compute the time-resolved x-ray absorption spectrum (XAS) of photoexcited solids. By applying this formalism to the photodoped half-filled and quarter-filled two-orbital Hubbard models in the Mott insulating regime we clarify how the time-resolved XAS signal reflects the nonequilibrium population of different local states. Apart from the missing broadening associated with continuum excitations, the atomic XAS spectrum computed with the nonthermal state populations provides a good approximation to the full nonequilibrium DMFT result. This suggests a route to combine the accurate DMFT description of nonequilibrium states of solids with cluster calculations of the XAS signal.

The unidirectional spin Hall magnetoresistance (USMR) effect is useful to detect the direction of magnetization in a ferromagnetic metal/nonmagnetic metal bilayer, which is the typical geometry used in a spin-orbit torque switching device. In this Letter, we demonstrate the enhancement of the USMR effect by reducing the Curie temperature $T_{\mathrm{C}}$ of the ferromagnetic (FM) layer in a FM/platinum bilayer. The USMR ratio was maximized when the thickness of each FM layer was consistent with the spin diffusion length. We found that the maximum USMR ratio can be doubled by replacing ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ ($T_{\mathrm{C}}=854$ K) with ${\mathrm{Ni}}_{85}{\mathrm{Cu}}_{15}$ ($T_{\mathrm{C}}=527$ K) as the FM. This enhancement of the USMR effect is attributed to an increase of electron-magnon scattering in accordance with Bloch law. We also found that the use of ferromagnets with face-centered-cubic structures, such as Ni, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, and ${\mathrm{Ni}}_{85}{\mathrm{Cu}}_{15}$, increased the USMR effect relative to that of body-centered-cubic Fe.

We study the transition between the two-dimensional topological insulator (TI) featuring quantized edge conductance and the trivial Anderson insulator induced by strong disorder. We discover a distinct scaling behavior of the TI near the phase transition where the longitudinal conductance approaches the quantized value by a power law with system size, instead of an exponential law in a clean TI. This region is thus called the weak quantization topological insulator. By using the self-consistent Born approximation, we associate the emergence of the weak quantization with the imaginary part of the effective self-energy acquiring a finite value at strong disorder. We use our analytical theory, supported by direct numerical simulations, to study the effect of disorder range on the topological Anderson insulator. Interestingly, while this phase is quite generic for uncorrelated or short-range disorder, it is strongly suppressed by long-range disorder, perhaps explaining why it has not been seen in solid state systems.

Non-Hermitian systems have gained a lot of interest in recent years. However, notions of chaos and localization in such systems have not reached the same level of maturity as in the Hermitian systems. Here, we consider non-Hermitian interacting disordered Hamiltonians and attempt to analyze their chaotic behavior or lack of it through the lens of the recently introduced non-Hermitian analog of the spectral form factor and the complex spacing ratio. We consider three widely relevant non-Hermitian models which are unique in their ways and serve as excellent platforms for such investigations. Two of the models considered are short-ranged and have different symmetries. The third model is long-ranged whose Hermitian counterpart has itself become a subject of growing interest. All these models exhibit a deep connection with the non-Hermitian random matrix theory of corresponding symmetry classes at relatively weak disorder. At relatively strong disorder, the models show the absence of complex eigenvalue correlation, thereby, corresponding to Poisson statistics. Our thorough analysis is expected to play a crucial role in understanding disordered open quantum systems in general.

We consider a model of a light-matter system, in which a system of fermions (or bosons) is coupled to a photonic mode that drives phase transitions in the matter degrees of freedom. Starting from a simplified analytical model, we show that the entanglement between light and matter vanishes at small and large coupling strength and shows a peak in the proximity of the transition. We perform numerical simulations for a specific model (relevant to both solid state and cold atom platforms) and show that the entanglement displays critical behavior at the transition and features maximum susceptibility, as demonstrated by a maximal entanglement capacity. Remarkably, light-matter entanglement provides direct access to critical exponents, suggesting another approach to measure universal properties without direct matter probes.

We describe a density functional method which aims at computing the ground state electron density and the spectral function at the same time. One basic ingredient of our method is the construction of the spectral function from the first four spectral moment matrices. The second basic ingredient is the construction of the spectral moment matrices from density functionals. We call our method moment functional based spectral density functional theory (MFbSDFT), because it is based on density functionals for the spectral moments and because it allows us to compute the spectral function. If it is implemented in second variation our method consumes only a fraction more computer time than a standard DFT calculation with the PBE functional. We show that MFbSDFT captures correlation effects such as the valence-band satellite in Ni and the formation of lower and upper Hubbard bands in ${\mathrm{SrVO}}_3$. For the purpose of constructing the spectral function from the first four $N\times N$ spectral moment matrices we describe an efficient algorithm based on the diagonalization of one Hermitian $2N\times 2N$ matrix.

Motivated by recent experimental observations of correlated metallic phases and superconductivity in rhombohedral trilayer graphene (RTG), we perform an unbiased study of electronic ordering instabilities in hole-doped RTG. Specifically, we focus on electronic states energetically proximate to Van Hove singularities (VHSs), where a large density of states promotes different interaction-induced symmetry-breaking electronic orders. To resolve the Fermi surface near VHSs, we construct a fermionic hot-spot model and demonstrate that a perpendicular electric field can tune different nesting structures of the Fermi surface. Subsequently, we apply a renormalization group analysis to describe the low-energy phase diagrams of our model under both short-range repulsive interactions as well as realistic (long-range) Coulomb interactions. Our analysis shows instabilities towards either intervalley coherent metallic phases or superconducting phases. The dominant pairing channel depends crucially on the nature of Fermi surface nesting—repulsive Coulomb interaction favors spin-singlet $d$-wave pairing for relatively small displacement field and spin-singlet $i$-wave pairing for larger displacement field. We argue that the phase diagram of RTG can be well-understood by modeling the realistic Coulomb interaction as the sum of repulsive density-density interaction and ferromagnetic spin-triplet intervalley coherence (IVC) Hund's coupling, while phonon-mediated electronic interactions have a negligible effect on this system, in sharp contrast to twisted graphene multilayers.

The Bloch wave functions have been playing a crucial role in the diagnosis of topological phases in noninteracting systems. However, the Bloch waves are no longer applicable in the presence of finite Coulomb interaction and alternative approaches are needed to identify the topological indices. In this paper, we focus on three-dimensional higher-order topological insulators protected by $C_{4}T$ symmetry and show that the topological index can be computed through eigenstates of inverse Green's function at zero frequency. If there is an additional $S_4$ rotoinversion symmetry, the topological index $P_3$ can be determined by eigenvalues of $S_4$ at high-symmetry momenta, similar to the Fu-Kane parity criterion. We verify this method using many-body exact diagonalization in higher-order topological insulators with interaction. We also discuss the realization of this higher-order topological phase in tetragonal lattice structure with $C_{4}T$-preserving magnetic order. Finally, we discuss the boundary conditions necessary for the hinge states to emerge and show that these hinge states exist even when the boundary is smooth and without a sharp hinge.

The group-IV diamond-structure elements are known to host a variety of planar defects, including {001} planar defects in C and {001}, {111}, and {113} planar defects in Si and Ge. Among the {001} planar defects, the Humble defect, known for some time to occur in Ge, has recently also been observed in Si-Ge alloys, but the details of its electronic structure remain poorly understood. Here we perform first-principles density-functional calculations to study Humble defects in both Ge and ${\mathrm{Ge}}_{0.8}{\mathrm{Si}}_{0.2}$. We also measure the Si ${\mathrm{L}}_{2,3}$-edge electron energy-loss spectra both at the defect and in a bulk-like region far from the defect and compare with theoretical calculations on corresponding Si sites in our first-principles calculations. We find that inclusion of core-hole effects in the theory is essential for reproducing the observed ${\mathrm{L}}_{2,3}$ edge spectra, and that once they are included, the results provide a set of fingerprints for different types of local atomic bonding environments in ${\mathrm{Ge}}_{0.8}{\mathrm{Si}}_{0.2}$. Our first-principles calculations reveal that the Humble defects have a tendency to enlarge the electronic band gap, which may have potential uses in band engineering. The use of hybrid functionals for an improved description of the band gap in these systems is also discussed.

We have investigated magnetocapacitance (MC) performance in superlattices (SLs) of pure ferroelectric and ferromagnetic sublayers for multiferroic applications. ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}\mathrm{Nb}{\mathrm{O}}_3$ (KNN)/${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) SLs with LSMO sublayer thicknesses of 5, 7, 9, and 11 unit cell (u.c.) keeping KNN thickness fixed at 6 u.c. were deposited on (001)-oriented $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates alternately for 15 times using pulsed laser deposition (PLD) technique. The crystallinity and layer by layer growth of both constituents were confirmed from X-ray diffraction as well as transmission electron microscopy (TEM). SLs with LSMO layers of 5 and 7 u.c. experience high amount of strain while the in-plane lattice parameter almost matches with the substrate for 9 and 11 u.c. The spin-glass behavior in the SLs below 50 K and the maximum Curie temperature ($T_C$) of $\sim 206$ K in the thicker film imply the presence of defects as evident by TEM. The defects induced large magnetoresistance (MR) of $\sim -98\%$ was realized at 100 K in the SLs with 9 and 11 u.c. However, the insulating nature in the SLs with 7 and 9 u.c. and insulator to metal transition for 11 u.c. were observed in the transport study. The capacitance as well as magnetocapacitance showed high value at low frequency and reduced monotonically upon increase in the frequency which were well explained by the MR coupled Maxwell-Wagner relaxation model. Finally, the large MC of $\sim 766$% at standard frequency of 1 kHz with magnetic field of 9 T at 100 K in the SL with optimum LSMO sublayer thickness of 9 u.c. opens a path to design new artificial multiferroics with significant magnetoelectric coupling.

Spontaneous order of layer pseudospins in two-dimensional bilayers is common in quantum Hall systems, where it is responsible for hysteretic responses to gate fields in states with Ising order and giant drag voltages in states with XY (spontaneous interlayer phase coherence) order. In this article we predict that layer pseudospin order will also occur in double-moiré strongly correlated two-dimensional electron systems. We comment on similarities and differences in the competition between the two types of order in quantum Hall and double-moiré systems, and relate our findings to previous work on Falicov-Kimball models of electronic ferroelectrics.

Quantum states of a novel Bose-Einstein condensate, in which both fermion-pair and exciton condensations are simultaneously present, have recently been realized theoretically in a model Hamiltonian system. Here we identify quantum phase transitions in that model between fermion-pair and exciton condensations based on a geometric analysis of the convex set of ground-state two-particle reduced density matrices (2-RDMs). The 2-RDM set provides a finite representation of the infinite parameter space of Hamiltonians that readily reveals a fermion-pair condensate phase and two distinct exciton condensate phases, as well as the emergence of first- and second-order phase transitions as the particle number of the system is increased. The set, furthermore, shows that the fermion-exciton condensate (FEC) lies along the second-order phase transition between the exciton and fermion-pair condensate phases. The detailed information about the exciton and fermion-pair phases, the forces behind these phases, as well as their associated transitions provides additional insight into the formation of the FEC condensate, which we anticipate will prove useful in its experimental realization.

Spin-orbit coupling plays a pivotal role in condensed matter physics. For instance, spin-orbit interactions affect the magnetization and transport dynamics in solids, while spins and momenta are locked in topological matter. Alternatively, spin-orbit entanglement may play an important role in exotic phenomena, like quantum spin liquids in 4d and 5d systems. An interesting question is how electronic states mixed by spin orbit coupling interact with electromagnetic fields, which may hold potential to tune their properties and reveal interesting physics. Motivated by our recent discovery of large gyrotropic signals in some Jahn-Teller manganites, here we explore the interaction of light with spin-mixed $t_{2g}-e_g$ states in a $d^4$ transition metal. We show that spin-orbit mixing enables electronic transitions that are sensitive to circularly polarized light, giving rise to a gyrotropic response that increases with spin-orbit coupling. Interestingly, photoexcited transitions that involve spin reversal are behind such gyrotropic resonances. Additionally, we find that the interaction with the electromagnetic field depends strongly on the relative orientation of the propagation of light with respect to Jahn-Teller distortions and spin quantization. We suggest that such interactions offer the opportunity to use electromagnetic waves at optical wavelengths to entangle orbital and spin degrees of freedom. Our approach, which includes a group-theoretical treatment of spin-orbit coupling, has wide applicability and provides a versatile tool to explore the interaction of electromagnetic fields with electronic states in transition metals with arbitrary spin-orbit coupling strength and point-group symmetries.

In topological semimetals, nodes appear at symmetry points in the Brillouin zone as a result of band inversion, and yield quasirelativistic massless fermions at low energies. ${\mathrm{Cd}}_3{\mathrm{As}}_2$ is a three-dimensional topological semimetal that hosts two Dirac cones responsible for a variety of quantum phenomena. In this work, we demonstrate the strain tuning of the Dirac nodes of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ through a combination of magneto-optical infrared spectroscopy and high-resolution x-ray-diffraction studies performed on epitaxial films. In these thin films, we observe a giant enhancement of the node separation in momentum space by close to a factor of 4. A combination of experimental measurements and theoretical modeling allows us to relate the origin of this enhancement to a strengthening of the topological band inversion driven by lattice strain. Our results demonstrate how strain can be used as a knob to tune the topological properties of semimetals and to potentially enhance their performance and response for various applications.

We generalize our recent results for the hard-wall boundary and interface charges in one-dimensional single-channel continuum [S. Miles et al., Phys. Rev. B 104, 155409 (2021)] and multichannel tight-binding [N. Müller et al., Phys. Rev. B 104, 125447 (2021)] models to the realm of the multichannel continuum systems. Using the technique of boundary Green's functions, we give a rigorous proof that the change in boundary charge upon the shift of the system towards the boundary by the distance $x_{\phi }\in [0,L]$ (where $L$ is a potential periodicity) is given by a perfectly linear function of $x_{\phi }$ plus an integer-valued topological invariant $I$, the so-called boundary invariant. We provide two equivalent representations for $I\left(x_{\phi }\right)$: the winding-number representation and the bound-state representation. The winding-number representation allows one to write $I$ as a winding index of a particular functional of bulk Green's function. The corresponding integration contour is chosen in the complex frequency plane to encircle the occupied part of the spectrum residing on the real axis. In turn, in the bound-state representation, $I$ is expressed through the sum of the winding number of the boundary Green's function and the number of bound states supported by the cavity of size $x_{\phi }$ below the chemical potential. We observe that during a single cycle in the variation of $x_{\phi }$, the boundary invariant exhibits exactly $\nu$ downward jumps, each by a unit of electron charge, whenever $\nu$ energy bands are completely filled leading to the value $I\left(L\right)=-\nu$. Additionally, for translationally invariant models interrupted by a localized impurity we derive the winding-number expression for the excess charge accumulated on the said impurity. We observe that the charge accumulated on a single repulsive impurity is restricted to the values $-N_c,\cdots ,0$, where $N_c$ is the number of channels (spin or orbital components) in the system. For systems with weak potential amplitudes, we additionally develop Green's-function-based low-energy theory, allowing one to analytically access the physics of multichannel continuum systems in the low-energy approximation.

We report anisotropic resistivity measurements of the heavy-fermion superconductor ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ in magnetic fields up to 16 T and temperatures down to 0.35 K. The measured ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ resistivity shows a signature corresponding to the suggested quadrupole-density-wave order state at $T_0\sim 0.5$ K for both measured directions. For a magnetic field applied along the tetragonal $a$ axis, $T_0$ is enhanced with magnetic field reaching $\sim 1.75$ K at 16 T. Further, a magnetic field-induced transition occurs at ${\mu }_0{H}_m\sim 8.1$ T corresponding to a change to a new broken symmetry state. For a magnetic field applied along the $c$ axis, $T_0$ is suppressed below our base temperature $\sim 0.35$ K by ${\mu }_{0}H\sim 4.5$ T, a field close to the previously reported field-induced transition within the superconducting state suggested to be from an even-parity to an odd-parity state. Our results indicate that the multiple superconducting phases in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ are intimately tied to the suppression of the proposed quadrupole-density-wave phase at $T_0$.

The antiferroelectric ${\mathrm{NaNbO}}_3$ is a candidate material for application in high-energy density dielectric capacitors. Various doping strategies have been used for installing the desired narrow double P-E loop behavior in this lead-free material. However, to unravel the influence of doping on the double P-E loops, we first need to understand the influence of doping on the defect equilibrium. In this study, we therefore calculate the formation energies, charge transition levels, and doping behavior of single defects and defect complexes in orthorhombic ${\mathrm{NaNbO}}_3$ doped with Ca, Sn, and Sr by means of density functional theory (DFT). The results show that substitutions with dopants have formation energies as low as Na and O vacancies. Additionally, all defect complexes show negative binding energy, which means they are stable and very likely to form. Lastly, we show how defect quenching influences the Fermi level, which reveals how the synthesis conditions can tailor the final defect distribution.

We here report on the study of the crystallographic and magnetic properties of layered perovskites ${\mathrm{Ca}}_3{\mathrm{Mn}}_{2-x}{\mathrm{Ru}}_x{\mathrm{O}}_7$ ($x\le 0.9$). We observe a solid solution between Mn and Ru atoms in the whole series and all samples present the same orthorhombic structure independently of the Ru content. Different magnetic structures, depending on the Ru content in the sample, have been determined using neutron powder diffraction. For low Ru doping ($x\le 0.1$), there is a dominant $G$-type antiferromagnetic ordering in the perovskite bilayers but, differently from undoped ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$, the magnetic moments are located on the $ab$ plane. For higher Ru concentration ($x\ge 0.3$), the predominant $G$-type ordering is preserved along the $y$ axis while an $A$-type component is developed along the $x$ axis and its intensity increases as Ru content does. This component is characterized by a ferromagnetic ordering in the $a$ direction of one of the $\mathrm{Mn}\left(\mathrm{Ru}\right){\mathrm{O}}_6$ layers, coupled antiferromagnetically with the neighbor $\mathrm{Mn}\left(\mathrm{Ru}\right){\mathrm{O}}_6$ layer within the same bilayer. The study of the macroscopic magnetic properties shows that ferromagneticlike correlations are enhanced with increasing Ru content as deduced from the shift to higher temperature of the onset of the magnetic transition temperature. The magnetic transitions take place in two steps. At higher temperatures (140–200 K), short-range magnetic correlations are established. Tiny spontaneous magnetization is observed in the hysteresis loops with small coercive field. At $T_{\mathrm{N}}\approx 115–125\mathrm{K}$, long-range antiferromagnetic ordering is developed. The ferromagnetic component remains with a strong increase of coercivity. We discuss in the paper the possible origins of this ferromagnetic contribution.

We address the critical properties of the many-body localization (MBL) phase transition in one-dimensional systems subject to spatially correlated disorder. Rather than starting from a microscopic model, we analyze the transition within a strong-randomness renormalization group (RG) framework. We introduce disorder directly at the level of scaling variables appearing in the RG and consider a general family of spatial correlations, parameterized by how strong the fluctuations of the disordered couplings are when coarse-grained over a region of size $\ell$. For uncorrelated randomness, the characteristic scale for these fluctuations is $\sqrt{\ell }$; more generally they scale as ${\ell }^{\gamma }$. We discuss both positively correlated disorder ($1/2<\gamma <1$) and anticorrelated, or “hyperuniform,” disorder ($\gamma <1/2$). We argue that anticorrelations in the disorder are generally irrelevant at the MBL transition. Moreover, assuming the MBL transition is described by the recently developed renormalization-group scheme of Morningstar et al. [Phys. Rev. B 102, 125134 (2020)], we argue that even positively correlated disorder leaves the critical theory unchanged, although it modifies certain properties of the many-body localized phase.

A single ferromagnetic kagome layer is predicted to realize a Chern insulator with quantized Hall conductance, which upon stacking can become a Weyl semimetal with a large anomalous Hall effect (AHE) and magneto-optical activity. Indeed, in the kagome bilayer material ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$, a large AHE was detected. In order to directly probe the responsible band structure features, we measure the optical Hall conductivity spectra in addition to the diagonal optical conductivity over a broad frequency range. Since the former is the energy selective measure of the intrinsic contributions to the AHE, we identify their common origin with the help of momentum- and band-decomposed optical conductivity spectra obtained from first principles calculations. We find that low-energy transitions, tracing “helical volumes” in momentum space reminiscent of the formerly predicted helical nodal lines, substantially contribute to the AHE, which is further increased by contributions from multiple higher-energy interband transitions. Our study also reveals that in this kagome magnet, local Coulomb interactions lead to remarkable band reconstructions near the Fermi level.