The ZrSiS-type materials have attracted intensive attention due to the existence of various topological fermions. The magnetic version of the ZrSiS-type materials, $\mathit{Ln}\mathrm{SbTe} \left(Ln=\mathrm{lanthanides}\right)$, is an ideal candidate to explore novel exotic states due to the interaction between magnetism and topology. In this work, we report the experimental study on structural, magnetic, thermodynamic, and electronic properties for $\mathrm{Sm}{\mathrm{Sb}}_x{\mathrm{Te}}_{2-x}$ with various Sb content. The revealed evolutions of these properties with tuning the compositions would provide useful insights for the fundamental topological physics and the future applications.

Weyl semimetals are promising quantum materials that offer unique topological properties. Lately, it has been shown that laser-driven electron dynamics have characteristic signatures in two-dimensional and three-dimensional Dirac semimetals. The transition from Dirac to Weyl semimetal requires the breaking of either inversion or time-reversal symmetry. The present paper shows that the laser-driven electron dynamics in a Weyl semimetal with broken time-reversal symmetry has intriguing features in its high-harmonic spectrum. It is found that the parity and magnitude of the nonzero Berry curvature's components control the direction and strength of the anomalous current, which leads to the generation of the anomalous odd harmonics. We demonstrate that the nontrivial topology of the Berry curvature in time-reversal symmetry-broken quantum materials can be probed by measuring the polarization of the emitted anomalous odd harmonics. Our findings unequivocally illustrate that laser-driven electron dynamics leads to the generation of nonlinear anisotropic anomalous Hall effect in time-reversal symmetry-broken quantum materials on an ultrafast timescale.

Spintronic terahertz (THz) emission has been investigated in ferromagnetic (FM) metal/nonmagnetic (NM) metal, FM/semiconductor, and FM/topological insulator systems. However, THz emission in the heterojunction of FM and $3d$ transition light metal is rarely reported. Large spin Hall angles have been observed in a light metal alloy of the $3d$ transition metal NiCu, allowing for the emission of high-performance THz waves without the use of heavy metal. Here, we present a comprehensive study of THz emission from a CoFeB/NiCu heterostructure, with peak intensity of emitted THz pulses up to half that of CoFeB/Pt. The THz radiation from CoFeB/NiCu is shown to be generated by the spin-to-charge conversion in NiCu via the inverse spin Hall effect. The evolution of the THz signal of $\mathrm{CoFeB}/{\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ across ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ Curie temperature is also explored, and it is discovered that the THz emission signal in this sample is temperature independent and unaffected by the magnetic state of ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$. Our findings highlight the great potential of light metal alloys containing $3d$ transition elements as spintronic THz emitters. Moreover, it has been demonstrated that THz emission spectroscopy is a helpful tool for investigating inverse spin Hall effect or anomalous Hall effect in ferromagnets.

The modern theory of polarization establishes the bulk-boundary correspondence for the bulk polarization. In this paper, we attempt to extend it to a sum rule of the bulk quadrupole moment by employing a many-body operator introduced in Kang et al. [B. Kang, K. Shiozaki, and G. Y. Cho, Phys. Rev. B 100, 245134 (2019)] and Wheeler et al. [W. A. Wheeler, L. K. Wagner, and T. L. Hughes, Phys. Rev. B 100, 245135 (2019)]. The sum rule that we propose consists of the alternating sum of four observables, which are the phase factors of the many-body operator in different boundary conditions. We demonstrate its validity through extensive numerical computations for various noninteracting tight-binding models. We also observe that individual terms in the sum rule correspond to the bulk quadrupole moment, the edge-localized polarizations, and the corner charge in the thermodynamic limit on some models.

We have investigated the electronic and magnetic properties of the perovskite ${\mathrm{NaOsO}}_3$ within density functional theory using the generalized gradient approximation while taking into account strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. ${\mathrm{NaOsO}}_3$ is a G-type AFM Mott insulator which is characterized by AFM spins canted away from the $c$ axis. This magnetic configuration explains the weak ferromagnetism in ${\mathrm{NaOsO}}_3$. Despite the large strength of spin-orbit coupling, it has only a small effect on the electronic and magnetic properties of ${\mathrm{NaOsO}}_3$. We have also investigated theoretically the resonant inelastic x-ray scattering (RIXS) spectrum at the Os $L_3$ edge. The experimentally measured RIXS spectrum of ${\mathrm{NaOsO}}_3$ in addition to the elastic scattering peak at 0 eV possesses a sharp feature below 2 eV corresponding to transitions within the Os $t_{2g}$ levels, a strong intense peak at around 2.8 eV which is from $t_{2g}\to e_g$ transitions, and a wide structure stretching from 6 eV to 12 eV that corresponds to ligand-to-metal charge transfer excitations, d-d transitions to Os $t_{2g}$ and $e_g$ manifolds from the Os 5$d_{\mathrm{O}}$ states derived from the “tails” of oxygen 2$p$ states.

We have investigated the electronic and magnetic properties of the pyrochlore osmate ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ within density functional theory using the generalized gradient approximation with taking into account strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ is a Mott insulator with all-in-all-out magnetic order. The optical spectra, x-ray absorption spectra, and x-ray magnetic circular dichroism at the Os $L_{2,3}$ edges in the pyrochlore osmate ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ have been investigated theoretically from first principles. The calculated results are in good agreement with experimental data. We have also investigated theoretically the resonant inelastic x-ray scattering (RIXS) spectrum at the Os $L_3$ edge. The experimentally measured RIXS spectrum of ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ in addition to the elastic scattering peak at 0 eV possesses a sharp feature at 0.9 eV corresponding to transitions within the Os $t_{2g}$ levels, a strong intense peak at around 4.5 eV which is from $t_{2g}\to e_g$ transitions, and a wide structure stretching from 5.5 eV to 15 eV that corresponds to ligand-to-metal charge transfer excitations.

We elaborate that single-layer graphene with periodic vacancies can have a band structure containing nodal lines or nodal loops, opening the possibility of graphene-based electronic or spintronic devices with novel functionalities. The principle is that by removing carbon atoms such that the lattice becomes nonsymmorphic, every two sublattices in the unit cell will map to each other under glide plane operation. This mapping yields degenerate eigenvalues for the glide plane operation, which guarantees that the energy bands must stick together pairwise at a boundary of the Brillouin zone. Moving away from the Brillouin zone boundary causes the symmetry-enforced nodal lines to split, resulting in accidental nodal lines caused by the crossings of the split bands. Moreover, the density of states at the Fermi level may be dramatically enhanced if the nodal lines cross the Fermi level. The nodal lines occur in a variety of vacancy configurations even in the presence of Rashba spin-orbit coupling. Finally, our theory also explains the nodal loops surrounding the entire Brillouin zone of chevron-type nanoporous graphene fabricated in a recent experiment.

Rare-earth hexaborides offer a rich tapestry for exploring the interplay of topological orders, magnetism, and electron-correlation effects associated with protected quantum phenomena. Here, using first-principles modeling, we identify in cerium hexaboride ($\mathrm{Ce}{\mathrm{B}}_6$) a topological state that harbors a pseudonodal surface (PNS) along with double-Weyl fermions. An analysis of the electronic states in the vicinity of the Fermi level reveals the presence of $M_z$-mirror-symmetry-protected band crossings that involve itinerant ($d$) and localized ($f$) states of Ce. These band crossings are found to remain nearly gapless as one goes away from the mirror plane, driving the formation of PNS states. We also demonstrate the presence of coexisting double-Weyl fermions with chiral charges of $\pm 2$ on the $C_{4z}$ rotational axis. By analyzing the Berry curvature field associated with the topological states, we predict that $\mathrm{Ce}{\mathrm{B}}_6$ supports a giant anomalous Hall conductivity of $-1840 {\mathrm{\Omega }}^{-1}{\text{cm}}^{-1}$ in the slightly electron-doped regime, which would be larger than the values reported to date in magnetic Weyl semimetals. Our study thus indicates that $\mathrm{Ce}{\mathrm{B}}_6$, which combines ferromagnetic order with topological physics in a heavy-fermion matrix, would provide a promising playground for investigating topological magnetic states with giant anomalous Hall responses.

We show that, contrary to popular assumptions, predictions from machine learning potentials built upon high-dimensional atom-density representations almost exclusively occur in regions of the representation space which lie outside the convex hull defined by the training set points. We then propose a perspective to rationalize the domain of robust extrapolation and accurate prediction of atomistic machine learning potentials in terms of the probability density induced by training points in the representation space.

Since the seminal work of Anderson, localization has been recognized as a standard mechanism allowing quantum many-body systems to escape ergodicity. This idea has acquired even more prominence in the last decade as it has been argued that localization—dubbed many-body localization (MBL) in this context—can sometimes survive local interactions in the presence of sufficiently strong disorder. A conventional signature of localization is in the statistical properties of the spectrum—spectral statistics—which differ qualitatively from those in the ergodic phase. Although features of the spectral statistics are routinely used as numerical diagnostics for localization, their derivation from first principles in the presence of nontrivial interactions has been lacking. Here we provide the example of a simple class of quantum many-body systems—which we dub strongly localized quantum circuits—that are interacting and localized and where the spectral statistics can be characterized exactly. Furthermore, we show that these systems exhibit a cascade of three different regimes for spectral correlations depending on the energy scale: at small, intermediate, and large scales they behave as disconnected patches of three decreasing sizes. We argue that these features appear in generic MBL systems, with the difference that only at the smallest scale do they become Poissonian.

By incorporating the orbital magnetic moment, we investigate the magneto-optical transport of Weyl semimetals within a semiclassical approximation. In the linear or nonlinear response regime, an analytic expression for the magnetoconductivity is obtained, where a new term due to the orbital magnetic moment is added, and leads to a partial cancellation for the total magnetoconductivities. This suppressed feature is further manifested by analyzing the linear and quadratic contribution in the magnetic field to magnetoconductivities. This result may be used to explain the deviation between the recent theory and experiment.

Enhancing valley splitting in SiGe heterostructures is crucial for developing silicon spin qubits. Complex SiGe heterostructures, sharing a common feature of four-monolayer (4 ML) Ge layer next immediately to the silicon quantum well (QW), have been computationally designed in 2013 to have giant valley splitting approaching 9 meV and hence could be used to overcome the challenge of valley splitting towards the experimental realization of Si quantum computing. However, none of them has been successfully fabricated, perhaps due to their complexity. Here, we remarkably simplify the originally designed complex SiGe heterostructures by laying out the Si QW directly on the SiGe substrate followed by capping a ${\left({\mathrm{Ge}}_4{\mathrm{Si}}_4\right)}_n$ superlattice (SL) barrier with a sacrifice in valley splitting (VS), which is reduced from a maximum value of 8.7 to 5.2 meV. Even the smallest number of SL periods ($n=1$) will also give a sizable VS of 1.6 meV, which is large enough for developing stable spin qubits. We further develop an effective Hamiltonian model to reveal the physical mechanism underlying the enhanced valley splitting by the ${\left({\mathrm{Ge}}_4{\mathrm{Si}}_4\right)}_n$ SL barriers. We surprisingly find that the presence of the SL barrier will reduce rather than enhance the VS in most cases. The only exception is the ${\left({\mathrm{Ge}}_4{\mathrm{Si}}_4\right)}_n$ SL barriers, where their miniband states have such a strong coupling with Si QW valley states that they provide an even larger VS. These findings lay a solid theoretical foundation for overcoming the valley splitting issue of SiGe heterostructures in the experiment that is heading toward Si quantum computing.

We develop a unified theory for second sound in two-dimensional materials. Previously studied drifting and driftless second sound are two limiting cases of the theory, corresponding to the drift and diffusive part of the energy flux, respectively. We find that due to the presence of quadratic flexural phonons, the drifting second sound does not exist in the thermodynamic limit, while the driftless mode is less affected. This is understood as a result of infinite effective inertia of flexural phonons, due to their constant density states and divergent Bose-Einstein distribution in the long wavelength limit. Consequently, the group velocity of the drifting mode is smaller than that of the driftless mode. However, upon tensile strain, the velocity of the drifting mode becomes larger. Both of them increase with tensile strain due to the linearization of the flexural phonon dispersion. Our results clarify several puzzles encountered previously and pave the way for exploring wavelike heat transport beyond the hydrodynamic regime.

We theoretically propose and numerically demonstrate that perfect feeding of a polaritonic system with full electromagnetic energy under one-port beam incidence, referred to as polaritonic critical coupling, can be achieved in a hybrid dielectric metasurface–${\mathrm{WS}}_2$ monolayer structure. Polaritonic critical coupling, where critical coupling and strong coupling are simultaneously attained, is determined by the relative damping rates of the cavity resonance, ${\gamma }_Q$, provided by a symmetry-protected quasibound states in the continuum cavity, and excitonic resonance of the ${\mathrm{WS}}_2$ monolayer, ${\gamma }_X$. We reveal that the population of the polariton states can be tuned by the asymmetry parameter of the quasibound states in the continuum. Furthermore, polaritonic critical coupling is achieved in the designed system while ${\gamma }_Q={\gamma }_X$, and only strong coupling is achieved while ${\gamma }_Q\ne {\gamma }_X$. This work enriches the study of polaritonic physics with controlled absorbance and may guide the design and application of efficient polariton-based light-emitting or lasing devices.

Photonic bound states in the continuum (BIСs) were investigated depending on material and radiation losses on the example of the phase-change chalcogenide ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ and two types of resonators in the form of a cylinder and a ring which support quasi-BIC according to the Friedrich-Wintgen mechanism. The complex refractive index for the amorphous and crystalline phases of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ was measured experimentally. It was found that as the material losses increase, the quasi-BIC becomes practically a dark mode, while the usual resonator eigenmodes continue to remain intense. At the same time the strength of the mode coupling has been not affected, that is, the Rabi splitting remains unchanged in the studied range of losses. Photonic mode switching during the amorphous-crystalline phase transition was also investigated and two mechanisms were identified, including an efficient Fano resonance mechanism that can be used in real devices.

In this paper we study genuine multipartite correlations (GMC's) in a boundary time crystal (BTC). Boundary time crystals are nonequilibrium quantum phases of matter in contact to an environment, for which a macroscopic fraction of the many-body system breaks the time translation symmetry. We analyze both (i) the structure (orders) of GMC's among the subsystems, as well as (ii) their build-up dynamics for an initially uncorrelated state. We find that, in the thermodynamic limit (and only in such a limit), multipartite correlations of all orders grow indefinitely in time in the BTC phase, further displaying a persistent oscillatory behavior around their mean growth. The orders of the correlations show a power-law decaying hierarchy among its $k$ partitions. Moreover, in the long-time limit the GMC's are shown extensive with the system size, contrasting to the subextensive scaling in the non time crystals (ferromagnetic) phase of the model. We also discuss the classical and quantum nature of these correlations with basis on multipartite entanglement witnesses, specifically, the analysis of the quantum Fisher information (QFI). Both GMC and QFI are able to capture and distinguish the different phases of the model. Our paper highlights the genuine many-body properties of these peculiar nonequilibrium phases of matter.

The density matrix renormalization group and quantum Monte Carlo methods are used to describe coupled trimer chains in a magnetic field $h$. The Hamiltonian contains exchange terms involving the intratrimer coupling $J_1$ (taken as the unit of energy) and the intertrimer coupling $J_2$, plus the Zeeman interaction for a magnetic field $h$ along the $z$ direction. Results for the magnetization per trimer $m$ are calculated in regimes of positive and negative values of the ratio $J=J_2/J_1$, from which the rich field-induced ground state phase diagram $h$ versus $J$ is derived, with the presence of a Luttinger liquid, the 1/3 plateau ($m=1/2$), and the one of fully polarized magnetization ($m=3/2$). Also, zero-field Lanczos calculation of the spin-wave dispersion from the 1/3 plateau for $S^z=1$ is shown at the previous regimes of $J$ values. In addition, we also report on the decay of correlation functions of trimers along open chains, as well as the average two-magnon distribution. The ground state is ferrimagnetic for $0<J\le 1$, and is a singlet for $-1\le J<0$. In the singlet phase, the spin correlation functions along the legs present an antiferromagnetic power-law decay, similar to the spin-1/2 linear chain, thus suggesting that the ground state is made of three coupled antiferromagnetically oriented chains. In the singlet phase, the dimensionless thermal magnetic susceptibility per site normalized by $1/\left|J\right|$ gets closer to $1/{\pi }^2$ as the temperature $T\to 0$. For the ferrimagnetic phase, we fit the susceptibility to the experimental data for the compound ${\mathrm{Pb}}_3{\mathrm{Cu}}_3{\left({\mathrm{PO}}_4\right)}_4$ and estimate the model exchange couplings: $J_1=74.8\text{K}$ and $J=0.4$. These values imply a range of energies for the magnon excitations that are in accord with the data from neutron scattering experiments on ${\mathrm{Pb}}_3{\mathrm{Cu}}_3{\left({\mathrm{PO}}_4\right)}_4$ for two excitation modes. The 1/3 plateau closes only at $1/\left|J\right|=0$ with $J<0$.

We present an ab initio time-dependent density functional theory study of the ultrafast demagnetization of elemental ferromagnets subject to terahertz (THz) laser pumping compared to optical laser pumping. We find that THz radiation can cause as much demagnetization as optical radiation for significantly lower-energy transfer to the sample. We show that the demagnetization efficiency of the THz pulse is related to a coherent coupling of the radiation to the electron charge dynamics. In fact, our simulations reveal that the low-frequency THz electric field enforces in-phase electron charge oscillations around the atomic centers, leading to a highly efficient spin-orbit torque switching of the magnetization. Unlike for optical pumping, at the end of the pulse a major fraction of the excited electrons is driven back to equilibrium acquiring a thermalized Fermi-Dirac distribution.