${\mathrm{Y}}_2{\mathrm{Co}}_3$ is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the $b$ axis and some canting towards the $ac$ plane. In this study, we report the magnetic structure of ${\mathrm{Y}}_2{\mathrm{Co}}_3$ to be an A-type AFM structure with ferromagnetic (FM) interactions within the distorted kagome plane and an interplane antiferromagnetic interaction, as determined by single-crystal neutron diffraction. The magnetic moments align along the $b$ axis, with minimal canting towards the $c$ axis, at odds with the previous interpretation of bulk magnetization measurements. The magnetic moments on the two distinct Co sites are [0, -0.68(3), 0] ${\mu }_B$ and [0, 1.25(4), 0.07(1)] ${\mu }_B$. We attribute the previously reported “noncollinear” behavior to the considerable temperature dependence of itinerant AFM exchange interactions, induced by thermal contraction along the $b$ axis. Additionally, our examination of lattice constants through pressure studies reveals compensating effects on FM and AFM interactions, resulting in negligible pressure dependence of $T_{\text{N}}$.

Employing time-periodic light to modulate physical properties of materials represents an effective strategy, known as Floquet engineering, which enables dynamic manipulation of electronic structures. In this paper, we investigate the dynamic behavior of multi-Weyl topological semimetals irradiated by circularly polarized light (CPL) in the off-resonant regime. Utilizing a minimal low-energy effective model, we reveal distinct evolutions of multi-Weyl points (WPs) under CPL incident from different directions. Since the stability of multi-WPs is usually ensured by a specific crystal symmetry, each multi-WP remains intact when this symmetry is preserved under the CPL, whereas the multi-WP splits into WPs with lower charge under symmetry-breaking CPL. This finding is explicitly exemplified by the celebrated double-Weyl semimetal ${\mathrm{HgCr}}_2{\mathrm{Se}}_4$. Furthermore, we have carried out first-principles analysis within the framework of Floquet theory to present the dynamical evolution of electronic structures of two realistic materials, namely, ${\mathrm{BaNiIO}}_6$ and cP-C24, hosting double and quadruple WPs, respectively. Remarkably, the first-principles results are in good consistence with our theoretical findings. In this paper, we reveal that light can trigger an abundance of topological states in multi-Weyl semimetals, which may hold promising prospects for application in optoelectronic devices.

We develop a quantum Boltzmann equation approach that incorporates microscopic impurity models into the theory of orbital transport, revealing how the impurity properties, including their symmetry, influence the relaxation of orbital momentum and the orbital Hall effect. Specifically, we demonstrate that when the impurity potential has axial symmetry, the relaxation is governed by the Dyakonov-Perel mechanism. In contrast, when this symmetry is broken, scattering can result in a rapid Elliott-Yafet relaxation of orbital momentum. The details of impurity potential also affect the intrinsic orbital Hall effect even when the impurity concentration is very small. Impurities that alter the orbital texture can also give rise to a skew-scattering contribution to the orbital Hall effect, although this does not necessarily dominate in materials that are nearly pristine because of the Dyakonov-Perel relaxation.

We propose a route for the realization of persistent spin texture (PST). We have shown from symmetry considerations that in nonpolar chiral systems, bands with specific orbital characters around a high-symmetry point with $D_2$ little group may admit a single spin-dependent term in the low-energy $\mathbf{k}·\mathbf{p}$ model Hamiltonian that naturally leads to PST. Considering a two-dimensional plane in the Brillouin zone, we have further argued that in such chiral systems, the PST is transpired due to the comparable strengths of the Dresselhaus and Weyl (radial) interaction parameters where the presence of these two terms is allowed by the $D{}_2$ symmetry. Finally, using first-principles density functional theory calculations, we have identified that the nonpolar chiral compounds ${\mathrm{Y}}_3{\mathrm{TaO}}_7$ and ${\mathrm{AsBr}}_3$ displays PST for the conduction band and valence band, respectively, around the $\mathrm{\Gamma }$ point having $D{}_2$ little group and predominantly Ta-$d_{xz}$ orbital character for ${\mathrm{Y}}_3{\mathrm{TaO}}_7$ and Br-$p{}_x$ orbital character for ${\mathrm{AsBr}}_3$, corroborating our general strategy. Our results for the realization of PST in nonpolar chiral systems thereby broaden the class of materials displaying PST that can be employed for application in spin orbitronics.

A significant limitation of wide-band-gap materials is their low hole mobility related to localized holes with heavy effective masses $\left(m_h^{*}\right)$. We identify in low-symmetric wide-band-gap compounds an anion-anion antibonding coupling (AAAC) effect as the intrinsic factor behind hole localization, which explains the extremely heavy $m_h^{*}$ and self-trapped hole (STH) formation observed in gallium oxide $(\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3)$. We propose a design principle for achieving light holes by manipulating AAAC, demonstrating that specific strain conditions can reduce $m_h^{*}$ in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ along $c^{*}$ from 4.77 $m_0$ to 0.38 $m_0$, making it comparable to the electron mass $(0.28 m_0)$ while also slightly suppressing the formation of self-trapped holes, evidenced by the reduction in the formation energy of hole polarons from −0.57 to −0.45 eV under tensile strain. The light holes show significant anisotropy, potentially enabling two-dimensional transport in bulk material. This study provides a fundamental understanding of hole mass enhancement and STH formation in novel wide-band-gap materials and suggests new pathways for engineering hole mobilities.

We theoretically predict that the motion of a polar crystalline layer between two graphene planes exerts Coulomb drag on electrons in graphene, inducing a DC drag current. The physical mechanism underlying this drag arises from intervalley scattering of charge carriers in graphene caused by the time-dependent potential of the moving crystalline layer. This drag effect manifests above a finite threshold doping of the graphene layers, which is determined by the lattice structure and the relative orientation of the crystalline layer with respect to the graphene lattice. Additionally, the drag current exhibits a nonlinear Hall effect due to the interplay between the induced nonequilibrium pseudospin and the intrinsic pseudospin-orbit coupling in graphene. In turn, the drag exerted on electrons in graphene produces a backaction on the crystalline layer, which can be described as an increase in its dynamic viscosity.

We investigate the effects of Holstein polarons formed due to the electron-phonon (e-p) coupling, on the quantum spin Hall (QSH) phase of a pseudospin-1 fermionic $\alpha \text{−}{T}_3$ lattice. The parameter $\alpha$ and the e-p coupling strength $\lambda$ have an interesting interplay, which demonstrates that at smaller values of $\alpha$, there is a single transition from a topological to a trivial phase as a function of $\lambda$, while the larger $\alpha$ values host two gap-closing transitions, namely, trivial-topological-trivial transitions, accompanied by a narrow semimetallic phase in-between. The topological properties are characterized by computing the ${\mathbb{Z}}_2$ invariant which confirms the existence of topological (trivial) phases, which are hence verified against the presence (absence) of counterpropagating helical edge modes in a nanoribbon. Furthermore, on the introduction of a planar magnetic field into the system, emergence of a second-order topological phase is observed. The in-plane field causes gapping out of the first-order edge states while maintaining the topological phase of the bulk intact, subsequently leading to the emergence of robust corner modes under suitable open boundary conditions. This resultant phase is adequately designated by computing the projected spin Chern number, a well-established invariant for the time-reversal symmetry-broken QSH phase. Further, we show that the e-p coupling yields a complete disruption of the corner modes as we tune it beyond a certain critical strength, giving rise to a second-order topological phase transition.

We present a study of spin-unpolarized and spin-polarized two-dimensional uniform electron liquids using variational and diffusion quantum Monte Carlo (VMC and DMC) methods with Slater-Jastrow-backflow trial wave functions. Ground-state VMC and DMC energies are obtained in the density range $1\le r_{\text{s}}\le 40$. Single-particle and many-body finite-size errors are corrected using canonical-ensemble twist-averaged boundary conditions and extrapolation of twist-averaged energies to the thermodynamic limit of infinite system size. System-size-dependent errors in Slater-Jastrow-backflow DMC energies caused by partially converged VMC energy minimization calculations are discussed. We find that, for $1\le r_{\text{s}}\le 5$, optimizing the backflow function at each twist lowers the twist-averaged DMC energy at finite system size. However, nonsystematic system-size-dependent effects remain in the DMC energies, which can be partially removed by extrapolation from multiple finite system sizes to infinite system size. The DMC energies in the thermodynamic limit are used to parametrize a local spin density approximation correlation functional for inhomogeneous electron systems. Our zero-temperature phase diagram shows a single transition from a paramagnetic fluid to a hexagonal Wigner crystal at $r_{\text{s}}=35\left(1\right)$, with no region of stability for a ferromagnetic fluid.

We demonstrate how exceptional points (EPs) naturally occur in the Luttinger liquid (LL) theory describing the low-energy excitations of a microscopic lattice model with sublattice-dependent electron-electron interaction. Upon bosonization, this sublattice dependence directly translates to a nonstandard sine-Gordon-type term responsible for the non-Hermitian matrix structure of the single-particle Green function (GF). As the structure in the lifetime of excitations does not commute with the underlying free Bloch Hamiltonian, non-Hermitian topological properties of the single-particle GF emerge—despite our Hermitian model Hamiltonian. Both finite temperature and a nontrivial Luttinger parameter $K\ne 1$ are required for the formation of EPs, and their topological stability in one spatial dimension is guaranteed by the chiral symmetry of our model. In the presence of the aforementioned sine-Gordon term, we resort to leading-order perturbation theory (PT) to compute the single-particle GF. All qualitative findings derived within LL theory are corroborated by comparison to both numerical simulations within the conserving second Born approximation and, for weak interactions and high temperatures, by fermionic plain PT. In certain parameter regimes, quantitative agreement can be reached by a suitable parameter choice in the effective bosonized model.

We study the joint dynamics of the phase bias $\varphi$ and the propagating Majorana fermions of the edge modes in Josephson junctions containing two-dimensional time-reversal invariant topological superconductors (TRITOPS). We consider TRITOPS-TRITOPS junctions, as well as junctions between topological and nontopological superconductors (TRITOPS-S). Both types of junctions are described by effective Dirac Hamiltonians with a $\varphi$-dependent mass. We analyze the effect of the phase fluctuations in the junction, as well as solitonic solutions of $\varphi$ generated by fluxons trapped in the junction. We show that these solitons generate a spatial-dependent mass with a sign change akin to the Jackiw-Rebbi model. This enables the formation of zero-energy fermionic states localized at the fluxon. For the TRITOPS-TRITOPS junction these consist of a Kramers pair of Majorana modes, while for the TRITOPS-S one, there is a single Majorana fermion. The localized bound states hybridize in soliton-antisoliton configurations. Depending on the occupation state, these modes generate an effective attraction or repulsion in the dynamics of the soliton-antisoliton collision.

Floquet topological insulators (FTIs) are materials which undergo topological phase transitions under a time periodic perturbation causing time reversal symmetry breaking. In this Letter, we propose a spin filter model based on a FTI realized by irradiating a honeycomb lattice with circularly polarized light, in the presence of intrinsic spin-orbit coupling. The main ingredient of our proposed mechanism of Floquet topological spin filter (FTSF) implementation is the presence of an on-site staggered potential which controls independently the topological phases of the two existent spin states. After giving a numerical example of the occurrence of the FTSF phase, we argue that the origin of the FTSF phase resides in the spatial inversion symmetry breaking due to the presence of a staggered potential. The light helicity degree of freedom may be used to select the filtered spin state. Moreover, due to the topological properties of our model, the spin will be purely filtered in a Hall transport experiment. We discuss also the experimental feasibility.

Many dipolar topological structures have been experimentally demonstrated on ${\left({\mathrm{PbTiO}}_3\right)}_n/{\left({\mathrm{SrTiO}}_3\right)}_n$ superlattices, such as flux closure, vortex, and skyrmion. In this work, we employ deep potential molecular dynamics (MD) to investigate the atomic-level dynamical response of the ${\left({\mathrm{PbTiO}}_3\right)}_{10}/{\left({\mathrm{SrTiO}}_3\right)}_{10}$ superlattice, which hosts polar vortex arrays, to variations in temperature and electric field. Our simulations reveal a unique phase transition sequence from ferroelectric-like to antiferroelectric-like to paraelectric in the in-plane direction as temperature increases. In the ferroelectric-like state, we observe field-driven reversible switching of in-plane polarization coupled with out-of-plane movements of vortex cores. In the antiferroelectric-like region, the polarization-electric field hysteresis loop exhibits a superparaelectric feature, showing nearly no loss. This behavior is attributed to a strong recovering force that drives the formation of polar vortex arrays, dictated by the electrical and mechanical boundary conditions within the superlattice. The ${\left({\mathrm{PbTiO}}_3\right)}_{10}/{{(\mathrm{SrTiO}}_3)}_{10}$ superlattice in the antiferroelectric-like state also demonstrates large in-plane susceptibility and tunability. The effect of Pb doping in the ${\mathrm{SrTiO}}_3$ layer on the topological structural transition in the superlattice is investigated. The weakened depolarization field in the ${\mathrm{PbTiO}}_3$ layers leads to new dipolar configurations, such as an enlarged skyrmion bubble within $c$ domains in ${\left({\mathrm{PbTiO}}_3\right)}_{10}/{\left({\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{TiO}}_3\right)}_{10}$, and we quantify their thermal and electrical responses using MD simulations. These quantitative atomistic insights will be useful for the controlled optimization of perovskite superlattices for various device applications.

There have been several criteria for the existence of topological edge states in 1D non-Hermitian two-band sublattice-symmetric tight-binding Hamiltonians. The generalized Brillouin zone (GBZ) approach uses the integration of the Berry connection over the GBZ contour in the complex wavevector space. An alternate ‘pole-zero’ approach uses algebraic properties of the off-diagonal matrix elements of the sublattice-symmetric Hamiltonian in off-diagonal form. Both correctly predict the presence or absence of edge states, but there has not been an explicit proof of their equivalence. Here we provide such an explicit proof and moreover we extend the pole-zero approach so that it also applies for sublattice-symmetric models when the Hamiltonian is not in off-diagonal form. We give numerical examples for these invariants.

In two-dimensional topological insulators, a disorder-induced topological phase transition is typically identified with an Anderson localization transition at the Fermi energy. However, in ${\mathbb{Z}}_2$ trivial, spin-resolved topological insulators it is the spectral gap of the spin spectrum, in addition to the bulk mobility gap, which protects the nontrivial topology of the ground state. In this work, we show that these two gaps, the bulk electronic and spin gap, can evolve distinctly on the introduction of quenched short-ranged disorder and that an odd-quantized spin Chern number topologically protects states below the Fermi energy from localization. This decoupling leads to a unique situation in which an Anderson localization transition occurs below the Fermi energy at the topological transition. Furthermore, the presence of topologically protected extended bulk states nontrivial bulk topology typically implies the existence of protected boundary modes. We demonstrate the absence of protected boundary modes in the Hamiltonian and yet the edge modes in the eigenstates of the projected spin operator survive. Our work thus provides evidence that a nonzero spin-Chern number, in the absence of a nontrivial ${\mathbb{Z}}_2$ index, does not demand the existence of protected boundary modes at finite or zero energy.

We studied the effects of point defects and Hg impurities in the electronic properties of bismuth iodide $\left({\mathrm{Bi}}_4{\mathrm{I}}_4\right)$. Our transport measurements after annealing at different temperatures show that the resistivity of ${\mathrm{Bi}}_4{\mathrm{I}}_4$ depends on its thermal history, suggesting that the formation of native defects and impurities can shape the temperature dependence of electrical resistivity. Our density functional theory calculations indicate that the bismuth and iodine antisites and the bismuth vacancies are the dominant native point defects. We find that bismuth antisites introduce resonant states in the band edges, while iodine antisites and bismuth vacancies lead to a $n$-type and a $p$-type doping of ${\mathrm{Bi}}_4{\mathrm{I}}_4$, respectively. The Hg impurities are likely to be found at Bi substitutional sites, giving rise to the $p$-type doping of ${\mathrm{Bi}}_4{\mathrm{I}}_4$. Overall, our findings indicate that the presence of native point defects and impurities can significantly modify the electronic properties, and, thus, impact the resistivity profile of ${\mathrm{Bi}}_4{\mathrm{I}}_4$ due to modifications in the amount and type of carriers, and the associated defect (impurity) scattering. Our results suggest possible routes for pursuing fine-tuning of the electronic properties of quasi-one-dimensional quantum materials.

Magnon-based spintronic devices require the modulation of magnon spin transport for their operations. We provide a proof-of-principle of an unconventional two-terminal nonlocal magnon transport device where the same contact is simultaneously used as injector and gate and an unconventional three-terminal nonlocal magnon transport device where the gate is positioned outside the magnon transport channel. In these devices we modulate the diffusive magnon transport of incoherent magnons in the van der Waals antiferromagnet Chromium thiophosphate, ${\mathrm{CrPS}}_4$. The nonlocal signals generated electrically by spin injection via the spin Hall effect (SHE) and thermally via the spin Seebeck effect are altered by a modulator electrode. The current through the modulator increases or decreases the magnon chemical potential via the SHE and changes the magnon temperature through Joule heating. We achieve up to ${\eta }_{\text{SHE}}^{1\omega }=25\%/\mathrm{mA}$ and ${\eta }_{\text{SHE}}^{2\omega }=16\%/\mathrm{mA}$ modulation efficiencies for the electrically and thermally generated magnon spin transport, respectively, for ${\mathrm{CrPS}}_4$ in the collinear state at in-plane fields $>7$ T at a temperature of 25 K.

Topological photonics enables unprecedented photon manipulation by realizing various topological states, such as corner, edge, and surface states. However, achieving a topological fiber state has remained elusive. Here, we demonstrate a topological fiber state in a Weyl gyromagnetic photonic crystal fiber. By applying an in-plane magnetic bias to a gyromagnetic photonic crystal fiber with broken parity-inversion symmetry, we create an asymmetrical Weyl band gap that supports one-way fiber states associated with type-II Weyl points. Dispersion and topological invariant calculations reveal a transition from Weyl surface states to one-way Weyl fiber states. Electromagnetic field simulations confirm the existence of these one-way Weyl fiber states and their robust transport, even in the presence of metallic obstacles along the transport path. Our findings offer an intriguing pathway for exploring topological states and guiding the design of topological fibers.

Spintronics has emerged as a viable alternative to traditional electronics-based technologies in the past few decades. While the discovery of topological phases of matter with protected spin-polarized states has opened up exciting prospects, recent revelation of intriguing nonrelativistic spin-splitting in antiferromagnetic (AFM) materials with unique symmetries facilitate a wide possibility of realizing both these features simultaneously. In this work, we report the coexistence of these two intriguing properties within a single material: GdAlSi. Single crystal of GdAlSi stabilizes in a body-centered tetragonal structure with a noncentrosymmetric space group $I{4}_{1}md$ (109), which is confirmed using detailed structural analysis through x-ray diffraction (XRD) and optical second harmonic generation (SHG) measurements. The magnetization data indicates AFM ordering with an ordering temperature ($T_N$) $\sim 32$ K. Ab initio calculations reveal GdAlSi to be a collinear AFM Weyl semimetal with an unconventional, momentum-dependent spin-splitting, also referred to as altermagnet. Angle-resolved photoemission spectroscopy measurements on GdAlSi single crystals subsequently hints the possible presence of Fermi arcs. Electric and magnetic multipole analysis provides a deeper understanding of the symmetry-mediated, momentum-dependent spin-splitting, which has strictly nonrelativistic origin. GdAlSi is possibly the first candidate material with noncentrosymmetric collinear AFM structure showing such momentum-dependent spin-splitting, as also confirmed by our detailed symmetry analysis, rendering GdAlSi a special and promising candidate material. We further propose a device harnessing these features, poised to enable practical and efficient topotronic applications.