A comprehensive study of magnetic, room-temperature Mössbauer spectroscopic, room-temperature Raman spectroscopic, magnetocaloric, and critical behavior properties of chemical vapor transport (CVT) grown single-crystalline ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ ($x\approx 0.3$) (F3GT) have been performed. The single crystal of F3GT has been formed in a van der Waals force bonded layered compound. The compound crystallizes in hexagonal crystal structure with space group $\mathit{P}{6}_3/\mathit{mmc}$. Temperature variation of dc and ac magnetic susceptibility of F3GT reveals a paramagnetic to ferromagnetic transition at $\approx 180\mathrm{K}$ ($T_{\mathrm{C}}$). Field variation of magnetic isotherm in the magnetically ordered state at 5 K reveals strong anisotropy with crystallographic $c$ axis as the easy axis of magnetization. A weak itinerant behavior of Fe spins is also ascertained from the magnetization studies. Magnetocaloric effect in terms of isothermal magnetic entropy change has been estimated to be $\approx -2.6$ J/kg-K near $T_{\mathrm{C}}$ for a field change of 100 kOe along crystallographic $c$ direction. The magnetic transition at $T_{\mathrm{C}}$ is found to be of second order. Critical behavior analysis performed for the easy magnetization axis direction in CVT-grown F3GT by modified Arrott plots and Kouvel-Fisher technique suggests that this system does not belong to a particular universality class. However, the critical exponents assume values $\beta =0.321, \gamma =1.179$, and $\delta =4.67$. Isothermal magnetization curves at different temperatures in the closed vicinity of ferromagnetic transition of F3GT obey the scaling hypothesis using the estimated critical exponents wherein the magnetization vs field data below and above magnetic transition collapse onto two branches of the universal curves. The critical behavior analyses confirm the weak itinerant ferromagnetic long-range-type interaction due to itinerant $3d$ electrons and the interaction is very close to the three-dimensional Heisenberg type, wherein the occurrence of long-range-type interaction between the magnetic spins decays as $J\left(r\right)\approx r^{-4.74}$ in F3GT.

Motivated by the recent experimental progress in studying conventional and unconventional superconductors in a pump-probe setup, we perform a comprehensive theoretical analysis of the nonlinear response of a diffusive BCS conventional superconductor to the action of an alternating electromagnetic field using a generalized Usadel equation. We analyze the response up to the second order of the perturbation in the amplitude of the vector potential $\vec{A}$, the superconducting order parameter $\mathrm{\Delta }$, and in the third order for the current $\vec{j}$. On the basis of this approach, we derive general expressions for the retarded (advanced) Green's functions, as well as the Keldysh function for an arbitrary number of harmonics of the incident field. Most importantly, we analyze the set of physical observables in a nonequilibrium superconductor, such as frequency and the temperature dependencies of the zero harmonic $\delta {\left(\mathrm{\Delta }\right)}_0$ (Eliashberg effect), the second harmonic $\delta {\left(\mathrm{\Delta }\right)}_{2\mathrm{\Omega }}$, as well as the third harmonic for the electric current $j\left(3\mathrm{\Omega }\right)$ under the action of monochromatic irradiation. For the same set of parameters, we also analyze the behavior of the reflectivity and the down-conversion intensity of a thin superconducting film, discussed recently in the context of parametric amplification of superconductivity. We derive these quantities microscopically and show the connection of the down-conversion intensity to the third-harmonic generation currents induced by the amplitude mode and the direct action of the electric field on the charge carriers.

Absence of local conserved quantities is often required, such as for thermalization or for the validity of response theory. Although many studies have discussed whether thermalization occurs in the Ising chain with longitudinal and transverse fields, rigorous results on local conserved quantities of this model have still been lacking. Here, we rigorously prove that, if all coupling constants are nonzero, this model has no conserved quantity spanned by local operators with support size up to half of the system size other than a trivial one, i.e., a linear combination of the Hamiltonian and the identity. The proof is given not only for the periodic boundary condition but also for the open boundary condition. We also discuss relation to the integrability of the model where the longitudinal field is set to zero. Our results provide the second example of spin models whose nonintegrability is rigorously proved.

Compounds with kagome lattice structure are known to exhibit Dirac cones, flatbands, and van Hove singularities, which host numerous versatile quantum phenomena. Inspired by these intriguing properties, we investigate the temperature and magnetic field-dependent electrical transports along with the theoretical calculations of ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$, a nonmagnetic charge-density wave (CDW) compound. At low temperatures, the compound exhibits Shubnikov–de Haas quantum oscillations, which help to design the Fermi-surface (FS) topology. This analysis reveals the existence of several small FSs in the Brillouin zone, combined with a large FS. Among them, the FS-possessing Dirac band is nontrivial and generates a nonzero Berry phase. In addition, the compound also shows the anomalous Hall-like behavior up to the CDW phase transition, and they might be correlated. Combining these interesting physical properties with the CDW phase, ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ presents a unique material example of the versatile $\mathrm{Hf}{\mathrm{Fe}}_6{\mathrm{Ge}}_6$ family and provides various promising opportunities to explore the series further.

Exploring the physics of low-dimensional spin systems and their pressure-driven electronic and magnetic transitions is a thriving research field in modern condensed matter physics. In this context, recently antiferromagnetic Cr-based compounds such as ${\mathrm{CrI}}_3$, ${\mathrm{CrBr}}_3$, and ${\mathrm{CrGeTe}}_3$ have been investigated experimentally and theoretically for their possible spintronics applications. Motivated by the fundamental and industrial importance of these materials, we theoretically studied the electronic and magnetic properties of a relatively less explored Cr-based chalcogenide, namely ${\mathrm{LaCrS}}_3$, where two-dimensional (2D) layers of magnetic ${\mathrm{Cr}}^{3+}$ ions form a rectangular lattice. We employed density functional theory $+$ Hubbard $U$ approach in conjunction with constrained random-phase approximation, where the latter was used to estimate the strength of $U$. Our findings at ambient pressure show that the system exhibits a semiconducting antiferromagnetic ground state with a gap of 0.5 eV and large Cr moments that correspond to a nominal $S=3/2$ spin state. The first nearest neighbor (NN) interatomic exchange coupling $\left(J_1\right)$ is found to be strongly antiferromagnetic, while the second NN couplings are relatively weaker ferromagnetic (FM), making this system a candidate for a 1D nonfrustrated antiferromagnetic spin-chain family of materials. Based on orbital resolved interactions, we demonstrated the reason behind two different types of interactions among first and second NNs despite their very similar bond lengths. We observe a significant spin-orbit coupling effect, giving rise to a finite magnetocrystalline anisotropy, and Dzyaloshinskii-Moriya interaction. Further, we found that by applying uniaxial tensile strain along the crystallographic $a$ and $b$ axis, ${\mathrm{LaCrS}}_3$ exhibits a magnetic transition to a semiconducting FM ground state, while compression gives rise to the realization of a gapless semiconducting antiferromagnetic ground state. Thus our findings can enrich the versatility of ${\mathrm{LaCrS}}_3$ and make it a promising candidate for industrial applications.

The Haldane-Hubbard model is a prime example of the combined effects of band topology and electronic interaction. We revisit its spinful phase diagram at half filling as a consensus on the presence of SU(2) symmetry is currently lacking. To start, we utilize the Hartree-Fock mean-field method, which offers a direct understanding of symmetry breaking through the effective mass term that can acquire spin dependence. Our results, in agreement with previous studies, provide an instructive insight into the regime where the Chern number $C=1$, with only one spin species remaining topological. Besides that, we numerically study the phase diagram of the Haldane-Hubbard model via a large-scale infinite-density matrix renormalization group (iDMRG) method. The phase boundaries are determined by the Chern number and the correlation lengths obtained from the transfer-matrix spectrum. Unlike previous studies, the iDMRG method investigates the Haldane-Hubbard model on a thin and infinitely long cylinder and examines scenarios consistent with the two-dimensional thermodynamic limit. Here, the phase diagram we obtained qualitatively goes beyond the Hartree-Fock scope, particularly in the $C=1$ region, and serves as a quantitative benchmark for further theoretical and experimental investigations.

The experimental characterization of quantum spin liquids poses significant challenges due to the absence of long-range magnetic order, even at absolute zero temperature. The identification of these states of matter often relies on the analysis of their excitations. In this paper, we propose a method for detecting the signatures of the fractionalized excitations in quantum spin liquids using a tunneling spectroscopy setup. Inspired by the recent development of the quantum twisting microscope, we consider a planar tunneling junction, in which a candidate quantum spin-liquid material is placed between two graphene layers. By tuning the relative twist angle and voltage bias between the leads, we can extract the dynamical spin structure factor of the tunneling barrier with momentum and energy resolution. Our proposal presents a promising tool for experimentally characterizing quantum spin liquids in two-dimensional materials.

Recent theory and experiment have shown that gray arsenic has a Shockley surface state with a topological origin, and is spin polarized by the Rashba spin-orbital coupling. However, there has been no microscopic study of the scattering properties of this surface state, which could reveal its spin structures and quasiparticle dynamics. Here, with scanning tunneling microscopy/spectroscopy, and first-principles calculations, we show that the surface band has a significant out-of-plane spin component at the higher-energy part, unlike what a basic Rashba effect would produce. We also show that the spin-coherence length of gray arsenic at liquid-helium temperature to be longer than 70 nm, which is larger than its spin procession length and indicates the potential spintronic applications based on the Rashba state of gray arsenic.

The possibility to engineer a Kitaev chain in quantum dots coupled via superconductors has recently emerged as a promising path toward topological superconductivity and possibly non-Abelian physics. Here we show that it is possible to avoid some of the main experimental hurdles on this path by using only local proximity effect on each quantum dot in a geometry that resembles a two-dot version of the proposal in Fulga et al. [New J. Phys. 15, 045020 (2013)]. There is no need for narrow superconducting couplers, additional Andreev bound states, or spatially varying magnetic fields; it suffices with spin-orbit interaction and a constant magnetic field in combination with control of the superconducting phase to tune the relative strengths of elastic cotunneling and an effective crossed-Andreev-reflection-like process generated by higher-order tunneling. We use a realistic spinful, interacting model and show that high-quality Majorana bound states can be generated already in a double quantum dot.

We propose that the Fermi surface anomaly of symmetry group $G$ in any dimension is universally classified by $G$-symmetric interacting fermionic symmetry-protected topological (SPT) phases in $(0+1)$-dimensional spacetime. The argument is based on the perspective that the gapless fermions on the Fermi surface can be viewed as the topological boundary modes of Chern insulators in the phase space (position-momentum space). Given the noncommutative nature of the phase-space coordinates, we show that the momentum space dimensions should be counted as negative dimensions for SPT classification purposes. Therefore, the classification of phase-space Chern insulators (or, more generally fermionic SPT phases) always reduces to a $(0+1)$-dimensional problem, which can then be answered by the cobordism approach. In addition to the codimension-1 Fermi surface case, we also discuss the codimension-$p$ Fermi surface case briefly. We provide concrete examples to demonstrate the validity of our classification scheme, and make connections to the recent development of Fermi surface symmetric mass generation.

Recently, the magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ was discovered as a great platform to investigate the interplay between magnetism and topology, such as the anomalous Hall effect and the topological magnetoelectric effect. Here, we employ a first-principles approach to investigate the electronic structure of monolayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Our calculations reveal a large spin Hall conductivity $[\sim 844.8 (\hslash /\mathrm{e})\mathrm{S}/\mathrm{cm}]$ accompanying the anomalous Hall effect due to the anticrossing bands near the Fermi level. We also surprisingly find an emergent sixfold rotational symmetry for the spin Berry curvature in the large $\mathit{k}$ region of the Brillouin zone. Based on the symmetry analysis, we conclude that the spin polarization in the band structure is crucial to the emergent symmetry. Our results may provide valuable insights into understanding the large spin Hall conductivity in magnetic topological insulators.

Angle-resolved photoemission spectra of alkali metals exhibit a puzzling, nondispersing peak in the apparent density of states near the Fermi energy. We argue that the holes left behind a significant fraction of photoejected electrons are not wavepacketlike objects used to describe excitations of an equilibrium Fermi liquid, but instead, are relatively localized entities resulting from a photon-induced cavitation in the electron fluid. At the same time, these special localized holes can be thought of as vacancies in a transient Wigner solid. The corresponding contribution to the photoemission current is nondispersive and is tied to the Fermi level; it exhibits certain similarities to photoemission from localized core orbitals such as the presence of recoil currents. Calculated spectra are consistent with experiment. We briefly discuss the present findings in the context of quantum measurement.

The quantum nature of electron spin is crucial for establishing topological invariants in real materials. Since the spin does not in general commute with the Hamiltonian, some of the topological features of the material can be extracted from its study. In insulating materials, the spin operator induces a projected operator on valence states called the spin valence operator. Its spectrum contains information with regard to the different phases of the spin Chern class. If the spin valence spectrum is gapped, the spin Chern numbers are constant along parallel planes thus defining spin Chern insulating materials. If the spin valence spectrum is not gapped, the changes in the spin Chern numbers occur whenever this spectrum is zero. Materials whose spin valence spectrum is gapless will be denoted spin Weyl topological insulators and their definition together with some of their properties will be presented in this work. The classification of materials from the properties of the spin valence operator provides a characterization which complements the existing list of topological invariants.

Resonances are essential for understanding the interactions between light and matter in photonic systems. The real frequency response of the non-Hermitian systems depends on the complex-valued resonance frequencies, which are the poles of electromagnetic response functions. The zeros of the response functions are often used for designing devices since the zeros can be located close to the real axis, where they have significant impact on scattering properties. While methods are available to determine the locations of the poles, there is a lack of appropriate approaches to find the zeros in photonic systems. We present an approach to compute poles and zeros based on contour integration of electromagnetic quantities. This also allows to extract sensitivities with respect to geometrical or other parameters enabling efficient device design. The approach is applied to a topical example in nanophotonics, an illuminated metasurface, where the emergence of reflection zeros due to the underlying resonance poles is explored using residue-based modal expansions. The generality and simplicity of the theory allows straightforward transfer to other areas of physics. We expect that easy access to zeros will enable new computer-aided design methods in photonics and other fields.

We carry out an extensive numerical study of low-temperature electronic transport in quantum point contacts based on twisted bilayer graphene. Assuming ballistic electron dynamics, quantized plateaus in the conductance are observed in defect-free samples when the twisting angle is large enough. However, plateaus are smeared out and hardly noticeable on decreasing the angle. Close to the magic angle, the conductance around the charge neutrality point drops significantly and the quantization steps visible at higher angles are no longer appreciable. Furthermore, we consider the effects of a random distribution of vacancies on the quantum point contact. Whereas the electron-hole symmetry is broken in pristine samples, we find that this symmetry is restored upon increasing the concentration of vacancies. We explain this effect by a reduction of the effective interlayer coupling due to the presence of the vacancies.

The two-dimensional material ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ is a member of the class of insulating van der Waals (vdW) magnets. Here, using high resolution angle-resolved photoemission spectroscopy in a detailed temperature dependence study, we identify a clear response of the electronic structure to a dimensional crossover in the form of two distinct temperature scales marking onsets of modifications in the electronic structure. Specifically, we observe Te $p$-orbital-dominated bands to undergo changes at the Curie transition temperature $T_C$ while the Cr $d$-orbital-dominated bands begin evolving at a higher temperature scale. Combined with neutron scattering, density functional theory calculations, and Monte Carlo simulations, we find that the electronic system can be consistently understood to respond sequentially to the distinct temperatures at which in-plane and out-of-plane spin correlations exceed a characteristic length scale. Our findings reveal the sensitivity of the orbital-selective electronic structure for probing the dynamical evolution of local moment correlations in vdW insulating magnets.

High-temperature superconductivity has been observed in binary hydrides such as ${\mathrm{LaH}}_{10}$ at pressures close to $180\mathrm{G}\mathrm{Pa}$, whereby hydrogen cagelike structures have been identified as a common motif beneficial for high critical temperatures $T_{\text{c}}$. Efforts are now focused on finding hydride high-temperature superconductors at lower pressures. We present evidence for high-temperature superconductivity in binary ${\mathrm{La}}_4{\mathrm{H}}_{23}$ at a pressure of $P=95\mathrm{G}\mathrm{Pa}$, with A15-type arrangement of the lanthanum atoms in the type-I clathrate structure. We synthesize ${\mathrm{La}}_4{\mathrm{H}}_{23}$ from a lanthanum film capped with a palladium catalyst promoting the dissociation of hydrogen. In resistance measurements, we observe superconductivity with a transition temperature $T_{\text{c}}\sim 90\mathrm{K}$. The A15-type lanthanum sublattice is identified in x-ray diffraction on the same sample, in agreement with previous x-ray diffraction and structure prediction studies. Our study reinforces the favorability of cagelike networks of the hydrogen lattice for high-temperature superconductivity.

We study fermions on a finite chain, interacting repulsively when residing on the same and on nearest-neighbor sites, and subjected to a Wannier-Stark linearly varying potential. Using the density matrix renormalization-group numerical technique to solve this generalized extended Hubbard model, the ground state exhibits a staircase of (quasi) plateaus in the average local site density along the chain, decreasing from being doubly filled to empty as the potential increases. These “plateaus” represent locked-in commensurate phases of charge density waves together with band and Mott insulators. These phases are separated by incompressible regions with incommensurate fillings. These results differ from the many-body localization proposed for this model earlier. It is suggested that experimental variations of the slope of the potential and the range of the repulsive interactions will produce such a coexistence of phases which have been individually expected theoretically and observed experimentally for uniform systems.

Topological invariants are global properties of the ground-state wave function, typically defined as winding numbers in reciprocal space. Over the years, a number of topological markers in real space have been introduced, allowing to map topological order in heterogeneous crystalline and disordered systems. Notably, even if these formulations can be expressed in terms of lattice-periodic quantities, they can actually be deployed in open boundary conditions only, as in practice they require computing the position operator $\mathbf{r}$ in a form that is ill defined in periodic boundary conditions. Here, we derive a local Chern marker for infinite two-dimensional systems with periodic boundary conditions in the large supercell limit, where the electronic structure is sampled with one single point in reciprocal space. We validate our approach with tight-binding numerical simulations on the Haldane model, including trivial/topological superlattices made of pristine and disordered Chern insulators. The strategy introduced here is very general and could be applied to other topological invariants and quantum-geometrical quantities in any dimension.