Hydrogen-rich metal alloys and compounds have drawn interest from planetary geophysics and condensed matter physics communities because of their potential for deep hydrogen storage in planets and high-temperature superconductivity. We find that a small amount of Ni can alter the phase behavior in the FeH alloy system. Ni can stabilize the double hexagonal close-packed (dhcp) structure in FeH up to the liquidus at 33 GPa, which is in contrast with the stability of the face-centered cubic (fcc) structure in Ni-free FeH at the same conditions. Above 60 GPa, Ni suppresses the stability of the tetragonal ${\mathrm{FeH}}_2$ phase but stabilizes fcc FeH at higher temperatures. At the same pressure range, we find tetragonal ${\mathrm{FeH}}_2$ and cubic ${\mathrm{FeH}}_3$ to be stable at temperatures above 2500 K without Ni. Therefore, in planetary interiors, Ni will expand the stability field of dense close-packed structures in the FeH system. If the Ni content is low, then ${\mathrm{FeH}}_2$ and ${\mathrm{FeH}}_3$ can play an important role in the cores of hydrogen-rich planets. Also, our study demonstrates that a secondary alloying component can severely impact the high-pressure stability of polyhydrides.

The synthesis and characterization of vanadium-based kagome metals ${\mathrm{YV}}_6{\mathrm{Sn}}_6$ and ${\mathrm{GdV}}_6{\mathrm{Sn}}_6$ are presented. X-ray diffraction, magnetization, magnetotransport, and heat capacity measurements reveal an ideal kagome network of V ions coordinated by Sn and separated by triangular lattice planes of rare-earth ions. The onset of low-temperature magnetic order of Gd spins is detected in ${\mathrm{GdV}}_6{\mathrm{Sn}}_6$ and is suggested to be noncollinear, while V ions in both compounds remain nonmagnetic. Density functional theory calculations are presented modeling the band structures of both compounds, which can be classified as ${\mathbb{Z}}_2$ topological metals in the paramagnetic state. Both compounds exhibit high mobility, multiband transport and present an interesting platform for controlling the interplay between magnetic order associated with the $R$-site sublattice and nontrivial band topology associated with the V-based kagome network. Our results invite future exploration of other $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ ($R$ = rare earth) variants where this interplay can be tuned via $R$-site substitution.

We report on electrical transport measurements of ballistic Bi constrictions with top and bottom gate electrodes in which interference of their metalliclike surface states and quantized bulk states results in Fano resonance features in the conductance. Within the formed constrictions, the mean spacing between the bulk states is smaller than their coupling to the leads. Under these conditions, gradual lapses in the scattering phases of the bulk states in response to a change in the gate voltage are inferred by analyzing the symmetry parameter of the Fano features. Comparison between the results of this analysis in response to the top and bottom gates also suggests that the surface states are localized at the interface between the Bi and the underlying ${\mathrm{SiO}}_2$ layer. Our findings advocate that nanofabrication of Bi could better harness its unique electronic properties to impart advanced functionalities.

Variational Monte Carlo studies employing projected entangled-pair states (PEPS) have recently shown that they can provide answers to long-standing questions such as the nature of the phases in the two-dimensional $J_1\text{−}{J}_2$ model. The sampling in these Monte Carlo algorithms is typically performed with Markov chain Monte Carlo algorithms employing local update rules, which often suffer from long autocorrelation times and interdependent samples. We propose a sampling algorithm that generates independent samples from a PEPS, bypassing all problems related to finite autocorrelation times. This algorithm is a generalization of an existing direct sampling algorithm for unitary tensor networks. We introduce an auxiliary probability distribution from which independent samples can be drawn, and combine it with importance sampling in order to evaluate expectation values accurately. We benchmark our algorithm on the classical Ising model and on variational optimization of two-dimensional quantum spin models.

The bulk photovoltaic effect (BPVE) has attracted increasing interest due to its potential to overcome the efficiency limit of traditional photovoltaics, and much effort has been devoted to understanding its underlying physics. However, previous work has shown that theoretical models of the shift current and the phonon-assisted ballistic current in real materials do not fully account for the experimental BPVE photocurrent, so other mechanisms should be investigated in order to obtain a complete picture of BPVE. In this paper, we demonstrate two approaches that enable the ab initio calculation of the ballistic current originating from the electron-hole interaction in semiconductors. Using ${\mathrm{BaTiO}}_3$ and monolayer ${\mathrm{MoS}}_2$ as two examples, we show clearly that for them the asymmetric scattering from an electron-hole interaction is less appreciable than that from an electron-phonon interaction, indicating more scattering processes need to be included to further improve the BPVE theory. Moreover, we provide a first-principles approach for material prediction and design in order to search for materials with a larger ballistic current due to electron-hole interactions.

We apply the dynamical large-$N$ Schwinger boson technique as an impurity solver for the dynamical mean-field theory (DMFT) calculations of the Kondo lattice model. Our approach captures the hybridization physics through the DMFT self-consistency that is missing in the pure Schwinger boson calculations with independent electron baths. The resulting thermodynamic and transport properties are in qualitative agreement with more rigorous calculations and give the correct crossover behavior over a wide temperature range from the local moment regime to the Fermi liquid. Our method may be further extended to combine with the density functional theory for efficient material calculations.

Structural-property relationship, the connection between materials’ structures and their properties, is central to the materials research. Especially at reduced dimensions, novel structural motifs often generate unique physical properties. Motivated by a recent work reporting a novel half-auxetic effect in monolayer ${\mathrm{PdB}}_4$ with a hypercoordinated structure, here we extensively explore similar two-dimensional (2D) transition-metal boride structures $M{\mathrm{B}}_4$ with $M$ covering $3d$ and $4d$ elements. Our investigation screens out one stable candidate, the monolayer ${\mathrm{RhB}}_4$. We find that monolayer ${\mathrm{RhB}}_4$ also shows half-auxeticity, i.e., the material always expands in a lateral in-plane direction in response to an applied strain in the other direction, regardless of whether the strain is positive or negative. We show that this special mechanical character is intimately tied to the hypercoordinated structure with the $MC\text{©}{\mathrm{B}}_8$ structural motif. Furthermore, regarding electronic properties, monolayer ${\mathrm{RhB}}_4$ is found to be an example of an almost ideal 2D spin-orbit Dirac-point semimetal. The low-energy band structure is clean, with a pair of fourfold degenerate Dirac points robust under spin-orbit coupling located close to the Fermi level. These Dirac points are $enforced$ by the nonsymmorphic space-group symmetry which is also determined by the lattice structure. Our work deepens the fundamental understanding of structural-property relationship in reduced dimensions. The half-auxeticity and the spin-orbit Dirac points will make monolayer ${\mathrm{RhB}}_4$ a promising platform for nanomechanics and nanoelectronics applications.

We present an $LC$ circuit model that supports a tilted “Dirac cone” in its spectrum. The tilt of the Dirac cone is specified by the parameters of the model consisting of mutual inductance between the neighboring sites and a capacitance $C_0$ at every lattice site. These parameters can be completely measured by impedance spectroscopy. Given that a tilted Dirac cone can be described by a background spacetime metric, the impedance spectroscopy can perfectly provide (local) information about the metric of the spacetime. Nonuniform spatial dependence of the mutual inductance or capacitance induces a nontrivial geometrical structure on the emergent spacetime. Our work extends the range of usefulness of circuit models to emulate nontrivial spacetime structures.

We predict a plasmonic excitation that exhibits chiral properties in the absence of external magnetic field. This excitation, that we call an interedge chiral Berry plasmon (IE CBP), is bound to an interface between two two-dimensional electron systems with different anomalous Hall conductivities. Crucially, the IE CBP is a unidirectional mode whose propagation direction and frequency are susceptible to external control, in sharp contrast to state-of-the-art magnetic field-free devices. We believe that the IE CBP represents a unique platform for tunable backscattering-immune signal transmission in the absence of magnetic field.

To gain insight into the thermodynamic response of shock-compressed Ag states corresponding to the face-centered-cubic (fcc) to body-centered-cubic (bcc) transformation and melting observed using in situ x-ray diffraction (XRD) measurements [Phys. Rev. Lett. 124, 235701 (2020)], we present longitudinal sound speed results and their analysis in Ag at peak stresses ranging from 60 to 300 GPa. The measured sound speeds increased linearly with density compression to 171 GPa, showing that the sound speeds (and longitudinal moduli) in the fcc and bcc phases are very similar. Between 171 and 218 GPa, the sound speed dropped significantly, consistent with the melting reported using XRD measurements in shock-compressed Ag. From 218 to 300 GPa, the increasing sound speeds and Hugoniot states provide a determination of the liquid phase Ag response. In particular, determination of the Grüneisen parameter (Γ) showed that the density–Grüneisen parameter product (ρΓ) for liquid Ag is constant, but differs significantly from that for solid Ag at ambient conditions. Thus, the Mie-Grüneisen equation of state can be used to describe the Hugoniot and off-Hugoniot response of liquid Ag.

The interplay among anisotropic magnetic terms, such as the bond-dependent Kitaev interactions and single-ion anisotropy, plays a key role in stabilizing the finite-temperature ferromagnetism in the two-dimensional compound ${\mathrm{CrSiTe}}_3$. While the Heisenberg interaction is predominant in this material, a recent work shows that it is rather sensitive to the compressive strain, leading to a variety of phases, possibly including a sought-after Kitaev quantum spin liquid [C. Xu et al., Phys. Rev. Lett. 124, 087205 (2020)]. To further understand these states, we establish the quantum phase diagram of a related bond-directional spin-$3/2$ model by the density-matrix renormalization group method. As the Heisenberg coupling varies from ferromagnetic to antiferromagnetic, three magnetically ordered phases, i.e., a ferromagnetic phase, a ${120}^{\circ }$ phase, and an antiferromagnetic phase, appear consecutively. All the phases are separated by first-order phase transitions, as revealed by the kinks in the ground-state energy and the jumps in the magnetic order parameters. However, no positive evidence of the quantum spin liquid state is found and possible reasons are discussed briefly.

We investigate spin pumping current injected by the nutation resonances of a ferromagnet (FM) or an antiferromagnet (AFM) into an adjacent metal. Comparing the dc spin pumping current between the normal precession and the nutation resonances, we find that the ratio of spin pumping current at the nutation resonance to the precession resonance is more pronounced in AFMs. We further show that the spin pumping current injected by the nutation resonance is opposite in sign as compared with the normal precession mode. This could offer a useful experimental signature for identifying such nutation resonances. Analyzing the nature of the nutational eigenmodes, we show that the sign change in spin current is rooted in a reversal of the precession sense for the nutation mode(s). Furthermore, the nutational modes in AFMs are found to be dominated by precession of one of the two sublattices only.

A comprehensive study of thermodynamic properties of three samples of bimetallic molecular magnets ${\left[{\mathrm{Co}}^{\mathrm{II}}{\left(\mathrm{pyrazole}\right)}_4\right]}_{2x}{\left[{\mathrm{Fe}}^{\mathrm{II}}{\left(\mathrm{pyrazole}\right)}_4\right]}_{2(1-x)}\left[{\mathrm{Nb}}^{\mathrm{IV}}{\left(\mathrm{CN}\right)}_8\right]·4{\mathrm{H}}_2\mathrm{O}$ with $x=1$ (${\mathrm{Co}}_2\mathrm{Nb}$), 0.5 (CoFeNb), and 0 (${\mathrm{Fe}}_2\mathrm{Nb}$) is reported. The three samples display the same crystallographic structure crystallizing in the tetragonal system with space group $I{4}_1/a$ . Their heat capacities are measured in the temperature range 0.36–100 K without applied field as well as in the field of ${\mu }_{0}H=0.1$, 0.2, 0.5, 1, 2, 5, and 9 T. The results imply the presence of the second-order phase transitions to magnetically ordered phases at 4.87(8), 7.1(2), and 8.44(3) K for $x=1$, 0.5, and 0, respectively. The corresponding thermodynamic functions are analyzed to discuss the stability of the mixed compound and the magnetocaloric effect (MCE). The Gibbs energy of mixing is found to be positive but smaller in magnitude than the energy of thermal fluctuations indicating that the mixed sample is most probably metastable in the full detected temperature range. The enthalpy of mixing is negative, which points to favoring a direct neighborhood of the Co(II) and Fe(II) ions in the solid solution CoFeNb. The negative values of the entropy of mixing are explained by considering the enhanced rigidity of the crystal lattice of the solid solution sample. To extract the magnetic contribution to the heat capacity, an approach based on a reasonable frequency spectrum is adopted. Taking advantage of the in-field heat capacity measurements, MCE was described in terms of the isothermal entropy change $\mathrm{\Delta }{S}_{\mathrm{M}}$ and the adiabatic temperature change $\mathrm{\Delta }{T}_{\mathrm{ad}}$. The magnitudes of these quantities are typical for the class of molecular magnets. The values of $|\mathrm{\Delta }{S}_{\mathrm{M}}{|}^{\max }$ detected for ${\mu }_0\mathrm{\Delta }H=5\mathrm{T}$ amount to 7.04, 5.26, and 4.93 $\mathrm{J}{\mathrm{K}}^{\text{–}1}\mathrm{mo}{\mathrm{l}}^{\text{–}1}$ for ${\mathrm{Co}}_2\mathrm{Nb}$, CoFeNb, and ${\mathrm{Fe}}_2\mathrm{Nb}$, respectively, and are on the order of those obtained for the same field change in the isostructural compounds. The values of $\mathrm{\Delta }{T}_{\mathrm{ad}}$ detected for ${\mu }_0\mathrm{\Delta }H=5\mathrm{T}$ amount to 4.16, 2.47, and 2.01 K for ${\mathrm{Co}}_2\mathrm{Nb}$, CoFeNb, and ${\mathrm{Fe}}_2\mathrm{Nb}$, respectively, and are larger or comparable with those observed for the isostructural compounds. Temperature dependences of exponent $n$, quantifying the field dependence of $\mathrm{\Delta }{S}_{\mathrm{M}}$, display minima close to the transition temperatures, implying through their values that the studied compounds belong to the universality class of the three-dimensional Heisenberg model. The regeneration Ericsson cycles employing the studied compounds as the working substance were considered. Most surprisingly, the Ericsson cycle operating between the temperatures corresponding to the full width at half maximum of the $|\mathrm{\Delta }{S}_{\mathrm{M}}|$ signal ($T_{\mathrm{C}}$, $T_{\mathrm{H}}$) turns out to be totally ineffective. Through shifting the temperature of the hot reservoir $T_{\mathrm{H}}$ down to the temperature $T_{\max }$ corresponding to $|\mathrm{\Delta }{S}_{\mathrm{M}}{|}^{\max }$, the coefficient of performance is rendered positive and comparable with that of the Carnot cycle. A detailed analysis indicates that the regeneration Ericsson cycle operating between $T_{\mathrm{C}}$ and $T_{\max }$ should be most efficient for the maximal studied value of the applied field $(=9\mathrm{T})$ with irrelevant differences between the studied compounds.

We study magnon-mediated optical transitions in micrometer-sized axially symmetric yttrium iron garnet (YIG) particles, which act as optomagnonic cavities, by means of electromagnetic calculations, treating the magneto-optical coupling to first order in perturbation theory, in the framework of a fully dynamic approach. Such particles with engineered shape anisotropy exhibit high-quality-factor Mie resonances in the infrared part of the spectrum, with a separation of few gigahertz, which matches the typical frequencies of magnons. This allows for optical transitions mediated by spin waves, while the micrometer volume favors stronger overlap between the optical modes and the precessing magnetization. Our results predict that photon-magnon coupling strengths of tens of kilohertz could be realized with cylindrical or spheroidal particles, since mainly the reduced volume, but also shape anisotropy, can lead to strong, up to four orders of magnitude, enhancement of the coupling strengths compared to submillimeter YIG spheres.

We studied the acoustic properties of liquid oxygen up to 90 T by means of ultrasound measurements. We observed a monotonic decrease of the sound velocity and an asymptotic increase of the sound attenuation when applying magnetic fields. The unusual attenuation, twenty times as large as the zero-field value, suggests strong fluctuations of the local molecular arrangement. We assume that the observed fluctuations are related to a liquid-liquid transition or crossover, from a small-magnetization to a large-magnetization liquid, which is characterized by a local-structure rearrangement. To investigate higher-field properties of liquid oxygen, we performed single-turn-coil experiments up to 180 T by means of the acoustic, dilatometric, magnetic, and optical techniques. We observed only monotonic changes of these properties, reflecting the absence of the proposed liquid-liquid transition in our experimental conditions.

Spintronics is postulated on the possibility to employ the magnetic degree of freedom of electrons for computation and couple it to charges. In this view the combination of the high frequency of spin manipulations offered by antiferromagnets, with the wide tunability of the electronic properties peculiar of semiconductors, provides a promising and intriguing platform. Here we explore this scenario in $\alpha$-MnTe, which is a semiconductor antiferromagnetically ordered at room temperature. Relying on a Raman mechanism and femtosecond laser pulses, we drive degenerate modes of coherent optical phonons, which modulate the chemical bonds involved in the superexchange interaction. The spectrally resolved measurements of the transient reflectivity reveal a coherent modulation of the band gap at the frequency of 5.3 THz. The detection of the rotation of the polarization, typically associated with magneto-optical effects, shows coherent and incoherent contributions. Modeling how the ionic motion induced by the phonons affects the exchange interaction in the material, we calculate the photoinduced THz spin dynamics: the results predict both a coherent and incoherent response, the latter of which is consistent with the experimental observation. Our work demonstrates that the same phonon modes modulate both the charge and magnetic degree of freedom, suggesting the resonant pumping of phonons as a viable way to link spin and charge dynamics even in nonlinear regimes.

In the lacunar spinel ${\mathrm{GaV}}_4{\mathrm{S}}_8$, the interplay of spin, charge, and orbital degrees of freedom produces a rich phase diagram that includes an unusual Néel-type skyrmion phase composed of molecular spins. To provide insight into the interactions underlying this complex phase diagram, we study the spin excitations in ${\mathrm{GaV}}_4{\mathrm{S}}_8$ through inelastic neutron scattering measurements on polycrystalline and single crystal samples. Using linear spin wave theory, we describe the spin wave excitations using a model where ${\mathrm{V}}_4$ clusters decorate an fcc lattice. The effective cluster model includes a ferromagnetic interaction and a weaker antisymmetric Dzyaloshinskii-Moriya interaction between the neighboring molecular spins. Our work clarifies the spin interactions in ${\mathrm{GaV}}_4{\mathrm{S}}_8$ and supports the picture of interacting molecular clusters.

Extreme in-plane upper critical magnetic fields $B_{\mathrm{c}2//\mathrm{ab}}$ strongly violating the Pauli paramagnetic limit have been observed in the misfit layer ${\left(\mathrm{LaSe}\right)}_{1.14}\left(\mathrm{Nb}{\mathrm{Se}}_2\right)$ and ${\left(\mathrm{LaSe}\right)}_{1.14}{\left(\mathrm{Nb}{\mathrm{Se}}_2\right)}_2$ single crystals with $T_{\mathrm{c}}=1.23$ and 5.7 K, respectively. The crystals show a two-dimensional to three-dimensional transition at temperatures slightly below $T_{\mathrm{c}}$ with an upturn in the temperature dependence of $B_{\mathrm{c}2//\mathrm{ab}}$, a temperature-dependent huge superconducting anisotropy and a cusplike behavior of the angular dependence of $B_{\mathrm{c}2}$. Both misfits are characterized by a strong charge transfer from LaSe to ${\mathrm{NbSe}}_2$. As shown in our previous work, ${\left(\mathrm{LaSe}\right)}_{1.14}{\left(\mathrm{Nb}{\mathrm{Se}}_2\right)}_2$ is electronically equivalent to the highly doped ${\mathrm{NbSe}}_2$ monolayers. Then, the strong upper critical field can be attributed to the Ising coupling recently discovered in atomically thin transition metal dichalcogenides with strong spin-orbit coupling and a lack of inversion symmetry. A very similar behavior is found in ${\left(\mathrm{LaSe}\right)}_{1.14}\left(\mathrm{Nb}{\mathrm{Se}}_2\right)$, where the charge transfer is nominally twice as big, which could eventually lead to complete filling of the ${\mathrm{NbSe}}_2$ conduction band and opening superconductivity in LaSe. Whatever the particular superconducting mechanism would be, a common denominator in both misfits is that they behave as a stack of almost decoupled superconducting atomic layers, proving that Ising superconductivity can also exist in bulk materials.