In this work, we demonstrate an anomalous non-Hermitian skin effect which decays from both ends of a system consisting of two coupled Hermitian chains induced by nonreciprocal interchain cross-coupling. An intriguing feature of the system is that its eigenenergy spectrum in the thermodynamic limit deviates from the generalized Brillouin zone (GBZ), contrary to that of conventional non-Hermitian systems. The thermodynamic-limit energy spectrum is however restored to the GBZ by the presence of even an infinitesimal amount of gain and/or loss in the system. In this case, the system exhibits a critical non-Hermitian skin effect (CNHSE) similar to that of coupled non-Hermitian chains, whereby the eigenspectrum starts to approach the GBZ beyond some critical size. Furthermore, the non Hermitian skin effect becomes less pronounced as the system size exceeds this critical size. We analytically explain these peculiar features which highlight the important role of gain and loss terms as well as interchain coupling in tuning non-Hermitian skin modes. Finally, we model the implementation of the model based on the topolectrical platform, using realistic circuit parameter values, to demonstrate the experimental feasibility of the model, thus suggesting a new avenue for the modulation of the skin mode characteristics of open systems.

Quantum oscillations provide a valuable tool for probing the Fermi surfaces and their topology in solids, offering critical insights into transport and electronic properties. Notably, Onsager's relation establishes a direct relation between the oscillation frequency and the Fermi surface area. However, estimating the oscillation phase accumulated along quantum orbits in the computation is still challenging due to multiple contributions from the Berry phase, orbital, and spin magnetic moments. Particularly, the wave function's gauge freedom, which allows a unitary rotation among degenerate states, must be carefully treated. In this work, we develop a gauge-independent Wilson loop scheme to evaluate all oscillation phase contributions and apply it to the new kagome metal ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$. We find that the spin-orbit coupling dramatically alters the oscillation phases compared to the spinless case. Additionally, the oscillation phases of representative quantum orbits exhibit a distinct three-dimensional signature despite the cylinderlike Fermi surface geometry. Our work unveils the Fermi surface topology of ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$ and paves the way for theoretical investigations of quantum oscillations in practical materials.

Layer stacking presents a promising avenue for manipulating the physical properties of two-dimensional materials. The van der Waals layered semiconductor CrSBr, which exhibits A-type antiferromagnetism and a relatively high Néel temperature, has been successfully exfoliated into atomically thin sheets. In this paper, we investigate the structural, lattice dynamical, electronic, magnetic, and optical properties of four distinct stacking structures of CrSBr bilayers using first-principles calculations and Monte Carlo simulations. Our findings show that, though the most energetically favorable bilayer structure retains the stacking pattern of the bulk counterpart, three other high-symmetry stacking structures can be achieved by sliding one of the layers along three distinct directions, with energy cost comparable to that observed in ${\mathrm{MoS}}_2$ bilayer. All these four bilayers exhibit semiconductor behavior with A-type antiferromagnetic ordering, similar to the bulk material, and demonstrate closely aligned Néel temperatures. Moreover, these bilayers exhibit relatively low lattice thermal conductivities, pronounced anisotropy, and a strong dependence on stacking patterns. This behavior is attributed to significant phonon-phonon scattering arising from avoided crossings between acoustic and optical phonons, as well as the presence of flat optical phonon bands in the low-frequency region. While the electronic structures and optical properties of these bilayers show weak dependence on the stacking pattern for antiferromagnetic ordering, they undergo significant changes for ferromagnetic ordering, influencing the band gap, valence and conduction band splitting, and effective mass. Furthermore, we found that antiferromagnetic ordering can transition to ferromagnetic under intense light illumination. Thus, the integration of layer stacking and light illumination offers an effective means to control the heat transfer, magnetic, and optical properties of CrSBr bilayers.

High-pressure electride (HPE) superconductors often maintain structural symmetry over a wide pressure range, whereas their interstitial anion electrons (IAEs) undergo significant changes in topology and energy levels, potentially resulting in topological Lifshitz transitions in their Fermi surfaces. These Lifshitz transitions can play critical roles in influencing superconducting properties. However, their impact on the superconductivity of HPEs remains largely unexplored. To address this gap, we investigate a novel HPE, $\mathrm{L}{\mathrm{i}}_4\mathrm{Pd}$, which features distinct Fermi surfaces, including a hole ellipsoid and a warped electron rectangular cylinder. This combination satisfies the conditions for a Kohn anomaly, suggesting that $\mathrm{L}{\mathrm{i}}_4\mathrm{Pd}$ may be a potential HPE superconductor. Our theoretical calculations reveal that under high pressures, the Fermi surface of $\mathrm{L}{\mathrm{i}}_4\mathrm{Pd}$ undergoes a Lifshitz transition that affects the momentum distribution of electrons on the Fermi surface, leading to changes in the energy provided by phonons for forming Cooper pairs. Moreover, this transition modifies the phonon frequencies involved in electron-phonon coupling. The superconducting transition temperature of $\mathrm{L}{\mathrm{i}}_4\mathrm{Pd}$ is calculated to be 34.5 K at 250 GPa, which is comparable to that of $\mathrm{L}{\mathrm{i}}_6\mathrm{P}$ (39.3 K at 270 GPa). Our findings indicate that the formation of closed Fermi surfaces with extended spatial scales is a general feature that could be leveraged to design new classes of high-temperature superconductors among HPEs.

In systems with unconventional Rashba bands, we propose that a finite-momentum pairing state can emerge without the need for an external magnetic field. We analyze the phase transition from a zero-momentum Bardeen-Cooper-Schrieffer (BCS) state to a finite-momentum pairing state using a microscopic interaction model. We demonstrate that the coexistence of zero-momentum and Larkin-Ovchinnikov (LO)-type pairings in different channels provides a well understanding of recent experimental observations of pair-density-wave (PDW) states. Furthermore, we propose that an s-wave BCS superconductor-unconventional Rashba metal (SC-URM) junction can generate an LO-type pairing state via an orbital-selective proximity effect. This nontrivial state can be detected by measuring the Josephson current in an SC-URM-SC junction or through Josephson scanning tunneling microscopy/spectrosscopy (JSTM/S). Our results reveal that the internal multiorbital degrees of freedom play a crucial role in facilitating finite-momentum pairing states.

Van-Hove (vH) singularities in the vicinity of the Fermi level facilitate the emergence of electronically mediated Fermi surface instabilities. This is because they provide a momentum-localized enhancement of density of states promoting selective electronic scattering channels. High-temperature topological superconductivity has been argued for in graphene at vH filling which, however, has so far proven inaccessible due to the demanded large doping from pristine half filling. We propose compensated metallicity as a path to unlock vH-driven pairing close to half filling in an electronic honeycomb lattice model. Enabled by an emergent multipocket fermiology, charge compensation is realized by strong breaking of chiral symmetry from intrasublattice hybridization, while retaining vH dominated physics at the Fermi level. We conclude by proposing tangible realizations through quantum material design.

We explore the collective response in an excitonic insulator phase in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ using a semirealistic model including relevant lattice and electronic instabilities. We calculate order-parameter susceptibility and Raman response within a time-dependent Hartree-Fock approach. Contrary to the standard expectations, the amplitude mode frequency does not coincide with the single-particle gap but has a higher frequency. We find a phase mode that is massive because the excitonic condensation breaks a discrete symmetry only and that becomes heavier as the electron-lattice coupling is increased. These features are expected to apply to generic realistic excitonic insulators. We discuss scenarios under which the phase mode does not appear as a sharp in-gap resonance.

The critical skin effect, an intriguing phenomenon in non-Hermitian systems, displays sensitivity to system size and manifests distinct dynamical behaviors. In this Letter, we propose a scheme to achieve the critical non-Hermitian skin effect of exciton polaritons in an elongated microcavity system. We show that by utilizing longitudinal-transverse spin splitting and spin-momentum-locked gain, a critical non-Hermitian skin effect can be achieved in a continuous system without the need for an underlying lattice. We find that a phase transition can be induced by changing the cavity detuning with respect to the exciton energy. We identify a measurable order parameter associated with this phase transition and demonstrate the corresponding critical behavior. Our work offers a flexible approach to manipulating non-Hermitian phases of exciton polaritons, thereby expanding the potential applications of polaritonic devices.

Numerous experimental and theoretical studies have shown that alkali and alkaline metals transform into various structures with large open interstitials under pressure, an intricate behavior that is caused by the electron transfer among $s, p$, and $d$ atomic and quasiatomic orbitals. However, little is known about the behavior of these metals beyond this pressure region. Using first-principles calculations and crystal structure search methods, we show that Cs returns to simple close-packed structures under ultrahigh pressure, including the transition from double hcp to fcc structure at 183 GPa and a further transition into hcp at 655 GPa. More significantly, under ultrahigh pressure, the $5p$ electrons are activated and form strong chemical bonds between neighboring Cs atoms that govern the properties of Cs under very high pressure, including the transition from fcc to hcp.

The topological states in non-Hermitian Floquet systems possess intriguing physical phenomena. Unlike crystals, quasicrystals offer a unique platform for exploring topological states. Here, we generalize the non-Hermitian Floquet higher-order topological states to quasicrystals. We develop the non-Hermitian Floquet multipole numbers as a real-space topological invariant to effectively characterize higher-order topology. We establish the physical connection between the different delocalization properties of non-Hermitian Floquet 0 and $\pi$ modes and the lattice arrangement of quasicrystals as well as non-Hermitian skin effect. By constructing effective models and evolution equations, we reveal the modulation mechanism and evolution characteristics of non-Hermitian Floquet $\pi$ modes in quasicrystals with 12-fold rotational symmetry. Our results demonstrate the flexible tunability of non-Hermitian Floquet topological states in quasicrystals, offering a different avenue for investigating unique physical phenomena and applications of quasicrystals within non-Hermitian Floquet systems.

Born dynamical charges (${\mathbf{Z}}^{\mathrm{dyn}}$) play a key role in the lattice dynamics of most crystals, including both insulators and metals in the nonadiabatic (“clean”) regime. Very recently, the so-called static Born charges, ${\mathbf{Z}}^{\text{stat}}$, were introduced [G. Marchese et al., Nat. Phys. 20, 88 (2024)] as a means to modeling the long-wavelength behavior of polar phonons in overdamped (“dirty”) metals. Here we present a method to calculate ${\mathbf{Z}}^{\text{stat}}$ directly at the zone center by applying the “$2n+1$” theorem to the long-wavelength expansion of the charge response to a phonon. Furthermore, we relate ${\mathbf{Z}}^{\text{stat}}$ to the charge response to a uniform strain perturbation via an exact sum rule, where the quantum capacitance of the material plays a crucial role. We showcase our findings via extensive numerical tests on simple metals aluminum and copper, polar metal ${\mathrm{LiOsO}}_3$, and doped semiconductor ${\mathrm{SrTiO}}_3$. Based on our results, we critically discuss the physical significance of ${\mathbf{Z}}^{\text{stat}}$ in light of their dependence on the choice of the electrostatic reference, and on the length scale that is assumed in the definition of the macroscopic potentials.

We report on the anomalous transverse magnetothermoelectric properties of the topological spin semimetallic equiatomic quaternary Heusler alloys (EQHAs) FeCrRhSi and FeCrRhGe over a wide temperature range between $130\mathrm{and}295\mathrm{K}$. Magnetic measurements reveal ferromagnetic behavior above room temperature with high Curie temperatures ($T_C>400\mathrm{K}$) in these samples, where energetically competing ferromagnetic (FM) and antiferromagnetic (AFM) phases coexist at low temperatures, giving rise to field-induced metamagnetic transitions. The electrical resistivity of both samples exhibits semimetallic characteristics throughout the measured temperature range, with low-temperature resistivity upturns driven by Kondo-like scattering. Throughout this range, the longitudinal Seebeck coefficient is negative in sign for both FeCrRhSi and FeCrRhGe, indicating that electrons are the majority carriers for thermally driven charge transport. Notably, both samples exhibit giant values of the anomalous Nernst coefficient $\left(S_{zy}^{\mathrm{ANE}}\right)$ at room temperature, with $S_{zy}^{\mathrm{ANE}}$ values of $\left(0.272\pm 0.02\right)\mu \mathrm{V}{\mathrm{K}}^{-1}$ for FeCrRhSi and $\left(0.295\pm 0.02\right)\mu \mathrm{V}{\mathrm{K}}^{-1}$ for FeCrRhGe. These values are nearly three times larger than those reported for other EQHAs in the literature. An in-depth analysis reveals that intrinsic Berry curvature drives such large anomalous Nernst effects (ANEs) in our EQHAs. We show that in these samples, the contribution of the transverse thermoelectric conduction to the observed large ANE dominates over that of anomalous Hall effect acting on the charge carrier flow driven by the longitudinal Seebeck effect. The ratio of the anomalous off-diagonal transverse thermoelectric conductivity (${\alpha }_{zy}^A$) to the anomalous Hall conductivity (${\sigma }_{zy}^{\mathrm{AHE}}$) $|{\alpha }_{zy}^A/{\sigma }_{zy}^{\mathrm{AHE}}|$ lies between $0.2\left(\frac{k_B}{e}\right)$ and $\left(\frac{k_B}{e}\right)$ at room temperature, further highlighting the intrinsic Berry curvature as the underlying physical origin of the observed large ANEs.

In this paper, we expand upon the source-free (SF) exchange correlation (XC) functional developed by Sangeeta Sharma and coworkers to plane-wave density functional theory (DFT) based on the projector augmented wave (PAW) method. This constraint is implemented by the current authors within the VASP source code, using a fast Poisson solver that capitalizes on the parallel three-dimensional fast Fourier transforms (FFTs) implemented in VASP. Using this modified XC functional, we explore the improved convergence behavior that results from applying this constraint to the GGA-$\text{PBE}+U+J$ functional. In the process, we compare the noncollinear magnetic ground state computed by each functional and their SF counterpart for a select number of magnetic materials in order to provide a metric for comparing with experimentally determined magnetic orderings. We observe significantly improved agreement with experimentally measured magnetic ground-state structures after applying the source-free constraint. Furthermore, we explore the importance of considering probability current densities in spin-polarized systems, even under no applied field. We analyze the XC torque as well, in order to provide theoretical and computational analyses of the net XC magnetic torque induced by the source-free constraint. Along these lines, we highlight the importance of properly considering the real-space integral of the source-free local magnetic XC field. Our analyses on probability currents, net torque, and constant terms draw additional links to the rich body of previous research on spin-current density functional theory (SCDFT), and pave the way for future extensions and corrections to the SF corrected XC functional.

Ferrimagnets are antiparallel-ordered magnetic states in a bipartite lattice with two alternating unequal spins, which exhibit both ferromagnetic and antiferromagnetic properties. Several theoretical studies have explored the magnetic properties of ferrimagnets, but the entanglement entropy of ferrimagnets with arbitrary spin combinations has not been studied. In this study, we analytically derive the intersublattice entanglement entropy of a ferrimagnetic spin chain using the method that has been applied to the antiferromagnetic case. The analytical results are numerically verified using the density-matrix renormalization group. Going beyond the results for antiferromagnets, the entanglement entropy of ferrimagnets for fixed anisotropy is shown to solely depend on the difference of spins relative to its geometric average, and the quantity is shown to be stable against small-parameter variations.

We investigate spin-orbit torque (SOT) effective fields in a (Ga,Mn)(As,P) ferromagnetic semiconductor (FMS) film with perpendicular magnetic anisotropy using second harmonic Hall voltage measurements. Both the first harmonic (${\mathrm{V}}_{1\omega }$) and second harmonic (${\mathrm{V}}_{2\omega }$) Hall voltage signals are recorded simultaneously as the external field is swept near the film plane along two current directions: [110] and $\left[1\overline{1}0\right]$ crystallographic directions. The ${\mathrm{V}}_{2\omega }$ signal displays distinct field-dependent characteristics, which are influenced by thermal effects, mainly the anomalous Nernst effect. In order to obtain accurate values of the SOT effective fields, thermal contributions are accounted for by careful analysis of the ${\mathrm{V}}_{2\omega }$ signal. The effective fields, ${\mathrm{H}}_{\mathrm{DLT}}$ (corresponding to dampinglike torque) and ${\mathrm{H}}_{\mathrm{FLT}}$ (corresponding to fieldlike torque), are found to be significantly larger for the $\left[1\overline{1}0\right]$ current direction compared to the [110] direction. This observation agrees with the relative strengths of net spin-orbit fields arising from the Rashba and Dresselhaus effects in (Ga,Mn)(As,P) film under tensile strain. Additionally, ${\mathrm{H}}_{\mathrm{DLT}}$ is consistently stronger than ${\mathrm{H}}_{\mathrm{FLT}}$ for both current directions, confirming the dominant role of the dampinglike torque in SOT magnetization switching experiments in FMS films.

Spin-orbit torques (SOTs) caused by spin currents generated in a ferromagnetic electrode enable a fast and deterministic magnetization switching. One SOT ($y$-SOT), polarized orthogonal to both electric current flowing in the electrode and easy axis of a ferromagnetic free layer, causes fast magnetization instability. The other SOT ($z$-SOT) points to the easy-axis direction and leads to the deterministic switching. Here, we evaluate the magnetization switching probability by these SOTs for various values of electric current density and the ratio of two SOTs from numerical simulation of the Landau-Lifshitz-Gilbert (LLG) equation. It is found that the switching probability is maximized when the electric current density is close to a critical value for the magnetization destabilization solely by the $y$-SOT. The origin of such a current dependence is investigated by analyzing temporal dynamics, spectra of the magnetization distribution, and a steady-state solution of the LLG equation. We reveal that the maximization of the switching probability originates due to two different switching behaviors. In the low current region, the magnetization in some trials remains near the initial state because of the weak $y$-SOT, and thus, the switching error occurs. The number of such trials, which represents the switching error, decreases as the electric current density increases because both $y$ and $z$-SOTs prompt the switching. In the large current region, the large $y$-SOT immediately tilts the magnetization toward the switched direction. However, since the $y$-SOT prefers an in-plane magnetized state, the switching error due to a probabilistic return to the initial state after turning off the electric current density occurs. The switching error in this region tends to increase with increasing current density. As a result, the switching probability is optimized when the electric current density is close to the critical value.

In this study, we demonstrate that, even in systems nominally lacking structural inversion asymmetry—such as Pt/Co/Pt films, which exhibit negligible net Dzyaloshinskii-Moriya interaction—intralayer symmetry breaking within the magnetic layer can induce inversion symmetry breaking. This intralayer symmetry breaking leads to an antisymmetric velocity contribution, resulting in different velocities for left- and right-handed chiral domain walls (DWs). Our experimental data and micromagnetic simulation results verify that the antisymmetric component of chiral DW velocity becomes more dominant as the magnetic layer thickness increases. Our findings introduce a new degree of freedom for engineering inversion symmetry breaking in such systems and enhance the understanding of diverse chiral phenomena in magnetic materials.

We theoretically study the creation of a helical-like structure in a soft magnetic material. This forms when a slowly rotating magnetic field is applied, in plane, to a very soft, low-exchange film (50-1000 atomic layers of Cu-doped Permalloy), which is magnetically pinned at the bottom. Both a self-consistent local mean-field theory as well as a spin dynamic approach (implementing the Landau-Lifshitz equation) are employed to find the stable twisted structures. Results from the two methods match well. We find that multiple full rotations of the magnetization—or “twists”—are possible in these films. At some point an instability occurs in the system at a critical applied field angle, preventing additional full rotations, and often causing one or more twists to be annihilated. The magnetic structure that forms depends on the thickness of the film, the strength of the applied field, and the exchange coupling strength. Analytic energy minimization calculations give an accurate estimate for how many twists will fit into a given film. We study the energy stored and released by the twisted structure, dynamic effects such as the unwinding frequency if the applied field is removed, and potential applications including energy storage This model presents a substantial advancement compared to previous similar studies, as it only fixes one end of the system and applies the rotating magnetic field to the entire structure, enabling a reduction in the strength of the applied field.

The coexistence of ferroelectricity and superconductivity is a forefront research topic in condensed matter physics, and achieving this coexistence in a single-phase material is particularly challenging, as the former is typically restricted to insulators while the latter demands metallicity. Here, we show that coexistence of robust ferroelectricity and superconductivity can be realized by properly hole doping a fertile ferroelectric insulator that possesses antibonding states as its highest valence bands. Using a prototypical two-dimensional ferroelectric insulating SnS monolayer as an example, we demonstrate that a moderate level of hole doping (∼0.30 hole per formula unit) not only effectively converts the system into a metal, but counterintuitively also enhances its ferroelectricity rather than weakens or even completely destroys the ferroelectricity as commonly encountered. The underlying physical origin is traced to the shifting of the Fermi level into the occupied antibonding bands, which renders the system metallicity and simultaneously also strengthens the in-plane bonding nature associated with polar displacement. We further show that the resultant ferroelectric metal can harbor superconductivity with a pronounced superconducting transition temperature of ∼7 K, due to the emergence of strong Fermi-surface nesting upon hole doping and softening of the out-of-plane acoustic phonon mode that couples effectively with the itinerant electrons. These findings provide a generic approach for achieving ferroelectric superconductivity with appealing device potentials.