Metallic quantum criticality is frequently discussed as a source for non-Fermi liquid behavior, but controlled theoretical treatments are scarce. Here we identify and study a novel magnetic quantum critical point in a two-dimensional antiferromagnet coupled to a three-dimensional environment of conduction electrons. Using sign-problem-free quantum Monte Carlo simulations and an effective field-theory analysis, we demonstrate that the quantum critical point is characterized by marginal Fermi liquid behavior. In particular, we compute the electrical resistivity for transport across the magnetic layer, which effectively acts like a Kondo impurity. Due to the presence of the marginal Fermi liquid excitations, the resistivity exhibits a linear decrease with temperature at criticality, in contrast to the usual quadratic decrease. Experimental realizations in Kondo heterostructures are discussed.

A recent experiment reported the presence of robust zero-energy states with peculiar properties in a trilayer heterostructure $\mathrm{MnTe}/{\mathrm{Bi}}_2{\mathrm{Te}}_3/\mathrm{Fe}(\mathrm{Te},\mathrm{Se})$. In this study, we provide a comprehensive understanding of the magnetic and electronic properties of this heterostructure. We propose that magnetic Mn-Bi antisite defects are formed in the topmost sublayer of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and remain hidden beneath the MnTe layer. Moreover, we reveal that these defects can give rise to two types of quasiparticles. First, the defect itself generates a Yu-Shiba-Rusinov state, and second, a pair of Majorana zero modes emerge from the superconducting phase domain wall induced by the defect. These two types of quasiparticles exhibit contrasting responses to magnetic fields, temperature, and other factors. The coexistence and mutual cooperation of both types of quasiparticles can account for the experimental observations. Furthermore, we propose a simpler heterostructure that possesses advantages in generating and finely modulating Majorana zero modes.

Experimental results that ${\mathrm{BaIn}}_2{\mathrm{As}}_2$ and $\mathrm{Ca}(\mathrm{Sr}){\mathrm{In}}_2{\mathrm{As}}_2$, which are the same class of alkali metal compounds, belong to different structural phases have puzzled the current materials physics community. Here, we investigate the pressure-induced structural phase transition of $A{\mathrm{In}}_2{\mathrm{As}}_2$ and its accompanying improvement in mechanical and thermal properties. Firstly, the structural stability of the materials and their structural phase transitions under pressure are characterized by enthalpy and double checking by phonon dispersion spectrum. We also confirm the structural phase transitions of the hexagonal and monoclinic phases from a group-theoretic point of view, associating their symmetry operations using transformation matrices. In terms of mechanical properties, we propose an effective scheme for pressure modulation of the anisotropy of $A{\mathrm{In}}_2{\mathrm{As}}_2$ materials and to induce the transformation of $A{\mathrm{In}}_2{\mathrm{As}}_2$ from isotropic to anisotropic (hexagonal) and from brittle to ductile (hexagonal and monoclinic). Meanwhile, we find the negative Poisson's ratio phenomenon under compression and tension, which is favorable for a wide range of applications of this series of materials in aerospace, medicine, sensors, etc. In terms of thermal properties, applying pressure will enhance the structural phase transition temperature of $A{\mathrm{In}}_2{\mathrm{As}}_2$ materials to near room temperature. We further give direct evidence of phonon softening based on group velocity calculations and reveal that phonon softening prevents the heat capacity from reaching the Dulong-Petit limit. Our study provides a theoretical basis for selecting stable structural phases and pioneering thermodynamic property studies of the thermoelectric topological candidate material $A{\mathrm{In}}_2{\mathrm{As}}_2$.

The magnetic properties of two-dimensional single-layer ${\mathrm{CrI}}_3$ at finite temperatures are self-consistently calculated within the nonlinear spin wave formalism, where the Heisenberg exchange interaction and the single-ion magnetic anisotropy energy are calculated from first principles. The lattice strain modulates the exchange interactions and determines the magnetic ground state of the single-layer ${\mathrm{CrI}}_3$ as the ferromagnetic or antiferromagnetic configuration. In both cases, the magnon-magnon interaction at finite temperature softens the magnon spectra. The Curie temperature for the ferromagnetic state varies nonmonotonically with decreasing lattice constant, and the maximum value appears at the compressive strain of $-2.1\%$. The Néel temperature for the antiferromagnetic order linearly increases with increasing compressive strain. The exchange interactions between the next-nearest and third-nearest-neighbor spins are found to play an important role in magnetism. Neglecting these exchange interactions results in a significant deviation in estimating the critical temperature.

Recently, by exhausting low-energy $k·p$ effective models around all band crossings in the 230 space groups, it was found that the maximal topological charge of the twofold-degenerate Weyl point (WP) is 4 only in the spinless setting. However, such a WP has yet to be realized in phonon spectra of crystalline material. In this paper, we perform high-throughput database calculations for charge-4 Weyl phonons in nonmagnetic materials and select the most ideal charge-4 WP per high-symmetry point allowing charge-4 phonons in each material. Moreover, the $k·p$ effective model around the charge-4 WP should be expanded to at least third order, and by explicitly solving for nodal lines, four nearly straight nodal lines carrying “quasiquantized” Berry phase can be anticipated. We select the three best candidates, BaPtSi, SrIrP, and ${\mathrm{Ta}}_3{\mathrm{Al}}_2\mathrm{C}$, to verify the predicted nearly nodal lines and also showcase the phonon surface states associated with the charge-4 WP and the coexisting spin-1 WPs and charge-2 Dirac points. Our results provide a platform of materials with diverse chemical compositions hosting charge-4 WPs, expected to assist experimentalists in selecting suitable materials for further study.

We explore how disorder and interactions conspire in lattice models with sequentially activated hopping to produce $K$-body (or many-body) localized phases. Specifically, we show the rich variety of dynamics produced in the finely tuned interacting Floquet models considered recently [M. Wampler and I. Klich, SciPost Phys. 14, 145 (2023)] is stabilized prethermally (or via many-body localization in some cases) in regions in parameter space near the special points where classical-like dynamics emerges. We also find additional parameter space regions where approximate classical-like behavior can occur away from the special Diophantine points and show how disorder may further stabilize them. Furthermore, the regions in parameter space where Hilbert space fragmentation occurs in the clean system (leading to Krylov subspaces exhibiting frozen dynamics, cellular automation, and subspaces exhibiting signs of ergodic behavior) may also be stabilized by the addition of disorder.

In this paper, we investigate the physical properties of the type-II multiferroic ${\mathrm{GdMn}}_2{\mathrm{O}}_5$ material by means of neutron scattering, electric polarization, and magnetization measurements. A complex (T,H) phase diagram shows up, with especially a field-induced magnetic transition around 11 T at low temperature. The high-field phase is accompanied by an additional electric polarization along both the $a$ and $b$ directions, as authorized by symmetry, but never observed experimentally up to now. While the magnetic properties recover their initial states after driving the field back to zero, the polarization along $a$ shows a significant increase. This behavior is observed for all directions of the magnetic field. It constitutes a novel and striking manifestation of the magnetoelectric coupling, resulting in the establishment of a new ground state at zero magnetic field.

The significantly enhanced superconductivity in the FeSe monolayer on oxide substrates like ${\mathrm{SrTiO}}_3$(001), for which the electron doping from the substrate to the FeSe monolayer is considered a key factor, has attracted extensive interest in the past decade. Here, based on the first-principles electronic structure calculations, we propose that ${\mathrm{Cu}}_3\mathrm{N}$(001) is a promising hole-doping substrate for tuning the electronic and magnetic properties of the epitaxially grown FeSe monolayer. Due to the in-plane lattice strain and the electron redistribution at the $\mathrm{FeSe}\text{/}{\mathrm{Cu}}_3\mathrm{N}$ interface, strong magnetic frustration between the dimer and stripe antiferromagnetic states may exist in the FeSe monolayer. According to the charge transfer analysis, the ${\mathrm{Cu}}_3\mathrm{N}$ substrate can dope $\sim 0.02$ hole per Fe atom to the FeSe monolayer, and the hole doping level can be partially modulated by the external electric field and/or the Cu vacancies in the substrate. These results indicate that $\mathrm{FeSe}\text{/}{\mathrm{Cu}}_3\mathrm{N}$ is a prospective platform for exploring the hole-doped superconductivity in the FeSe-based interfacial system, which may serve as a model system in contrast to electron-doped $\mathrm{FeSe}\text{/}{\mathrm{SrTiO}}_3$.

Half-integer quantized flux vortices appear in honeycomb lattices when the signs of an odd number of couplings around a plaquette are inverted. We show that states trapped at these vortices can be isolated by applying inhomogeneous strain to the system. A vortex then results in localized midgap states lying between the strain-induced pseudo-Landau levels, with $2n+1$ midgap states appearing between the $n\mathrm{th}$ and the ($n+1$)th level. These states are well-defined spectrally isolated and spatially localized excitations that could be realized in electronic and photonic systems based on graphenelike honeycomb lattices. In the context of Kitaev's honeycomb model of interacting spins, the mechanism improves the localization of non-Abelian anyons in the spin-liquid phase, and reduces their mutual interactions. The described states also serve as a testbed for fundamental physics in the emerging low-energy theory, as the correct energies and degeneracies of the excitations are only replicated if one accounts for the effective hyperbolic geometry induced by the strain. We further illuminate this by considering the effects of an additional external magnetic field, resulting in a characteristic spatial dependence that directly maps out the inhomogeneous metric of the emerging hyperbolic space.

Optical magneto-spectroscopy methods (Raman scattering, far-infrared transmission, and photoluminescence) have been applied to investigate the properties of the ${\mathrm{NiPS}}_3$ semiconducting antiferromagnet. The fundamental magnon gap excitation in this van der Waals material has been found to be split into two components, in support of the biaxial character of the ${\mathrm{NiPS}}_3$ antiferromagnet. Photoluminescence measurements in the near-infrared spectral range show that the intriguing 1.475 eV excitation unique to the ${\mathrm{NiPS}}_3$ antiferromagnetic phase splits upon the application of the in-plane magnetic field. The observed splitting patterns are correlated with properties of magnon excitations and reproduced with the simple model proposed. Possible routes towards a firm identification of the spin-entangled 1.475 eV optical excitation in ${\mathrm{NiPS}}_3$, which can hardly be recognized as a coherent Zhang-Rice exciton, are discussed.

We have determined the quasiparticle dispersion for semimetallic $1\mathit{T}\text{−}{\mathrm{TiTe}}_2$ at 20 K using time-of-flight momentum microscopy with tunable soft-x-ray excitation and high-resolution momentum microscopy with 6.4 eV ultraviolet excitation. In particular, we have studied the quasiparticle interactions of the electronic states of the Te $5p$–hole pockets with high Fermi velocity near the $\mathrm{\Gamma }$ point. Kinks in the otherwise parabolic dispersions suggest the onset of many-body interactions at a binding energy of about 30 meV. We attribute these kinks to electron-phonon coupling. Our study complements previously published results on the Ti $3d$ electron pockets near the $M$ and $L$ points. The electron-phonon coupling parameters ($\lambda =0.3$–0.8) are in agreement with previously reported values. An apparently nonzero real part of the self-energy at the Fermi level for one of the two bands may be caused by residual charge-density-wave fluctuations.

The generalized Kadanoff-Baym ansatz (GKBA) is an approximation to the Kadanoff-Baym equations (KBEs) that neglects certain memory effects that contribute to the Green's function at nonequal times. Here we present arguments and numerical results to demonstrate the practical insignificance of the quantities neglected when deriving the GKBA at conditions at which KBE and GKBA are appropriate. We provide a mathematical proof that places a scaling bound on the neglected terms, further reinforcing that these terms are typically small in comparison to terms that are kept in the GKBA. We perform calculations in a range of models, including different system sizes and filling fractions, as well as experimentally relevant nonequilibrium excitations. We find that both the GKBA and KBE capture the dynamics of interacting systems with moderate and even strong interactions well. We explicitly compute terms neglected in the GKBA approximation and show, in the scenarios tested here, that they are orders of magnitude smaller than the terms that are accounted for, i.e., they offer only a small correction when included in the full KBEs.

Phonons have provided an ideal platform for a variety of intriguing physical states, such as non-Abelian braiding and the Haldane model. It is promising that phonons will realize the complicated nodal states accompanying unusual quantum phenomena. Here, we propose the hybrid nodal surface and nodal line ($\mathrm{NS}+\mathrm{NL}$) phonons beyond the single-genre nodal phonons. We categorize the $\mathrm{NS}+\mathrm{NL}$ phonons into two- and four-band situations based on symmetry analysis and compatibility relationships. Combining database screening with first-principles calculations, we identify the ideal candidate materials for realizing all categorized $\mathrm{NS}+\mathrm{NL}$ phonons. Our calculations and tight-binding models further demonstrate that the interplay between NS and NL induces unique phenomena. In space group (SG) 113, the quadratic NL acts as a hub of the Berry curvature between two NSs, generating ribbonlike surface states (SSs). In SG 128, the NS serves as the counterpart of the Weyl NL, in which NS-NL mixed topological SSs are observed. Our findings extend the scope of hybrid nodal states and enrich the phononic states in realistic materials.

Current-induced spin polarization (CISP), which appears when the inversion symmetry is broken, has been experimentally shown to generate spin currents. We have demonstrated in our recent theory that the antiparallel CISP in a double-quantum-well structure with the inversion symmetry can also create spin currents. In this paper we consider the sublattice-staggered CISP in a group-IV buckled atomic layer and theoretically study the spin current extracted from the spin-polarized atomic layer into another with identical structure. We derive a formula for the interlayer spin current in both the presence and the absence of the inversion symmetry and show that the interlayer spin current, in the inversion-symmetric case, additionally has a contribution from intersubband spin matrix elements due to spin degeneracy which is comparable in magnitude to that from intrasubband matrix elements. We calculate the interlayer spin current as a function of the interlayer twist angle using the tight-binding model and the Boltzmann equation in the relaxation-time approximation and choosing silicene as an example atomic layer. We show that the interlayer spin current created by the sublattice-staggered CISP in inversion-symmetric silicene is comparable in magnitude to that obtained from silicene with a 1 V/nm out-of-plane electric field applied to break the inversion symmetry. We find that the calculated interlayer spin current exhibits various features by twisting two layers. Most interesting is that the twist, which breaks the mirror symmetry to make the system chiral, gives rise to the component of the interlayer spin current with in-plane spin direction perpendicular to the CISP in addition to that parallel to the CISP.

The Berry curvature (BC), manifesting the band structure geometry, is extensively employed to describe the intrinsic transport properties, such as the intrinsic anomalous Hall effect and the spin Hall effect. The BC also can cause the intrinsic antidamping spin-orbit torque (SOT), but is regardless of the fieldlike SOT which is usually of extrinsic origins. Here, we study the current-induced SOT and reveal a new intrinsic fieldlike SOT in addition to the intrinsic antidamping SOT. The topological origin of the intrinsic SOTs can be understood with the BC defined in the mixed momentum-magnetization parameter space $(\mathit{k},\mathit{M})$, rather than the conventional BC, which is especially important for systems with complicated spin-orbit interaction. We apply our theory to a magnetic semi-Dirac semimetal and find that the interplay between the peculiar spin textures and anisotropic band structure generates the intrinsic fieldlike SOT, which emerges even in the absence of band gap and for the Fermi energy far away from the Dirac points, quite different from the intrinsic antidamping SOT or anomalous Hall effect. Our findings provide another perspective to understand the physics of the current-induced SOTs.