${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ is a potential Kitaev magnet with a monoclinic layered crystal structure. Recent investigations of the $C_3$-symmetric sister compound ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ have uncovered a unique triple-$\mathbf{q}$ magnetic ground state, as opposed to a single-$\mathbf{q}$ (zigzag) one, prompting us to examine the influence of the reduced structural symmetry of ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ on its ground state. Neutron diffraction data obtained on a twin-free crystal reveal that the ground state remains a multi-$\mathbf{q}$ state, despite the system's strong non-$C_3$-symmetric anisotropy. This robustness of multi-$\mathbf{q}$ orders suggests that they are driven by a common mechanism in the honeycomb cobaltates, such as interactions beyond the bilinear order. Spin-polarized neutron diffraction results show that the ordered moments are entirely in plane, with each staggered component orthogonal to the propagating wave vector. The inferred ground state appears more compatible with the so-called XXZ easy-plane anisotropy than the Kitaev anisotropy, and features unequal ordered moments reduced by strong quantum fluctuations.

Inelastic scattering processes typically introduce friction among carriers and reduce the transport properties of photons, phonons, and electrons. However, we predict that in contrast to the role in reducing thermal conductivity, four-phonon scattering dominates the near-field radiative heat transfer (NFRHT) in both boron arsenide (BAs) and boron antimonide. Including four-phonon scattering results in a nearly 400-fold increase in the total heat flux between two BAs thin films compared to three-phonon scattering alone. This nonintuitive enhancement arises from the large number of NFRHT channels activated by four-phonon scattering, outcompeting the effect of decreased coupling strength of surface phonon polaritons at the resonance frequency. Additionally, we point out that four-phonon scattering decreases NFRHT in certain other systems.

In the present work, we investigated the magnetic ground state of $\mathrm{Ce}\mathit{NM}{\mathrm{Sb}}_2$ [$\mathit{NM}$ (noble metal): Cu, Ag, and Au] compounds using electronic-structure calculations while following a full-potential linearized-augmented-plane-wave method. Due to the lack of mirror symmetry about the $ab$ plane, the two Ce atoms—which are located at crystallographically equivalent sites—in a unit cell should be treated inequivalently, and, as a consequence, a $1\times 1\times 2$ supercell must be constructed to accommodate the antiferromagnetic (AFM) configuration. The magnetic configuration of the ground state of AFM ${\mathrm{CeAuSb}}_2$ is that the magnetic moments of the two Ce atoms in a conventional unit cell are aligned ferromagnetically and those of an adjacent cell are aligned oppositely along the easy axis. The ground states, including ferromagnetic ${\mathrm{CeAgSb}}_2$ and AFM ${\mathrm{CeCuSb}}_2$, are understood in terms of the exchange interaction $J$ of neighboring Ce atoms. Our results clearly confirm a recent experimental finding, in which, under an external magnetic field ($\lesssim 3\mathrm{T}$) along the $c$ axis, a spin-density wave (SDW) with a wave vector ($\eta ,\eta ,\frac{1}{2}$), where $\eta \approx 0.136$, is observed in an AFM ${\mathrm{CeAuSb}}_2$ compound. The Fermi surface (FS) on the $ab$ plane exhibits nesting along the (110) direction. The nesting vector $\mathbf{q}=(\zeta ,\zeta ,0)(2\pi /a)$, with $\zeta \sim 0.13$, is very similar to the experimental result aside from $\frac{1}{2}$ along the $c$ axis. We argue that the modulation along the $c$ axis is attributed to the period doubling along the $c$ axis due to the lack of mirror symmetry. Although our calculated generalized susceptibility $\chi \left(\mathbf{q}\right)$ exhibits a pair of peaks along the (110) direction at $\zeta \sim 0.06$ and 0.13, which are related to FS nesting, the peak at $\zeta \sim 0.06$ can hardly be observed in the experiment because of the negligible oscillator strengths of interband transitions at a low $\mathbf{q}$ value. This implies that the observed SDW can be attributed to strong Fermi-surface nesting.

Study of boundary phase transition in toric code under cylinder geometry via bulk projective measurement is reported. As the frequency of local measurement for bulk qubits is increased, spin-glass type long-range order on the boundaries emerges indicating spontaneous-symmetry breaking (SSB) of $Z_2$ symmetry. From the lattice gauge theory viewpoint, this SSB is a signal of a transition to Higgs phase with symmetry-protected topological order. We numerically elucidate the properties of this phase transition in detail, especially its criticality, and give a physical picture using nonlocal gauge-invariant symmetry operators. Phase transition in the bulk is also studied and its relationship to the boundary transition is discussed.

Characterizing the optical response of magneto-optic and magnetic materials usually relies on semiclassical models (e.g., Lorentz oscillator model) involving few parameters or models based on a detailed quantum mechanical description of the underlying response. These models typically involve a few parameters that are estimated via fitting the experimental data to provide a qualitative understanding of the underlying physics. Such a few-parameters fitting approach falls short of accurately capturing all elements of the complex-valued permittivity tensor across a range of wavelengths. Accurate characterization of the permittivity tensor elements across a broad range of wavelengths is invariably imperative for designing optical elements such as isolators, circulators, etc. Here, we propose and demonstrate a ubiquitous and accessible method based on a combination of spectroscopic ellipsometry and spectroscopic magneto-optic Kerr effect (MOKE) measurements coupled with rigorous numerical parameter extraction techniques. To this end, we use the combined MOKE ellipsometry measurements conducted at different angles of incidence with a gradient-descent minimization algorithm to provide the inverse solution to the complete dielectric permittivity tensor. Further, we demonstrate model reverification to ensure the estimated dielectric permittivity values reliably predict the measured experimental data. Our method is a simplified bench-top counterpart to the otherwise complex measurement systems.

The second harmonic voltage has been widely used to measure electrically driven spin torques in ferromagnet–heavy metal heterostructures. Based on the macrospin model, the third harmonic voltage is proposed for studying thermally driven spin torques. Our experiments clearly show that two distinct physical mechanisms dominate such thermal spin torques. The first refers to thermal expansion or contraction, which generates an effective field through the magnetoelastic effect. The second comes from the thermally reduced magnetization of the ferromagnetic layer, which modulates the magnetoresistance of the heterostructure. Our study illustrates the versatile applicability of the third harmonic voltage in investigating the nonlinear effects in ferromagnet–heavy metal heterostructures.

We study the behavior of the conductance and the current noise in three-terminal configurations of edge modes of a quantum Hall system in the $\nu =2$ filling factor with normal and $s$-wave superconducting contacts. We discuss the impact of spin-orbit coupling in the quantum Hall system and the possibility of effectively inducing triplet pairing in the edge states. We show that the presence of these correlations imprints very clear signatures in both the nonlinear conductance and noise in these type of devices.

Quantum spin-Hall insulators (QSHIs) possess nontrivial band topology. Using first-principles many-body perturbation theory ($GW+\text{Bathe-Salpeter}$ equation), we show that excitonic insulators (EIs) can exist in QSHIs AsO and ${\mathrm{Mo}}_2{\mathrm{TiC}}_2{\mathrm{O}}_2$ with nonvanishing band gaps. Their single-particle topological properties can be described by the same low-energy model, and we show that the EI phase in this model is of spin-triplet type due to the nontrivial band geometry. The rotational symmetry is broken by the $s$-wave EI order parameter, and the anisotropic absorption together with possible lattice reconstruction can be used as signatures for this spin-triplet topological EI phase. Large amounts of spin-triplet excitons emerge spontaneously when an EI is achieved and exciton spin-Hall effect may be observed.

The reactive Hall constant $R_H$, described by reactive (nondissipative) conductivities, is analyzed within linear response theory in the presence of spin-orbit interaction. Within a two-dimensional tight-binding model the effect of Van Hove singularities is studied. Along the same line a formulation of the Edelstein constant is proposed and studied as a function of coupling parameters and fermion filling.

Previous work has shown that time-reversal symmetric Weyl semimetals with a quadrupolar arrangement of first-order Weyl nodes exhibit a mixed crystalline-electromagnetic response. For systems with higher-order Weyl nodes, which are attached to both surface and hinge Fermi arcs, additional phenomena appear on surfaces of codimension $n>1$, such as electromagnetic responses of the hinges. Here we construct a model possessing a quadrupole of higher-order Weyl nodes to study the interplay between higher-order topology and mixed crystalline-electromagnetic responses. We show that the higher-order nature of the Weyl nodes yields a dipole of Dirac nodes on certain surfaces, leading to a mixed crystalline-electromagnetic surface response that binds charge to dislocations and momentum density to magnetic fields. In addition, we show that the model possesses a bulk quadrupole moment of crystal momentum that provides a link between the bulk and surface responses of the system.

In this paper, the crystal and electronic structures are investigated using first-principles theory for metal-semiconductor van der Waals heterostructures $1H$-transition-metal dichalcogenides (${\mathrm{NbS}}_2, {\mathrm{NbSe}}_2, {\mathrm{TaS}}_2, {\mathrm{TaSe}}_2, {\mathrm{MoS}}_2, {\mathrm{MoSe}}_2, {\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$). The theoretical investigations reveal that chalcogen sublayers play crucial roles in the crystal and electronic structures though these sublayers are minor parts in the pristine crystals. Actually, selenide sublayers energetically stabilize the commensurate heterobilayers, whereas moiré structure gives a much lower electronic energy with sulfide sublayers. Moreover, the chalcogen sublayers dominate the charge distribution via the difference of work functions in the component layers, i.e., the charge transfer between the metallic and semiconducting layers in the heterobilayers. The theoretical results suggest that the chalcogen sublayers can be the key to control the crystal and electronic structures in the metal-semiconductor vdW heterostructure of TMDC monolayers.

Ferrovalley, which refers to the valley polarization being nonvolatile and switchable, is highly desired for valleytronics applications but remains challenging due to rare candidate materials. Here we propose a strategy to realize ferrovalley with bilayer stacking (BSFV) in many candidate systems. As a special case of BSFV, sliding ferrovalley corresponds to the bilayers obtained by a direct AA stacking and subsequent in-plane sliding. Different from previous approaches, the BSFV strategy not only maintains time-reversal symmetry, but also keeps spatial-inversion symmetry in many cases. Importantly, switching of the valley polarization can be easily achieved by interlayer sliding. Group theory analysis is systematically performed over all kinds of lattices to identify those that can host BSFV. High-throughput screening is carried out and leads to 14 BSFV candidates with direct bandgap and 338 with indirect bandgap. First-principles verification of BSFV indicates that the valley polarization can be realized in, e.g., (i) the hexagonal $\mathrm{RhC}{\mathrm{l}}_3$ bilayer with a threefold rotation symmetry and 39 meV energy difference among valleys, and (ii) the square-latticed InI bilayer with a fourfold rotation symmetry and 326 meV energy difference among valleys. The presently proposed BSFV strategy offers a highly convenient approach for the realization of polarizers and the advancement of valleytronics applications.

We develop a theory of the magnetostriction for metals with small charge-carrier pockets of their Fermi surfaces. As an example, we consider LuAs that has a cubic crystal structure. The theory quite well describes the known experimental data on the magnetostriction of this metal. The obtained results also clearly demonstrate that the dilatometry can be used for detecting tiny Fermi surface pockets that are not discerned by traditional methods based on investigations of the Shubnikov–de Haas and de Haas–van Alphen effects.

Thermal radiation, universally governed by Planck's law for objects at nonzero temperatures, is known to deviate in the near-field regime. In the present paper, we uncover a distinctive thermal radiation behavior in Weyl semimetals, challenging the established principles of Planck's law, even in the far-field regime. The chiral anomaly inherent to these materials gives rise to unique electromagnetic phenomena—namely, the anomalous Hall effect, which cause a breakdown of Kirchhoff's law, and the chiral magnetic effect. We theoretically demonstrate that the latter effect can induce negative emissivity at low-frequency ranges, suggesting an energy transfer from cooler to warmer bodies. This unexpected emissivity opens up novel approaches for thermal energy management using Weyl semimetals, with significant potential for future technological innovations.

Quantum geometry, which encompasses both Berry curvature and the quantum metric, plays a key role in multiband interacting electron systems. We study the entanglement entropy of a region of linear size $\ell$ in fermion systems with nontrivial quantum geometry, i.e., whose Bloch states have nontrivial $k$ dependence. We show it scales as $S=\alpha {\ell }^{d-1}ln\ell +\beta {\ell }^{d-1}+\cdots$, where the first term is the well-known area-law violating term from the Fermi surface and $\beta$ contains the leading contribution from quantum geometry. We compute this for the case of uniform quantum geometry and cubic domains and provide numerical results for the Su-Schrieffer-Heeger model, two-dimensional (2D) massive Dirac cone, and 2D Chern bands. An experimental probe of the quantum geometric entanglement entropy is proposed using particle number fluctuations. We offer an intuitive account of the area-law entanglement related to the spread of maximally localized Wannier functions.

We applied the ab initio spin-polarized correlation matrix renormalization theory to the ferromagnetic state of the bulk bcc iron. We showed that it was capable of reproducing the equilibrium physical properties and the pressure-volume curve in good comparison with experiments. We then focused on the analysis of its local electronic correlations. By exploiting different local fluctuation-related physical quantities as measures of electronic correlation within target orbits, we elucidated the different roles of $t_{2g}$ and $e_g$ states in both spin channels and presented compelling evidence to showcase this distinction in their electronic correlation.

In this paper, we investigate topological modes of different physical systems defined on arbitrary two-dimensional spatial curved surfaces. We consider the shallow water equations, inhomogeneous Maxwell's equations, and the Jackiw-Rebbi model and show how the topological protection mechanism responds to the presence of curvature in different situations. We show the existence of a line gap in the considered models and study the condition on the curve, which can host topological modes.

Disorder-free localization (DFL) is an ergodicity-breaking mechanism that has been shown to occur in lattice gauge theories in the quench dynamics of initial states spanning an extensive number of gauge superselection sectors. Whether this type of DFL is intrinsically a quantum interference effect or can arise classically has hitherto remained an open question whose resolution is pertinent to further understanding the far-from-equilibrium dynamics of gauge theories. In this work, we utilize cellular automaton circuits to model the quench dynamics of large-scale quantum link model (QLM) formulations of $(1+1)\mathrm{D}$ quantum electrodynamics, showing excellent agreement with the exact quantum case for small system sizes. Our results demonstrate that DFL persists in the thermodynamic limit as a purely classical effect arising from the finite-size regularization of the gauge-field operator in the QLM formulation, and that quantum interference, though not a necessary condition, may be employed to enhance DFL.

We investigate the effect of ergodic inclusions in putative many-body localized systems. We consider the random field Heisenberg chain, which is many-body localized at strong disorder and we couple it to an ergodic bubble, modeled by a random matrix Hamiltonian. Recent theoretical work suggests that localized systems are unstable to ergodic bubbles, driving the delocalization transition. We tentatively confirm this by numerically analyzing the response of the on-site purities to the insertion of the bubble. For a range of intermediate disorder strengths, this response decays very slowly, or not at all, with increasing distance to the bubble. This suggests that at those disorder strengths, the system is actually delocalized in the thermodynamic limit. However, the signal is quite weak and artefacts in the numerics cannot be ruled out conclusively.

We initiate the study of the classical mechanics of nonrelativistic fractons in its simplest setting—that of identical one-dimensional particles with local Hamiltonians characterized by a conserved dipole moment in addition to the usual symmetries of space and time translation invariance. We introduce a family of models and study the $N$-body problem for them. We find that locality leads to a “Machian” dynamics in which a given particle exhibits finite inertia only if within a specified distance of another particle. For well-separated particles, this dynamics leads to immobility, much as for quantum models of fractons discussed before. For two or more particles within inertial reach of each other at the start of motion, we obtain an interesting interplay of inertia and interactions. Specifically, for a solvable “inertia only” model of fractons, we find that two particles always become immobile at long times. Remarkably, three particles generically evolve to a late time state with one immobile particle and two oscillating about a common center of mass with generalizations of such “Machian clusters” for $N>3$. Interestingly, these Machian clusters exhibit physical limit cycles in a Hamiltonian system even though mathematical limit cycles are forbidden by Liouville's theorem.