Magnetic properties of the antiferromagnet ${\mathrm{Zn}}_x{\mathrm{Cu}}_{2-x}{\mathrm{V}}_2{\mathrm{O}}_7$ (ZnCVO) with $x\approx 0.06$ have been thoroughly investigated on powder and single-crystal samples. The crystal structure determination using powder x-ray and neutron diffraction confirms that our ZnCVO samples are isostructural with $\beta -{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ ($\beta$-CVO) with a small deviation in the lattice parameters. Macroscopic magnetic property measurements also confirm the similarity between the two compounds. The ${\mathrm{Cu}}^{2+}$ spins were found to align along the crystallographic $c$ axis, antiparallel to their nearest neighbors connected by the leading exchange interaction $J_1$. Spin dynamics reveals a typical symmetric spin-wave dispersion with strong interactions in the $bc$ plane and weak interplane coupling. The exchange interaction analysis indicates that the spin network of ZnCVO is topologically consistent with the previous DFT prediction but the values of leading exchange interactions are contradictory. Furthermore, rather than the predicted 2D honeycomb structure, the spin network in ZnCVO could be better described by the anisotropic 2D spin network composed of $J_1, J_5$, and $J_6$ interactions, four bonds per one spin site, coupled by weak interplane interactions.

We show that the Bloch states in the conduction band of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ arise from the interplay between highly anisotropic hopping in subbands derived from the Ti $t_{2g}$ orbitals and spin-orbit coupling that mixes these orbitals. Because of the nearly flat-band characteristics for one of the principal axes, at sufficiently high doping these Bloch states become unstable with respect to electron interactions, resulting in Mott-like singlet correlations. These findings may be relevant to the anomalous electronic properties of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$, including its unusual superconductivity.

Thickness uniformity is regarded as an important parameter in designing thin film devices. However, some applications based on films with nonuniform thickness have recently emerged, such as gas sensors and optimized materials based on the gradual change of film composition. This work deals with superconducting Pb thin films with a thickness gradient prepared with the aid of a diffuse stencil mask. Atomic force microscopy and energy-dispersive x-ray spectroscopy show variations in the range 90 nm–154 nm. Quantitative magneto-optical images reveal interesting features during both the abrupt and the smooth penetration regimes of magnetic flux, as well as the thickness-dependent critical current density ($J_c$). In addition, we observe a gradual superconducting transition as the upper critical field is progressively reached for certain thicknesses. Furthermore, the hysteresis observed for triggering flux avalanches when increasing and decreasing magnetic fields is also accounted for by the $J_c$ profile evolution along the thickness gradient. Numerical simulations based on the thermomagnetic model are in fair agreement with the experimental data. These findings demonstrate that wedge-shaped films are a viable approach to investigate, in a continuous fashion, thickness-dependent properties of superconducting materials.

Second-order topological insulator can be engineered from two-dimensional materials with strong spin-orbit coupling and in-plane Zeeman field. In proximity to superconductor, topological superconducting phase could be induced in the two-dimensional materials, which host Majorana corner modes at the intersection between two zigzag edges. Two types of tight binding models in hexagonal lattice, which include $p_z$ or $p_{x,y}$ orbit(s) in each lattice site, are applied to engineer two-dimensional materials in topological superconducting phase. In both models, the condition that induces the second-order topological superconductor requires nonuniform value of either in-plane Zeeman fields or superconductor pairing parameters in two sublattices. The finite-size effect of the second model is smaller than that of the first model.

The theoretical study of many-body effects in the context of near-field radiative heat transfer (NFRHT) has already led to the prediction of a plethora of thermal radiation phenomena. Special attention has been paid to nonreciprocal systems in which the lack of the Lorentz reciprocity has been shown to give rise to unique physical effects. However, most of the theoretical work in this regard has been carried out with the help of approaches that consider either pointlike particles or highly symmetric bodies (such as spheres), which are not easy to realize and explore experimentally. In this work we develop a many-body approach based on the thermal discrete dipole approximation (TDDA) that is able to describe the NFRHT between nonreciprocal objects of arbitrary size and shape. We illustrate the potential and the relevance of this approach with the analysis of two related phenomena, namely the existence of persistent thermal currents and the photon thermal Hall effect, in a system with several magneto-optical bodies. Our many-body TDDA approach paves the way for closing the gap between experiment and theory that is hindering the progress of the topic of NFRHT in many-body systems.

We study the conductance afforded by a normal-metal probe which is directly contacting the helical edge modes of a quantum spin Hall insulator (QSHI). We show a $2e^2/h$ conductance peak at zero temperature in QSHI-based superconductor-ferromagnet hybrids due to the formation of a single Majorana bound state (MBS). In a corresponding Josephson junction hosting a pair of MBSs, a $4e^2/h$ conductance peak is found at zero temperature. The conductance quantization is robust to changes of the relevant system parameters and, remarkably, remains unaltered with increasing the distance between the probe and MBSs. In the low-temperature limit, the conductance peak is robust as the probe is placed within the localization length of MBSs. Our findings can therefore provide an effective way to detect the existence of MBSs in QSHI systems.

A group of recently discovered nonmagnetic metal kagome structures ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs) has aroused widespread interest both experimentally and theoretically due to their unusual charge density wave (CDW) and intertwined superconductivity. However, they all possess weak electron-phonon coupling (EPC) and low superconducting transition temperature $T_c$. Here, we perform high-throughput first-principles calculations on such kagome candidates with ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ prototype structure, and propose 24 dynamically stable kagome metals. The calculation based on Bardeen-Cooper-Schrieffer theory shows that most of these metals are superconductors with much stronger EPC than the reported ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ materials, and their $T_c$ are between 0.3 and 5.0 K. Moreover, several compounds, such as ${\mathrm{KZr}}_3{\mathrm{Pb}}_5$ with the highest $T_c$, are identified as ${\mathbb{Z}}_2$ topological metals with clear Dirac cone topological surface states near the Fermi level. In addition, we take ${\mathrm{NaZr}}_3{\mathrm{As}}_5$ as an example to demonstrate the possible CDW phases. Our results provide rich platforms for exploring various physics with the kagome structure, in which the coexistence of superconductivity and the nontrivial topological nature provides promising insights for the discovery of topological superconductors.

We study the time evolution processes of non-Hermitian systems under the open boundary condition and confirm that the unconventional reflection, dubbed the dynamical skin effect, exists in non-Hermitian systems analytically, and unveil the mechanism of its formation, which is caused by both the non-Hermitian skin effect and the Hermitian wave packet spreading. Furthermore, we find that in contrast to the uniform speed motion in Hermitian situations, the Gaussian wave packet can be accelerated and amplified during its time evolution in non-Hermitian systems. This additional motion is found to be responsible for the dynamic skin effect.

In this paper, we explore quantum criticality in the disordered Aubry-André (AA) model. For the pure AA model, it is well known that it hosts a critical point separating an extended phase and a localized insulator phase by tuning the strength of the quasiperiodic potential. Here we unearth that the disorder strength $\mathrm{\Delta }$ contributes an independent relevant direction near the critical point of the AA model. Our scaling analyses show that the localization length $\xi$ scales with $\mathrm{\Delta }$ as $\xi \propto {\mathrm{\Delta }}^{-{\nu }_{\mathrm{\Delta }}}$ with ${\nu }_{\mathrm{\Delta }}$ a new critical exponent, which is estimated to be ${\nu }_{\mathrm{\Delta }}\approx 0.46$. This value is remarkably different from the counterparts for both the pure AA model and the Anderson model. Moreover, rich critical phenomena are discovered in the critical region spanned by the quasiperiodic and the disordered potentials. In particular, in the extended phase side, we show that the scaling theory satisfies a hybrid scaling form as a result of the overlap between the critical regions of the AA model and the Anderson localization.

High-order harmonic generation (HHG), which has been widely studied in atomic gas, has recently been expanded to solids to study the highly nonlinear electronic response in condensed matter and produce coherent high-frequency radiation. Recently, attention has turned to topological materials and the use of HHG to characterize topological bands and invariants. However, the theoretical interpretation of the nonlinear electronic response in topological materials presents many challenges. In particular, the Bloch wavefunction phase of topological materials has undefined points in the Brillouin zone. This leads to singularities in the calculation of the interband and intraband transition dipole matrix elements of the semiconductor Bloch equations (SBEs). Here, we use the laser-electromagnetic velocity gauge $\mathit{p}·\mathbf{A}\left(t\right)$ to numerically integrate the SBEs and treat the singularity in the production of the electrical currents and HHG spectra with better numerical efficiency and more straightforward implementation. We used a prototype of Chern insulators (CIs), the Haldane model, to demonstrate our approach. The validity of the velocity gauge approach is demonstrated in the following way: for topologically trivial materials such as ${\mathrm{MoS}}_2$, qualitative agreement is achieved with the results of the length gauge approach and the time-dependent density functional theory. For the application of the velocity gauge approach to topological materials, Chern insulator is taken, using the two-band Haldane model. We found a good qualitative agreement between the velocity gauge and the length gauge approach in view of (i) the selection rules, (ii) the linear cutoff law scaling, and (iii) anomalous circular dichroism. We conclude that the velocity-gauge approach for HHG provides a theoretical tool to investigate topological materials.

In a polar solid, electrons or other charge carriers can interact with the phonons of the ionic lattice, leading to the formation of polaron quasiparticles. The optical conductivity and optical absorption spectrum of a material are affected by this electron-phonon coupling, most notably leading to an absorption peak in the midinfrared region. Recently, a model Hamiltonian for anharmonic electron-phonon coupling was derived [M. Houtput and J. Tempere, Phys. Rev. B 103, 184306 (2021)] that includes both the conventional Fröhlich interaction as well as an interaction in which an electron interacts with two phonons simultaneously. In this article, we calculate and investigate the optical conductivity of the anharmonic large polaron gas, and we show that an additional characteristic absorption peak appears due to this one-electron–two-phonon interaction. We calculate a semianalytical expression for the optical conductivity $\sigma \left(\omega \right)$ at finite temperatures and weak coupling using the Kubo formula. The electronic and phononic contributions can be split and treated separately, such that the many-body effects of the electron gas may be taken into account through the well-known dynamical structure factor $S(\mathbf{k},\omega )$. From the resulting optical conductivity, we calculate the polaron effective mass, an estimate for the electron-phonon scattering times, and the optical absorption spectrum of the anharmonic polaron gas. It is shown that the effects are negligible for four common III-V semiconductors (BN, AlN, BP, AlP) in the zinc-blende structure, which justifies the commonly used harmonic approximation in these materials. We show that alongside the well-known polaron absorption peak at the phonon energy $\hslash {\omega }_{\text{LO}}$, the one-electron–two-phonon interaction leads to an additional absorption peak at $2\hslash {\omega }_{\text{LO}}$. We propose this absorption peak as an experimentally measurable indicator for non-negligible one-electron–two-phonon interaction in a material, since the height of this peak is proportional to the strength of this anharmonic interaction.

The interplay between unitary dynamics and local quantum measurements results in unconventional nonunitary dynamical phases and transitions. In this paper we investigate the dynamics of $(d+1)$-dimensional hybrid stabilizer circuits, for $d=1,2,3$. We characterize the measurement-induced phases and their transitions using large-scale numerical simulations focusing on entanglement measures, purification dynamics, and wave-function structure. Our findings demonstrate the measurement-induced transition in $(d+1)$ spatiotemporal dimensions is conformal and has critical properties close to the percolation transition in $(d+1)$ spatial dimensions.

Several works have recently addressed the emergence of exceptional points (EPs), i.e., degeneracies of non-Hermitian Hamiltonians, in the long-wavelength dynamics of coupled magnetic systems. Here, by focusing on the driven magnetization dynamics of a van der Waals ferromagnetic bilayer, we show that exceptional points can appear over extended portions of the first Brillouin zone as well. Furthermore, we demonstrate that the effective non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in complex conjugate pairs, respects an unusual wavevector-dependent pseudo-Hermiticity. Finally, for both armchair and zigzag nanoribbon geometries, we discuss both the complex and purely real spectra of the topological edge states and their experimental implications.

We investigate the influence of surface states on the nonlinear Hall response driven by the Berry curvature dipole in non-centrosymmetric time-reversal invariant Weyl semimetals. To do so, we perform a tomography of the Berry curvature dipole in a slab system using a minimal two-band model. We find that in the type-I phase, the nonlinear Hall response is not particularly sensitive to the presence of Fermi arcs or other trivial surface states. However, in the type-II phase, we find that these surface states, be they topologically trivial or not, contribute substantially to the Berry curvature dipole, leading to a strong thickness dependence of the nonlinear Hall response. This feature depends on the nature of the surface states and, henceforth, on the slab geometry adopted. In order to assess the validity of this scenario for realistic systems, we performed Berry curvature dipole calculations by first principles on the ${\mathrm{WTe}}_2$, confirming the dramatic impact of surface states for selected slab geometries. Our results suggest that surface states, being topological or not, can contribute much more efficiently to the nonlinear Hall response than bulk states. This prediction is not limited to topological semimetals and should apply to topologically trivial noncentrosymmetric materials and heterostructures, paving the way to interfacial engineering of the nonlinear Hall effect.

Kagome-lattice materials have attracted tremendous interest due to the broad prospect for seeking superconductivity, quantum spin liquid states, and topological electronic structures. Among them, the transition-metal kagome lattices are high-profile objects for the combination of topological properties, rich magnetism, and multiple-orbital physics. Here we report an inelastic neutron scattering study on the spin dynamics of a kagome-lattice antiferromagnetic metal ${\mathrm{Fe}}_{0.89}{\mathrm{Co}}_{0.11}\mathrm{Sn}$. Although the magnetic excitations can be observed up to $\sim 250$ meV, well-defined spin waves are only identified below $\sim 90$ meV and can be modeled using Heisenberg exchange with ferromagnetic in-plane nearest-neighbor coupling $J_1$, in-plane next-nearest-neighbor coupling $J_2$, and antiferromagnetic (AFM) interlayer coupling $J_c$ under linear spin-wave theory. Above $\sim 90$ meV, the spin waves enter the itinerant Stoner continuum and become highly damped particle-hole excitations. At the $K$ point of the Brillouin zone, we reveal a possible band crossing of the spin wave, which indicates a potential Dirac magnon. Our results uncover the evolution of the spin excitations from the planar AFM state to the axial AFM state in ${\mathrm{Fe}}_{0.89}{\mathrm{Co}}_{0.11}\mathrm{Sn}$, solve the magnetic Hamiltonian for both states, and confirm the significant influence of the itinerant magnetism on the spin excitations.

The optical conductivity encodes the current response to a time dependent electric field. We develop a theory of the optical conductivity $\sigma \left(\omega \right)$ in the presence of a dc supercurrent. Current induced optical response is prohibited when current is conserved, an illustrative example of which is Galilean invariant systems. However, we show that lattice effects give rise to a pronounced current dependent peak in $\sigma \left(\omega \right)$ at the gap edge $\omega =2\mathrm{\Delta }$, which diverges in the clean limit. We demonstrate this in a model of a multiband superconductor. Our theory predicts the current induced peak in $\sigma \left(\omega \right)$ to scale quadratically with the super-current density, as was recently reported in experiments on NbN by Nakamura et al. [Phys. Rev. Lett. 122, 257001 (2019)]. This provides the potential for a new mechanism for direct activation of the Higgs mode with light.

Light-induced ferroelectric domain wall motion turns out to be a promising phenomenon to develop new photocontrolled devices. However, the physical origin of this light-matter coupling when material is irradiated with visible light remains unclear. Here, a phenomenological model predicting the motion of charged domain walls (CDWs) is developed. The photoinduced electronic reconstruction mechanism is proposed as the primary absorption mechanism, leading to a linear dependence for the polarization perturbation with the light intensity. Domain wall motion is then driven by the energetic difference between domains in a CDW array, such that the macroscopic polarization can be easily tuned.