We report the results of low-temperature magnetization and specific heat measurements of ${\mathrm{Sr}}_3{\mathrm{CoTa}}_2{\mathrm{O}}_9$ powder, in which ${\mathrm{Co}}^{2+}$ ions with effective spin-1/2 form a uniform triangular lattice in the $ab$ plane. It was found that ${\mathrm{Sr}}_3{\mathrm{CoTa}}_2{\mathrm{O}}_9$ undergoes successive antiferromagnetic transitions at $T_{\mathrm{N}1}=0.97\mathrm{K}$ and $T_{\mathrm{N}2}=0.79\mathrm{K}$ at zero magnetic field. As the magnetic field increases, both $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$ decrease monotonically. The obtained magnetic field vs temperature phase diagram together with a sharp magnetization anomaly at a saturation field of ${\mu }_0{H}_{\mathrm{s}}=2.3$ T indicates that ${\mathrm{Sr}}_3{\mathrm{CoTa}}_2{\mathrm{O}}_9$ is described as a spin-1/2 three-dimensional triangular-lattice antiferromagnet with a weak easy-axis anisotropy. We discuss the characteristics of the magnetic phase diagram, which approximates the phase diagram for the magnetic field perpendicular to the $c$ axis.

We report a study of ${\mathrm{NiBi}}_3$ single crystals by means of electrical-resistivity, magnetization, and muon-spin rotation and relaxation ($\mu \mathrm{SR}$) measurements. As a single crystal, ${\mathrm{NiBi}}_3$ adopts a needlelike shape and exhibits bulk superconductivity with $T_c\approx 4.1$ K. By applying magnetic fields parallel and perpendicular to the $b$ axis of ${\mathrm{NiBi}}_3$, we establish that its lower and upper critical fields as well as the magnetic penetration depths show slightly different values, suggesting a weakly anisotropic superconductivity. In both cases, the zero-temperature upper critical fields are much smaller than the Pauli-limit value, indicating that the superconducting state is constrained by the orbital pair breaking. The temperature evolution of the superfluid density, obtained from transverse-field $\mu \mathrm{SR}$, reveals a fully gapped superconductivity in ${\mathrm{NiBi}}_3$, with a shared superconducting gap ${\mathrm{\Delta }}_0=2.1k_{\mathrm{B}}{T}_c$ and magnetic penetration depths ${\lambda }_0$ = 223 and 210 nm for $H\parallel b$ and $H\perp b$, respectively. The lack of spontaneous fields below $T_c$ indicates that time-reversal symmetry is preserved in ${\mathrm{NiBi}}_3$. The absence of a fast muon-spin relaxation and/or precession in the zero-field $\mu \mathrm{SR}$ spectra definitely rules out any type of magnetic ordering in ${\mathrm{NiBi}}_3$ single crystals. Overall, our investigation suggests that ${\mathrm{NiBi}}_3$ behaves as a conventional $s$-type superconductor.

Ferroelectric materials can exhibit metastable phases when exposed to THz pulses, characterized by a polarization integration capability related to the amplitude and frequency of the pulses. These so-called “hidden” phases enable gradual switching of polarization that can be utilized in artificial synapses for nonconventional (neuromorphic) computing machines. In this work, we employ large-scale molecular-dynamics simulation based on an effective Hamiltonian approach, and we report on the discovery of hidden phases in Zr- and Nb-doped barium titanate ($\mathrm{BaTi}{\mathrm{O}}_3$). We investigate the formation and the stability of those phases at different stimuli and temperatures (20 and 200 K). Our results shed light on the compositional dependence of the properties of these phases, demonstrating the potential of lead-free relaxor ferroelectrics for near-room-temperature neuromorphic computing.

We study the response of a quantum magnet with spin-orbit coupling (SOC) to a Zeeman field by constructing effective actions and performing renormalization group (RG) analysis. There are several novel classes of quantum phase transitions at a low $h_{c1}$ and an upper critical field $h_{c2}$ driven by magnon condensations at commensurate (C) or incommensurate (IC) momenta $0<k_0<\pi$. The intermediate IC-skyrmion crystal (IC-SkX) phase is controlled by a line of fixed points in the RG flows labeled by $k_0$. We derive the relations between the quantum spin and the order parameters of the effective actions which determine the spin-orbital structures of the IC-SkX phase. We also analyze the operator contents near $h_{c1}$ and $h_{c2}$ which determine the exotic excitation spectra inside the IC-SkX. The intrinsic differences between the magnon condensations at the C and IC momenta are explored. The two critical fields $h_{c1}<h_{c2}$ and the intermediate IC-SkX phase could be a generic feature to any quantum magnets with SOC in a Zeeman field. Experimental implications to some materials or cold atom systems with SOC in a Zeeman field are presented.

Hematite is a canted antiferromagnetic insulator, promising for applications in spintronics. Here we present ab initio calculations of the tensorial exchange interactions of hematite and use them to understand its magnetic properties by parametrizing a semiclassical Heisenberg spin model. Using atomistic spin dynamics simulations, we calculate the equilibrium properties and phase transitions of hematite, most notably the Morin transition. The computed isotropic and Dzyaloshinskii–Moriya interactions result in a Néel temperature and weak ferromagnetic canting angle that are in good agreement with experimental measurements. Our simulations show how dipole-dipole interactions act in a delicate balance with first and higher-order on-site anisotropies to determine the material's magnetic phase. Comparison with spin-Hall magnetoresistance measurements on a hematite single crystal reveals deviations of the critical behavior at low temperatures. Based on a mean-field model, we argue that these differences result from the quantum nature of the fluctuations that drive the phase transitions.

Out-of-plane spin polarization (${\sigma }_z$) has potential applications for magnetic memory with high storage density and low energy consumption. Several noncollinear antiferromagnets have been confirmed to generate ${\sigma }_z$ due to their triangular spin configuration on kagome planes, including $\mathrm{Ir}{\mathrm{Mn}}_3$. Apart from the spin configuration of the (110)-oriented $\mathrm{Ir}{\mathrm{Mn}}_3$, we demonstrate that the interface of $\mathrm{Ir}{\mathrm{Mn}}_3$/permalloy also contributes to the generation of ${\sigma }_z$. With Cu insertion between $\mathrm{Ir}{\mathrm{Mn}}_3$ and permalloy, interfacial ${\sigma }_z$ vanishes, which further supports the interfacial origin of ${\sigma }_z$. We are not only convinced that the interface-relevant ${\sigma }_z$ is independent of exchange coupling between $\mathrm{Ir}{\mathrm{Mn}}_3$ and permalloy but also propose several possible origins of interface-relevant ${\sigma }_z$. Our findings enrich the understanding of generating ${\sigma }_z$ in antiferromagnets/ferromagnets bilayers. The interface-relevant ${\sigma }_z$ broadens the scope of material design and proposes a potential path to optimize magnetic memories with low power consumption.

It is shown that a term in the form of Dzyaloshinskii-Moriya interaction (DMI) contributes to the free energy of a ferromagnetic (FM) film on a paramagnetic (PM) (an FM above the critical temperature $T_c$) or superconducting (SC) substrate occurring in the London limit. This contribution results from magnetostatic interaction between the film and substrate under which the FM magnetization induces magnetization in the PM or supercurrent in the SC substrate and interacts with their stray fields. Strikingly, in the FM/PM system this effective DMI stabilizes chiral magnetic textures, e.g., magnetic skyrmions (MSk's) of the Néel type, which is in contrast to that in the FM/SC one. A strong temperature sensitivity of the effective DMI allows for tuning the coupling between the FM film and PM or SC substrate and thus controlling the MSk radius in FM/PM.

Recently, the non-Hermitian system has attracted great attention. However, the interplay between the non-Hermiticity and impurities in superconductors remains to be explored. In this work, we investigate the magnetic impurity chain on a three-dimensional non-Hermitian $s$-wave superconductor with the imbalanced pairing term. For the Yu-Shiba-Rusinov (YSR)-type state, we find that their fate is determined by the generalized in-gap nature of these states: These in-gap states and related topological phases survive only when the effective superconducting gap exists. With the increasing strength of the non-Hermitian part of the pairing term, the host superconductor is lead to a non-Hermitian gapless phase with exceptional surfaces, while these in-gap states and topological phases disappear. Our work generalizes the in-gap state nature of the YSR state to the non-Hermitian superconductor and reveals the relation between the exceptional surfaces and the YSR-type state, which provides a minimal starting point to explore the interplay between the non-Hermiticity and magnetic impurities in the $s$-wave superconductor.

Monolayers of two-dimensional (2D) van der Waals (vdW) materials grown on dielectric oxides exhibit unprecedented physical properties. For example, the superconducting transition temperature of an FeSe monolayer on ${\mathrm{SrTiO}}_3$ is by far higher than its bulk counterpart. However, how the charge distribution across the interface looks like and how it helps to improve these properties is hitherto unknown. Here, using momentum- and energy-resolved high-resolution electron spectroscopy, we probe the dynamic charge response of the $\mathrm{FeSe}/{\mathrm{SrTiO}}_3\left(001\right)$ heterostructures and, thereby, demonstrate the existence of a charge depletion layer at the interface. The presence of this depletion layer, accompanied with a considerably large charge transfer, leads to a renormalization of the ${\mathrm{SrTiO}}_3$ energy bands and a substantial band bending at the interface. Our discovery paves the way for designing novel 2D superconductors through interface engineering. The phenomenon is expected to occur at the interface of many vdW monolayers and dielectric oxides.

It is well known that phonons can overscreen the bare Coulomb electron-electron repulsion, turning it into the effective attraction that binds the Cooper pairs responsible for BCS superconductivity. Here, we use a simple lattice model to prove that the counterpart of this is also possible, whereby phonons overscreen the bare electron-hole attraction and may turn it repulsive at short distances, driving exciton dissociation in certain regions of the parameter space. We argue that this phonon-mediated short-range screening may play an important role in the physics of organic solar cell materials (and other materials with strong electron-phonon coupling) and could point the way to new strategies for optimizing their efficiencies.

Smearing techniques are widely used in first-principles calculations of metallic and magnetic materials where they improve the accuracy of Brillouin-zone sampling and lessen the impact of level-crossing instabilities. Smearing introduces a fictitious electronic temperature that smooths the discontinuities of the integrands; consequently, a corresponding fictitious entropic term arises, and needs to be considered in the total free-energy functional. Advanced smearing techniques—such as Methfessel-Paxton and cold smearing—have been introduced to guarantee that the system's total free energy remains independent of the smearing temperature, at least, up to the second order. In doing so, they give rise to nonmonotonic occupation functions (and, for Methfessel-Paxton, nonpositive definite), which can result in the chemical potential not being uniquely defined. We explore this shortcoming in detail and introduce a numerical protocol utilizing Newton's minimization method that is able to identify the desired Fermi energy. We validate the method by calculating the Fermi energy of $\sim 20 000$ materials and comparing it with the results of standard bisection approaches. In passing, we also highlight how traditional approaches, based on Fermi-Dirac or Gaussian smearing, are actually equivalent for all practical purposes, provided the smearing width is appropriately renormalized by a factor of $\sim 2.565$.

The Kohn-Sham (KS) system is an auxiliary system whose effective potential is unknown in most cases. It is in principle determined by the ground-state density and it has been found numerically for some low-dimensional systems by inverting the KS equations starting from a given accurate density. For solids, only approximate results are available. In this work, we determine accurate exchange-correlation (xc) potentials for Si and NaCl using the ground-state densities obtained from auxiliary field quantum Monte Carlo calculations. We show that these xc potentials can be rationalized as an ensemble of a few local functions of the density, whose form depends on the specific environment and can be well characterized by the gradient of the density and the local kinetic energy density. The KS band structure can be obtained with high accuracy. The true KS band gap turns out to be larger than the prediction of the local density approximation, but significantly smaller than the measurable photoemission gap, which confirms previous estimates. Finally, our findings show that the conjecture that very different xc potentials can lead to very similar densities and other KS observables is true also in solids, which questions the meaning of details of the potentials and, at the same time, confirms the stability of the KS system.

Topological quantum materials, which feature nontrivial band topology, have been one of the most attractive research topics in condensed-matter physics in recent decades. The low-dimensional topologically nontrivial materials are especially appealing due to the rich implications for topological physics and the potential applications in next-generation spintronic devices. Here, we report the crystal growth, anisotropic magnetotransport, Hall effect, and quantum de Haas–van Alphen (dHvA) oscillations of a quasi-one-dimensional ternary telluride ${\mathrm{NbNiTe}}_5$. The pronounced dHvA oscillations under $H\parallel b$ reveal three major oscillation frequencies $F_{\alpha }=136.41\mathrm{T}, F_{\beta }=240.34\mathrm{T}$, and $F_{\gamma }=708.03$ T and the associated light effective masses of charge carriers. From the angular dependence of dHvA oscillations, we have revealed the identified frequencies exhibit anisotropic character, all of which arise from the holelike Fermi surface sheets formed by band 1 ($F_{\alpha }$ and $F_{\beta }$) and band 2 ($F_{\gamma }$) by comparing with the Fermi surface calculations. First-principles calculations demonstrate that ${\mathrm{NbNiTe}}_5$ is a candidate of multiple topological material. In addition to the nonsymmorphic symmetry-protected nodal lines and band inversion (anticrossing) induced topological surface states, a ladder of topological gaps with the coexistence of strong and weak topology and a series of induced topological surface states are also identified.

The resistivity size effect of thin films due to atomically rough surfaces is investigated using first-principles quantum transport simulations with the disorder scattering treated by the nonequilibrium mean-field approach. Within the exact muffin-tin orbital–based first-principles method, the Madelung potential of film in device structure is constructed by implementing the boundary-condition correction. Cu(001), Co(0001), Ru(0001), and Mo(001) thin films are modeled with the thickness $d=1–10$ nm. The random surface roughness is represented by an alloy model, consisting of one monolayer of ${\text{M}}_x{\text{Va}}_{1-x}$ and ${\text{M}}_{1-x}{\text{Va}}_x$ on the respective top and bottom surfaces. The results of all metal films indicate that the first-principles resistivity ${\rho }_s$ induced by surface roughness scattering is proportional to $1/d$. Our simulated resistivity results are consistent with the experimental measurements of epitaxial metal layers. We find that, for the same thickness, Mo films present the highest ${\rho }_s$, significantly larger than the other metals. The thin-film resisitivity ${\rho }_s$ of Co is about 1.6 times that of Cu, while Ru results is slightly higher than Cu results. For all metal films, we obtain the parameter ${\gamma }_s$ characterizing the intensity of surface roughness scattering as a function of $x$. Furthermore, we find the proportionality constant ${\alpha }_s$ versus $x$ for the mean-free path ${\lambda }_s={\alpha }_s\times d$ for surface roughness scattering. Our results show that at the high $x>0.2, {\alpha }_s$ is rather close to a constant, with values of 4.5, 2.8, 2.1, and 1.0 for the respective Cu, Ru, Co, and Mo. We conclude that, compared to Co, Ru is competitive in resistivity size effect of surface roughness for an alternative to Cu interconnect for future technology nodes.

Bernal stacked bilayer graphene subject to a superlattice potential can realize topological and stacked flat bands [Ghorashi et al., Phys. Rev. Lett. 130, 196201 (2023)]. In the present work, we extend the study of a superlattice potential on graphene heterostructures to trilayer and quadrilayer graphene. Comparing Bernal- and chirally stacked multilayers reveals that the latter are more suitable for realizing stacks of many flat bands. On the other hand, Bernal-stacked graphene heterostructures can realize topological flat bands. Imposing two simultaneous superlattice potentials enhances the viability of both regimes.

We study ${\mathrm{\Gamma }}_3$ quadrupole orders in a face-centered cubic lattice. The ${\mathrm{\Gamma }}_3$ quadrupole moments under cubic symmetry possess a unique cubic invariant in their free energy in the uniform ($\mathbf{q}=\mathit{0}$) sector and the triple-$\mathbf{q}$ sector for the $X$ points $\mathbf{q}=(2\pi ,0,0),(0,2\pi ,0)$, and $(0,0,2\pi )$. Competition between this cubic anisotropy and anisotropic quadrupole-quadrupole interactions causes a drastic impact on the phase diagram both in the ground state and at finite temperatures. We show details about the model construction and its properties, the phase diagram, and the mechanism of the various triple-$\mathbf{q}$ quadrupole orders reported in our preceding letter [J. Phys. Soc. Jpn. 90, 043701 (2021)]. By using a mean-field approach, we analyze a quadrupole exchange model that consists of a crystalline-electric field scheme with the ground-state ${\mathrm{\Gamma }}_3$ non-Kramers doublet and the excited singlet ${\mathrm{\Gamma }}_1$ state. We have found various triple-$\mathbf{q}$ orders in the four-sublattice mean-field approximation. A few partially ordered phases are stabilized in a wide range of parameter space and they have a higher transition temperature than single-$\mathbf{q}$ orders. With lowering temperature, there occur transitions from these partially ordered phases into further symmetry broken phases in which previously disordered sites acquire nonvanishing quadrupole moments. The identified phases in the mean-field approximation are further analyzed by a phenomenological Landau theory. This analysis reveals results qualitatively consistent with the mean-field results and also shows that the cubic invariant plays an important role for stabilizing the triple-$\mathbf{q}$ states. The present mechanism for the triple-$\mathbf{q}$ states also takes effect in systems with different types of quadrupoles, and we discuss its implications for recent experiments in a few $f$- and $d$-electron compounds.

How fast quantum information scrambles such that it becomes inaccessible by local probes turns out to be central to various fields. Motivated by recent works on spin systems with nonlocal interactions, we study information scrambling in different variants of the Ising model. Our work reveals that nonlocal interactions can induce operator dynamics not precisely captured by out-of-time-order correlators (OTOCs). In particular, the operator size exhibits a slowdown in systems with generic power-law interactions despite a highly nonlinear light cone. A recently proposed microscopic model for fast scrambling does not show this slowdown, which uncovers a distinct analogy between a local operator under unitary evolution and the entanglement entropy following a quantum quench. Our work gives new insights on scrambling properties of systems in reach of current quantum simulation platforms and complements results on possibly observing features of quantum gravity in the laboratory.

Recently, the application of an external magnetic field to actively control the near-field radiative heat transfer (NFRHT) has emerged as an appealing and promising technique. Most prior studies have shown that the externally static magnetic field can only reduce the near-field flux transferred between two planar magneto-optical (MO) structures, yet so far the thermomagnetic effect in many-body systems consisting of such structures has not been revealed. Here, we investigate how the presence of an external static magnetic field modifies the nanoscale heat transfer in a MO many-body configuration comprising three noncontact slabs made of $n$-doped InSb. To this end, we first generalize a general Green's function approach for the calculation of the radiative energy transfer in many-body planar geometries composed of materials with complex optical anisotropy. Based on this approach, we show that the presence of a third MO body allows for either the reduction or even the enhancement of the NFRHT between MO slabs by applying external magnetic fields, which depends on the interplay between the zero-field surface waves and propagating hyperbolic modes induced by fields. Our study not only deepens the understanding of the active control of the nanoscale heat transfer via the applied magnetic field, but also paves the way for the magnetic regulation of energy fluxes in complex macroscopic many-body systems.

Applying pressure around megabar is indispensable in the synthesis of high-temperature superconducting hydrides, such as ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$. Stabilizing the high-pressure phase of hydride around ambient condition is a severe challenge. Based on the density-functional theory calculations, we give the first example that the structure of hydride ${\mathrm{CaBH}}_5$ predicted above 280 GPa can maintain its dynamical stability with pressure down to 1 GPa, by modulating the charge transfer from metal atoms to hydrogen atoms via the replacement of Ca with alkali metal atoms, e.g., Cs, in which the $[{\mathrm{BH}}_5{]}^{2-}$ anion shrinks along $c$ axis and expands in the $ab$ plane, experiencing an anisotropic virtual high pressure. This mechanism, namely charge transfer modulated virtual high-pressure effect, plays a vital role in enhancing the structural stability and leading to the reemergence of ambient-pressure-forbidden $[{\mathrm{BH}}_5{]}^{2-}$ anion around 1 GPa in ${\mathrm{CsBH}}_5$. Moreover, we find that ${\mathrm{CsBH}}_5$ is a strongly coupled superconductor, with transition temperature as high as 98 K, well above the liquid-nitrogen temperature. Our findings provide a novel mechanism to reduce the critical pressure required by hydrogen-rich compound without changing its crystal structure, and also shed light on searching ambient-pressure high-temperature superconductivity in metal borohydrides.

Motivated by Hall viscosity measurements in graphene sheets, we study hydrodynamic transport of electrons in a channel of finite width in external electric and magnetic fields. We consider electric charge densities varying from close to the Dirac point up to the Fermi-liquid regime. We find two competing contributions to the hydrodynamic Hall and inverse Nernst signals that originate from the Hall viscous and Lorentz forces. This competition leads to a nonlinear dependence of the full signals on the magnetic field and even a cancellation at different critical field values for both signals. In particular, the hydrodynamic inverse Nernst signal in the Fermi-liquid regime is dominated by the Hall viscous contribution. We further show that a finite channel width leads to a suppression of the Lorenz ratio, while the magnetic field enhances this ratio. All of these effects are predicted in parameter regimes accessible in experiments.