We have performed x-ray powder diffraction, magnetization, electrical resistivity, heat capacity, and inelastic neutron scattering (INS) to investigate the physical properties of the intermetallic series of compounds, ${\mathrm{CeCuBi}}_{2-x}{\mathrm{Sb}}_x$. These compounds crystallize in a tetragonal structure with space group $P4/nmm$ and present antiferromagnetic transition temperatures ranging from 3.6 to 16 K. Remarkably, the magnetization easy axis changes along the series, which is closely related to the variations of the tetragonal crystalline electric-field (CEF) parameters. This evolution was analyzed using a mean-field model, which included anisotropic nearest-neighbor interactions and the tetragonal CEF Hamiltonian. The CEF parameters were obtained by fitting the magnetic susceptibility data with the constraints given by the INS measurements. Finally, we discuss how this CEF evolution can affect the Kondo physics and the search for a superconducting state in this family.

The breakdown of the Born-Oppenheimer approximation is an important topic in chemical dynamics on metal surfaces. In this context, the most frequently used work horse is electronic friction theory, commonly relying on friction coefficients obtained from density-functional theory calculations from the early '80s based on the atom-in-jellium model. However, results are only available for a limited set of jellium densities and elements ($Z=1-18$). In this paper, these calculations are revisited by investigating the corresponding friction coefficients for the entire periodic table ($Z=1-92$). Furthermore, friction coefficients obtained by including the electron density gradient on the generalized gradient approximation level are presented. Finally, we show that spin polarization and relativistic effects can have sizable effects on these friction coefficients for some elements.

Recent experiments have studied the temperature and gate voltage dependence of nonlocal transport in bilayer graphene, identifying features thought to be associated with the two-dimensional semiconductor's bulk intrinsic valley Hall effect. Here, we use both simple microscopic tight-binding ribbon models and phenomenological bulk transport equations to emphasize the impact of sample edges on the nonlocal voltage signals. We show that the nonlocal valley Hall response is sensitive to electronic structure details at the sample edges and that it is enhanced when the local longitudinal conductivity is larger near the sample edges than in the bulk. We discuss recent experiments in light of these findings and also discuss the close analogy between electron pumping between valleys near two-dimensional sample edges in the valley Hall effect and bulk pumping between valleys due to the chiral anomaly in three-dimensional topological semimetals.

Single-layer $h-\mathrm{BN}$ is known to have edges with unique magnetism; however, in the commonly fabricated multilayer ${\text{AA}}^{\prime }-h-\mathrm{BN}$, edge relaxations occur that create interlayer bonds and eliminate the unpaired electrons at the edge. Recently, a robust method of growing the unconventional Bernal-stacked $h-\mathrm{BN}$ ($\mathrm{AB}-h-\mathrm{BN}$) has been reported. Here, we use theoretical approaches to investigate the nitrogen-terminated zigzag edges in $\mathrm{AB}-h-\mathrm{BN}$ that can be formed in a controlled fashion using a high-energy electron beam. We find that these “open” edges remain intact in bilayer and multilayer $\mathrm{AB}-h-\mathrm{BN}$, enabling researchers potentially to investigate these edge states experimentally. We also investigate the thermodynamics of the spin configurations at the edge by constructing a lattice model that is based on parameters extracted from a set of first-principles calculations. We find that the edge spins in neighboring layers interact very weakly, resulting in a sequence of independent spin chains in multilayer samples. By solving this model using Monte Carlo simulations, we can determine nm-scale correlation lengths at liquid-${\mathrm{N}}_2$ temperatures and lower. At low temperatures, these edges may be utilized in magnetoresistance and spintronics applications.

Excitonic insulator (EI) is an intriguing insulating phase of matter, where electrons and holes are bonded into pairs, so called excitons, and form a phase-coherent state via Bose-Einstein condensation (BEC). Its theoretical concept has been proposed several decades ago, but the followed research is very limited, due to the rare occurrence of EI in natural materials and the lack of manipulating methods of excitonic condensation. In this Rapid Communication, we report the realization of a doping-controlled EI-to-semimetal transition in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ (TNS) using in situ potassium deposition. Combining with angle-resolved photoemission spectroscopy (ARPES), we delineate the evolution of electronic structure through the EI transition with unprecedented precision. The results not only show that ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is an EI originated from a semimetal noninteracting band structure, but also resolve two sequential transitions, which could be attributed to the phase-decoherence and pair-breaking respectively. Our results unveil the Bardeen-Cooper-Schrieffer (BCS)-BEC crossover behavior of TNS and demonstrate that its band structure and excitonic binding energy can be tuned precisely via alkali-metal deposition. This paves a way for investigations of BCS-BEC crossover phenomena, which could provide insights into the many-body physics in condensed matters and other many-body systems.

We show that a metastable $\eta$-pairing superconducting phase can be induced by photodoping doublons and holes into a strongly repulsive fermionic Hubbard model. The doublon-hole condensate originates from an intrinsic doublon-hole exchange interaction and does not rely on the symmetry of the half-filled Hubbard model. It extends over a wide range of doublon densities and effective temperatures. Different nonequilibrium protocols to realize this state are proposed and numerically tested. We also study the optical conductivity in the superconducting phase, which exhibits ideal metallic behavior, i.e., a delta function at zero frequency in the conductivity, in conjunction with a negative conductivity at large frequencies. These characteristic optical properties can provide a fingerprint of the $\eta$-pairing phase in pump-probe experiments.

The $f$-sum rule and the Kohn formula are well-established general constraints on the electric conductivity in quantum many-body systems. We present their generalization to nonlinear conductivities at all orders of the response in a unified manner by considering two limiting quantum time-evolution processes: a quench process and an adiabatic process. Our generalized formulas are valid in any stationary state, including the ground state and finite-temperature Gibbs states, regardless of the details of the system such as the specific form of the kinetic term, the strength of the many-body interactions, or the presence of disorders.

The electronic structures of $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_3$ thin films with an oxygen vacancy on a $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrate are investigated using first-principles calculations. We find that the oxygen vacancy induces the insulator-metal transition as the film thickness is increased. For a 1.0-unit-cell-thick $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_3$ film, an insulating state with an energy gap of 1.2 eV is formed as a result of the large exchange and Coulomb interactions between Ni $3d$ orbitals, whereas a negative charge transfer energy causes restoration of the metallic state in films with a 2.0-unit-cell or larger thickness. Thermodynamic study reveals that an oxygen vacancy induced insulating film may be the preferable structure during the initial stage of film deposition. The low calculated formation energy of 0.26 eV for the oxygen vacancy indicates that the nonstoichiometric and insulating film may survive even the annealing process. Finally, work functions for films with various surfaces are discussed.

Two-dimensional $\mathrm{Mo}{\mathrm{S}}_2$ forms stable and several metastable allotropes of semimetallic, metallic, and semiconducting characters, depending on the experimental growth conditions. In this paper we consider intergrowth of the two most frequently reported metastable phases of $\mathrm{Mo}{\mathrm{S}}_2$ ($1T$ and $1T^{\prime }$) within the stable $1H$ phase to establish the effect of geometric and electronic reconstructions of the interface region between $1H$ and $T/1T^{\prime }$ phases using first-principles density functional calculations. We show that a complex structural reconstruction at the interfaces is responsible for the opening of an energy gap driving the electronic and geometric structures to resemble that of the $1T^{\prime }$ phase. It is also found that the size of the patches inside the $1H$ matrix crucially controls the geometry and electronic structure close to the Fermi level. These results establish that remarkable properties of chemically exfoliated $\mathrm{Mo}{\mathrm{S}}_2$ with patches of metastable structures are to be understood as arising fundamentally from intergrowths necessarily strained due to lattice mismatch across the interface between the $H$ and the distorted $T$ phase, rather than in terms of any pure metastable phase.

We study the electronic and structural properties of substitutional impurities of graphenelike nanoporous materials ${\mathrm{C}}_2\mathrm{N}$, $tg$-, and $hg\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ by means of density functional theory calculations. We consider four types of impurities; boron substitution on carbon sites [B(C)], carbon substitution on nitrogen sites [C(N)], nitrogen substitution on carbon sites [N(C)], and sulfur substitution on nitrogen sites [S(N)]. From cohesive energy calculations, we find that the C(N) and B(C) substitutions are the most energetically favorable and induce small bond modifications in the vicinity of the impurity, while the S(N) induces strong lattice distortions. Though all of the studied impurities induce defect levels inside the band gap of these materials, their electronic properties are poles apart depending on the behavior of the impurity as an acceptor [B(C) and C(N)] or a donor [N(C) and S(N)]. It is also observed that acceptor (donor) wave functions are composed only of $\sigma$ ($\pi$) orbitals from the impurity itself and/or neighboring sites, closely following the orbital composition of the valence (conduction) band wave functions of the pure materials. Consequently, acceptor wave functions are directed towards the pores and donor wave functions are more extended throughout the neighboring atoms, a property that could further be explored to modify the interaction between these materials and adsorbates. Moreover, impurity properties display a strong site sensitivity and ground state binding energies ranging from 0.03 to 1.13 eV in nonmagnetic calculations, thus offering an interesting route for tuning the optical properties of these materials. Finally, spin-polarized calculations reveal that all impurity configurations have a magnetic ground state with a total moment of $1.0{\mu }_B$ per unit supercell, which rises from the spin splitting of the impurity levels. In a few configurations, more than one impurity level can be found inside the gap and two of them could potentially be explored as two-level systems for single-photon emission, following similar proposals recently made on defect complexes on TMDCs.

Controlling shear band propagation is the key to obtain ductile metallic glasses. Here, we use a residual stress field to vary the direction of shear band propagation. We ascribe this behavior to the effect of the stress field on the activation of shear transformation zones (STZs) along their characteristic direction and we quantify this contribution to the energy of the process. Because of the progressively adverse orientation of the stress field, the energy stored as shear in the STZ decreases to a level where shear band propagation at alternative angles becomes energetically more favorable.

Phononic crystals show novel characteristics when it comes to acoustic and elastic wave propagation control. Nevertheless, most studies on phononic crystals are based on a plane wave assumption because this allows for application of Bloch theorem and analysis of the infinite system based on a single unit cell. However, the plane wave assumption is not valid in the near field of a source, where the wave front takes cylindrical or spherical form. Here, we overcome this limitation by introducing the concept of effective phononic crystals, which combine periodicity with varying isotropic material properties to force periodic coefficients in the elastic equations of motion in a non-Cartesian basis. The periodic coefficients allow for band structure calculation using Bloch theorem. Using the band structure, we demonstrate band gaps and topologically protected interface modes can be obtained for cylindrically propagating waves. Through effective phononic crystals, we show how behaviors of Cartesian phononic crystals can be realized in regions close to sources, where near-field effects are non-negligible.

Binary intermetallic ${\mathrm{Eu}}_2\mathrm{In}$ was recently reported to exhibit a giant anhysteretic magnetocaloric effect due to a first-order magnetic phase transition between paramagnetic and ferromagnetic states. Experimentally, the transition occurs with a small phase volume change, $\mathrm{\Delta }V/V$, of approximately 0.1% around $T_C$ of $ca.$ 55 K. We represent magnetic and compute magnetocaloric properties of a ${\mathrm{Eu}}_2\mathrm{In}$ compound using a microscopic description based on a model Hamiltonian that takes into account magnetic exchange and magnetoelastic interactions. In the model the thermodynamic nature of the transition is conveniently represented by a single magnetoelastic interaction parameter. A good agreement between the theoretical results and earlier published experimental data confirms the effectiveness of our approach.

Magnetocaloric effect (MCE) in an antiferromagnetic holmium (Ho) film is studied near the spin reorientation temperatures. A series of magnetization isotherms from 60 to 150 K around the Néel temperature, $T_N\approx 130\mathrm{K}$ were recorded for both in-plane and out-of-plane magnetic field orientations. A change in entropy, $\mathrm{\Delta }{S}_M$ of $\text{–}5\mathrm{J}/\mathrm{kg}\mathrm{K}$ was found near $T_N$ for an in-plane field orientation. A large change in $\mathrm{\Delta }{S}_M$ of −11.8 J/kg K due to a fan-helix spin transition at $T=90\mathrm{K}$ is observed for an in-plane field orientation. Spin transition at both $T_N$ and at the fan-helix transition exhibit larger MCE in the in-plane field orientations in comparison with the perpendicular orientation. The value of the refrigerant capacity extracted from the temperature dependence of $\mathrm{\Delta }{S}_M\left(T\right)$ is found to be larger for the “in- plane” orientation by a factor of two.

The superconducting density of states of the two-dimensional electron system at the ${\mathrm{LaAlO}}_3–{\mathrm{SrTiO}}_3$ interface exhibits broad quasiparticle peaks that are well described by the Dynes parameter $\mathrm{\Gamma }$. We measured $\mathrm{\Gamma }$ in a wide range of temperatures and gate voltages using tunneling spectroscopy. $\mathrm{\Gamma }$ exhibits a $T^q$ temperature dependence, with $q=2.2\pm 0.2$. We discuss several interpretations of $\mathrm{\Gamma }$ and conclude that $\mathrm{\Gamma }$ is an intrinsic parameter of the superconducting state that quantifies the effect of the pair-breaking processes in the superconductor. The strong temperature dependence at low temperatures rules out inelastic electron scattering and magnetic scattering as the origin of the pair breaking. We find that $\mathrm{\Gamma }$ can be explained by interband scattering and by scattering in a superconductor with nodes in the order parameter.

The demagnetizing factor can have an important effect on physical properties, yet its role in determining the behavior of nonellipsoidal samples remains to be fully explored. We present a detailed study of the role of spin symmetry in determining the demagnetizing factor of cuboids, focusing, as a model example, on the Ising dipolar ferromagnet ${\mathrm{LiHoF}}_4$. We distinguish two different functions: the demagnetizing factor as a function of intrinsic susceptibility $N\left(\chi \right)$ and the demagnetizing factor as a function of temperature $N\left(T\right)$. For a given nonellipsoidal sample, the function $N\left(\chi \right)$ depends only on dipolar terms in the spin Hamiltonian, but apart from in the limits $\chi \to 0$ and $\chi \to \infty$, it is a different function for different spin symmetries. The function $N\left(T\right)$ is less universal, depending on exchange terms and other details of the spin Hamiltonian. We apply a recent theory to calculate these functions for spherical and cuboidal samples of ${\mathrm{LiHoF}}_4$. The theoretical results are compared with $N\left(\chi \right)$ and $N\left(T\right)$ derived from experimental measurements of the magnetic susceptibility of corresponding samples of ${\mathrm{LiHoF}}_4$, both above and below its ferromagnetic transition at $T_{\mathrm{c}}=1.53$ K. Close agreement between theory and experiment is demonstrated, showing that the intrinsic susceptibility of ${\mathrm{LiHoF}}_4$ and other strongly magnetic systems can be accurately estimated from measurements on cuboidal samples. Our results further show that for cuboids, and implicitly for any sample shape, $N\left(\chi \right)$ below the ordering transition takes the value $N\left(\infty \right)$. This confirms and extends the scope of earlier observations that the intrinsic susceptibility of ferromagnets remains divergent below the transition, in contradiction to the implications of broken symmetry. We discuss the topological and microscopic origins of this result.

Recently it was realized that the zigzag magnetic order in Kitaev materials can be stabilized by small negative off-diagonal interactions called the ${\mathrm{\Gamma }}^{\prime }$ terms. To fully understand the effect of the ${\mathrm{\Gamma }}^{\prime }$ interactions, we investigate the quantum $K\text{−}\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }$ model on the honeycomb lattice using the variational Monte Carlo method. Two multinode $Z_2$ quantum spin liquids (QSLs) are found at ${\mathrm{\Gamma }}^{\prime }>0$, one of which is the previously found proximate Kitaev spin liquid called the PKSL14 state, which shares the same projective symmetry group (PSG) with the Kitaev spin liquid. A remarkable result is that a $\pi$-flux state with a distinct PSG appears at larger ${\mathrm{\Gamma }}^{\prime }$. The $\pi$-flux state is characterized by an enhanced periodic structure in the spinon dispersion in the original Brillouin zone (BZ), which is experimentally observable. Interestingly, two PKSL8 states are competing with the $\pi$-flux state and one of them can be stabilized by six-spin ring-exchange interactions. The physical properties of these nodal QSLs are studied by applying magnetic fields and the results depend on the number of cones. Our study infers that there exist a family of zero-flux QSLs that contain $6n+2,n\in \mathbb{Z}$ Majorana cones and a family of $\pi$-flux QSLs containing $4(6n+2)$ cones in the original BZ. It provides guidelines for experimental realization of non-Kitaev QSLs in relevant materials.

${\mathrm{SrPt}}_2{\mathrm{As}}_2$ undergoes a structural transition around 470 K (from a high-temperature tetragonal to a low-temperature orthorhombic phase) accompanied by a charge density wave transition and followed by a superconducting transition at much lower temperature (5 K). The superconducting gap symmetry of this noncentrosymmetric compound was investigated in detail by means of muon-spin rotation or relaxation $\left(\mu \mathrm{SR}\right)$ experiments in transverse-field (TF) configuration. The temperature-dependent magnetic penetration depth obtained from the $\mathrm{TF}\text{−}\mu \mathrm{SR}$ spectra suggests an isotropic $s$-wave-type superconducting gap. No observable differences are seen in the asymmetry time spectra collected in zero field above and below the superconducting transition temperature $T_c$. This strongly suggests the absence of spontaneous magnetic fields in the superconducting state, hence pointing to a preserved time-reversal symmetry below $T_c$.

We extend the basic theory of Andreev reflection (AR) in a normal metal/superconductor junction to the situation with an arbitrary time-dependent bias voltage $V\left(t\right)$ across the junction. The central element of the theory is the fact that the Fourier transform of the AR amplitude has a causal structure. As an example, the theory is used to describe the current response to short pulses of the bias voltage, which create coherent superposition of quasiparticle states with different energies. The current oscillates in time, with the gap frequency $\mathrm{\Delta }/\hslash$, and also as a function of the pulse area $\int V\left(t\right)dt$, with the period of the single-electron flux quantum $e/h$.