Achieving high-$T_c$ superconductivity in hydrogen-rich materials under ultrahigh pressure relies crucially on the metallization of strong $\sigma$ bonding or antibonding electrons. It is essential to discern the bonding characters of superconducting electrons in order to unveil the underlying high-$T_c$ pairing mechanism in these materials. In this study, we investigate the electronic and phonon properties of hydrogen sulfide (${\mathrm{SH}}_3$) under ultrahigh pressure to elucidate the origin of its high-$T_c$ superconductivity. Contrary to the prevailing belief attributing the high-$T_c$ superconductivity to the contribution of the metallized S-H $\sigma$ bonds, our analysis, based on the solution of the anisotropic Migdal-Eliashberg equation and the crystal orbital Hamilton population calculation, reveals that the H-H $\sigma$ antibonding states play a decisive role in the large electron-phonon coupling that leads to the superconducting pairing in ${\mathrm{SH}}_3$. Furthermore, by partially restricting the vibration of S atoms, we show that the S-H bonds only provide subsidiary contributions to the pairing interaction. These findings not only shed light on the importance of the H-H $\sigma$-antibonds in driving high-$T_c$ superconductivity in ${\mathrm{SH}}_3$, but crucially they pave the way for a profound understanding of the intricate connection between metallic H-H $\sigma$ antibonding states and high-$T_c$ superconductivity in a diverse array of hydrogen-rich materials subjected to high-pressure environments.

In this work, we present a systematic theoretical study on the positron annihilation characteristics (positron lifetimes and momentum distributions) in transition metals ($3d, 4d$, and $5d$ metals) and other elements (carbon, aluminum, silicon, and phosphorus). Our calculations agree well with available reference experimental and theoretical data. We show that clear patterns exist in the evolution of the positron annihilation characteristics in pure elements. The evolution of momentum distribution in one transition metal series is mainly contributed by the filling of $d$-band electrons. For the positron lifetimes, the lifetimes of transition metals evolve with their $d$-band filling in a similar behavior as their atomic volumes. A case study is performed to show qualitatively the effect of solute elements on the Doppler spectra of defects. It is demonstrated that vacancy-solute complexes depict similar annihilation characteristics as the corresponding pure solute elements, meaning that vacancy-solute complexes can be reliably identified if the Doppler spectra of the pure solute elements are known. For the positron lifetimes, we found that they have a linear relation with the atomic volumes of elements for the same transition metal series. This work is expected to improve understanding of the positron annihilation characteristics of transition metals. The results could be used to investigate and identify the microstructures in alloys and compounds, such as vacancy-solute complexes, solute clusters, precipitates, and vacancies in different sublattices of compounds.

We study thermalization dynamics in a fermion-phonon variant of the Sachdev-Ye-Kitaev model coupled to an external cold thermal bath of harmonic oscillators. We find that quantum critical fermions thermalize more efficiently than phonons, in sharp contrast to the behavior in the Fermi liquid regime. In addition, after a short prethermal stage, the system acquires a quasithermal distribution given by a time-dependent effective temperature, reminiscent of “hydrodynamic” relaxation. All physical observables relax at the same rate which scales with the final temperature through an exponent that depends universally on the low-energy spectrum of the system and the bath. Such relaxation rate is derived using a hydrodynamic approximation in full agreement with the numerical solution of a set quantum kinetic equations derived from the Keldysh formalism for nonequilibrium Green's functions. Our results hint toward further research on the applicability of the hydrodynamic picture in the description of the late time dynamics of open quantum systems despite the absence of conserved quantities in regimes dominated by conserving collisions.

The development of spin-orbitronic devices, such as magnetoelectric spin-orbit logic devices, calls for materials with a high resistivity and a high spin-charge interconversion efficiency. One of the most promising candidates in this regard is sputtered ${\mathrm{Bi}}_x{\mathrm{Se}}_{1-x}$. Although there are several techniques to quantify spin-charge interconversion, reported values to date for sputtered ${\mathrm{Bi}}_x{\mathrm{Se}}_{1-x}$ have often been overestimated due to spurious effects related to local currents combined with a lack of understanding of the effect of the interfaces and the use of approximations for unknown parameters, such as the spin diffusion length. In the present study, nonlocal spin valves are used to inject pure spin currents into ${\mathrm{Bi}}_x{\mathrm{Se}}_{1-x}$, allowing us to directly obtain its spin diffusion length as well as its spin Hall angle, from 10 K up to 300 K. These values, which are more accurate than those previously reported in sputtered ${\mathrm{Bi}}_x{\mathrm{Se}}_{1-x}$, evidence that the efficiency of this material is not exceptional. Indeed, the figure of merit for spin-charge interconversion, given by the product of these two parameters, is slightly under 1 nm. Our work demonstrates the importance of considering all material parameters and interfaces when quantifying the spin transport properties of materials with strong spin-orbit coupling.

Metamaterials, fabricated by assembling different compounds at the nanoscale, can have properties not found in naturally occurring materials, and therefore offer new avenues to develop novel devices. In the realm of spintronics, where the spin of the electrons is used to extend the capabilities of electronic devices, the quest for such new functional materials has expanded towards magnetic oxides. Here, finding methods to control their magnetic anisotropy is crucial to achieve higher memory density and longer stability. In order to address this challenge, we combined two oxides with a spinel crystal structure, synthesizing $\mathrm{Co}{\mathrm{Cr}}_2{\mathrm{O}}_4$/$\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4$ superlattices with layers only few unit cells thick. We show that the superlattices present a reorientation of the magnetic easy axis from in plane to out of plane when warmed up, at a temperature determined by the periodicity. We can describe this with a model that includes the strain-induced anisotropy of the two materials and their different temperature dependence. This approach to create artificial materials, involving engineering superlattices to tailor the magnetic anisotropy, can be generalized to a wide range of compounds that can be grown strained on suitable substrates.

The dimensionality effect on superconductivity has been a long-term interesting topic. Here we have systematically investigated the electronic and superconducting properties of ultrathin NbC films by using first-principles electronic structure calculations. We find that the superconducting transition temperatures of ultrathin NbC films show an interesting thickness-dependent oscillation, whose amplitude can reach as large as 10 K. Notably, the superconducting $T_{\text{c}}$ of trilayer NbC film is predicted to be 16.8 K, even higher than that (11.5 K) of bulk NbC. Further analyses indicate that the ultrathin NbC films with relatively high superconducting $T_{\text{c}}$'s all have strong phonon softening and enhanced electron-phonon coupling, for which the special phonon modes and electronic states as well as their real-space patterns are clarified. Our results provide an insight for modulating the superconductivity of transition metal carbides with the reduced dimensionality.

Despite being a layered, easily-exfoliated compound, copper monotelluride (CuTe) features an unusual quasi-one-dimensional charge density wave below $T_{\mathrm{CDW}}\approx 335\mathrm{K}$. Within a CuTe layer, the electrical resistivity depends sensitively on the direction of the electrical current. Here, we use magnetotransport to probe the metallic state of CuTe with two distinct in-plane current directions. When the current flows along the $a$ axis ($I//a$), the magnetoresistance exhibits a downward curvature as the magnetic field increases. On the other hand, when the current is along the $b$ axis ($I//b$), the magnetoresistance shows the opposite curvature. Our analysis uncovers a violation of Kohler scaling, but only for $I//a$. Shubnikov–de Haas oscillations are detected at low temperatures. Our results shed light on the nature of the metallic state in CuTe with the development of the charge density wave.

We present nuclear magnetic resonance (NMR), neutron diffraction, magnetization, and transport measurements on a single crystal and powder of ${\mathrm{CeVGe}}_3$. This material exhibits heavy fermion behavior at low temperatures, accompanied by antiferromagnetic (AFM) order below 5.8 K. We find that the magnetic structure is incommensurate with AFM helical structure, characterized by a magnetic modulated propagation vector of $(0,0,0.49)$ with in-plane moments rotating around the $c$ axis. The NMR Knight shift and spin-lattice relaxation rate reveal a coherence temperature $T^{*}\sim 15\mathrm{K}$, and the presence of significant antiferromagnetic fluctuations reminiscent of the archetypical heavy fermion compound ${\mathrm{CeRhIn}}_5$. We further identify a metamagnetic transition above $H_m\sim 2.5\mathrm{T}$ for magnetic fields perpendicular to $c$. We speculate that the magnetic structure in this field-induced phase consists of a superposition with both ferromagnetic and antiferromagnetic components, which is consistent with the NMR spectrum in this region of the phase diagram. Our results thus indicate that ${\mathrm{CeVGe}}_3$ is a hexagonal structure analog to tetragonal ${\mathrm{CeRhIn}}_5$.

We demonstrate a strong influence of the phonon environment on the coherent dynamics of the quantum dot (QD)-cavity system in the quantum strong coupling regime. This regime is implemented in the nonlinear QD-cavity QED and can be reliably measured by heterodyne spectral interferometry. We present a semianalytic asymptotically exact path integral-based approach to the nonlinear optical response of this system, which includes two key ingredients: Trotter's decomposition and linked-cluster expansion. Applied to the four-wave-mixing optical polarization, this approach provides access to different excitation and measurement channels, as well as to higher-order optical nonlinearities and quantum correlators. Furthermore, it allows us to extract useful analytic approximations and analyze the nonlinear optical response in terms of quantum transitions between phonon-dressed states of the anharmonic Jaynes-Cummings (JC) ladder. Being well described by these approximations at low temperatures and small exciton-cavity coupling, the exact solution deviates from them for stronger couplings and higher temperatures, demonstrating remarkable non-Markovian effects, spectral asymmetry, and strong phonon renormalization of the JC ladder.

Excitation with a massive spin reversal of the individual skyrmion/antiskyrmion type is theoretically studied in a quantum Hall ferromagnet, where the zeroth and first Landau levels are completely occupied only by electrons with spins aligned strictly in the direction determined by the magnetic field. The Wigner-Seitz parameter is not necessarily considered to be small. The microscopic model in use is based on a reduced basic set of quantum states [the so-called “single-mode (single-exciton) approximation”], which allows proper account to be taken for mixing of Landau levels, and substantiating the equations of the classical $O\left(3\right)$ nonlinear $\sigma$ model. The calculated “spin stiffness” determines the exchange gap for creating a pair of skyrmion and antiskyrmion. This gap is significantly smaller than the doubled cyclotron energy and the characteristic electron-electron correlation energy. Besides, the skyrmion–antiskyrmion creation gap is much smaller than the energy of creation of a separated electron–exchange-hole pair calculated in the limit case of a spin magnetoexciton corresponding to an infinitely large 2D momentum. At a certain magnetic field (related to the 2D electron density in the case of fixed filling factor $\nu$), the gap vanishes, which presumably points to a Stoner transition of the quantum Hall ferromagnet to a paramagnetic phase.

Perovskites down to the monolayer limit have emerged and have attracted increased interest due to their two-dimensional nature with potentially novel physical properties. Here, we investigate the phonon transport in the oxide perovskite ${\mathrm{SrTiO}}_3$ with thicknesses from the monolayer limit to 10 nm by constructing an accurate first-principles machine-learning potential and combining it with the Boltzmann transport equation and homogeneous nonequilibrium molecular dynamics simulations. Compared to its bulk counterpart, the phonon dispersion relation of monolayer ${\mathrm{SrTiO}}_3$ is insensitive to temperature, and the calculated in-plane thermal conductivity of monolayer ${\mathrm{SrTiO}}_3$ is much larger than that of bulk ${\mathrm{SrTiO}}_3$, which mainly results from the unique out-of-plane atomic vibrations in monolayer ${\mathrm{SrTiO}}_3$. The thermal conductivity of ${\mathrm{SrTiO}}_3$ thin film first decreases and then approaches the bulk value as thickness increases from the monolayer limit to 10 nm. The hardening of the out-of-plane acoustic phonon branch and the transition of low-frequency optical phonons can explain the observed trend in thermal conductivity transitions. Our study demonstrates that monolayer ${\mathrm{SrTiO}}_3$ has a higher thermal conductivity than its bulk counterpart with covalent bonds at the first-principles level of accuracy, and dimension reduction has a weak inhibition on phonon transport in freestanding atomically smooth ${\mathrm{SrTiO}}_3$ thin films, which furthers the understanding of phonon transport in two-dimensional perovskite thin films.

We introduce machine learning (ML) models that predict the electronic structure of materials across a wide temperature range. Our models employ neural networks and are trained on density functional theory (DFT) data. Unlike most other ML models that use DFT data, our models directly predict the local density of states (LDOS) of the electronic structure. This provides several advantages, including access to multiple observables such as the electronic density and electronic total free energy. Moreover, our models account for both the electronic and ionic temperatures independently, making them ideal for applications like laser heating of matter. We validate the efficacy of our LDOS-based models on a metallic test system. They accurately capture energetic effects induced by variations in ionic and electronic temperatures over a broad temperature range, even when trained on a subset of these temperatures. These findings open up exciting opportunities for investigating the electronic structure of materials under both ambient and extreme conditions.

In this paper, we report the physical properties of ${\mathrm{Fe}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystals. Fe-intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ exhibits superconductivity, pronounced Shubnikov–de Haas oscillations, high carrier mobility, and features of two-dimensional (2D) transport in the sample volume for $x$ as low as 0.02. The superconductivity of the samples is limited to the range $x=0.02–0.04$. We correlate all the transport properties with structural properties, namely, with mosaic structure of single crystals and point defects. This correlation in the context of the rich literature data allows us to build an intricate physical picture addressing the unique properties of the material under study within the framework of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ quasi-2D defect structure model. The ultimate aim of this paper is to show that many interesting properties of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ may be associated with inhomogeneous distribution of defects in the volume of the sample, which largely replicates the mosaicity of single crystals.

Despite extensive efforts it has remained elusive how the spin-relaxation and spin-precession processes influence the spin-orbit torque and spin memory loss at heavy metal (HM)–ferromagnetic insulator (FI) heterostructure. Here, we study the spin transport of the spin-orbit-coupled ferromagnets based on the SU(2) gauge field theory and reveal that the interfacial spin-orbit coupling is responsible for (i) noncollinear spin exchange, (ii) anisotropic spin relaxation, and (iii) interfacial magnetic field. We show that the noncollinear spin exchange can tune the damping- and field-like torques. The spin-loss conductance is analytically derived by considering interfacial Rashba, Dresselhaus, and strain-induced spin-orbit couplings. Our theory offers a deep understanding of the spin transport across the HM|FI interface.

We propose a paradigm of multiplexed dispersive qubit measurement performed while the qubits dephase. A Laplace transformation of the time-dependent cavity response allows to separate contributions from multiple qubits coupled to the same resonator mode, thus allowing for simultaneous single-shot readout. With realistic parameters for silicon spin qubits we find a competitive readout fidelity, while the measurement time compares favorably to conventional dispersive readout. We extend the multiplexed readout method to quantum nondemolition measurements using auxiliary qubits.

Spin and charge density wave order in the cuprates are known to compete with superconductivity. In the stripe order ${(\text{La},M)}_2{\mathrm{CuO}}_4$ family of cuprates, spin and charge order occur as unidirectional order that can be stabilized by symmetry-breaking structural distortions, such as the low-temperature tetragonal (LTT) phase. Here we examine the interplay between structure and the formation of charge density wave (CDW) order in the LTT phase of ${\mathrm{La}}_{1.475}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$ by applying uniaxial stress to distort the structure and influence the formation of CDW order. Using resonant soft x-ray scattering to measure both the CDW order and (0 0 1) structural-nematic Bragg peaks, we find that the application of uniaxial stress along the Cu-O bond direction suppresses the (0 0 1) peak and has the net effect of reducing CDW order, but does so only for CDW order propagating parallel to the applied stress. We connect these observations to previous work showing an enhanced superconducting transition temperature under uniaxial stress, providing insight into how CDW, superconductivity, nematicity, and structure are related and can be tuned relative to one another in cuprates.

We construct a quasiperiodic lattice model in curved spacetime to explore the crossover concerning both condensed matter and curved spacetime physics. We study the related Anderson localization and find that the model has a clear boundary of localized-extended phase separation, which leads to a swing mobility edge, i.e., the coexistence of localized, swing, and subextended phases. The swing mobility edge reported here features the phase-dependent eigenstate, that is, the eigenstate swing between the extended and localized states for different phase parameters of the quasiperiodic potential. Furthermore, a self-consistent segmentation method is developed to calculate the analytical expression of the critical point of phase separation, and the rich phase diagram is obtained by calculating the fractal dimension and scaling index in multifractal analysis.

In this paper, we address the skyrmion-bimeron dynamical transformation during the propagation of such structures along a magnetic nanotrack with anisotropy gradients. Using a micromagnetic approach and simulations, we observe that a skyrmion continuously transforms into a bimeron, and vice versa (in a reversible way). Furthermore, our results show that the topological charge and the velocity of the intermediary magnetic textures emerging during the transformation are constant. These results can contribute to understanding the dynamic transformation between solitonic magnetic textures with the same topological charge.

Magnetic properties of more than 20 Cantor alloy samples of varying composition were investigated over a temperature range of 5 K to 300 K and in fields of up to 70 kOe using magnetometry and muon spin relaxation. Two transitions are identified: a spin-glass-like transition that appears between $55\mathrm{K}$ and $190\mathrm{K}$, depending on composition, and a ferrimagnetic transition that occurs at approximately $43\mathrm{K}$ in multiple samples with widely varying compositions. The magnetic signatures at $43\mathrm{K}$ are remarkably insensitive to chemical composition. A modified Curie-Weiss model was used to fit the susceptibility data and to extract the net effective magnetic moment for each sample. The resulting values for the net effective moment were either diminished with increasing Cr or Mn concentrations or enhanced with decreasing Fe, Co, or Ni concentrations. Beyond a sufficiently large effective moment, the magnetic ground state transitions from ferrimagnetism to ferromagnetism. The effective magnetic moments, together with the corresponding compositions, are used in a global linear regression analysis to extract element-specific effective magnetic moments, which are compared to the values obtained by ab initio based density functional theory calculations. These moments provide the information necessary to controllably tune the magnetic properties of Cantor alloy variants.

Motivated by recent experiments [Nat. Phys. 16, 1227 (2020)], we present here a theoretical study of the DC Josephson effect in a system comprising two magnetic impurities coupled to their respective superconducting electrodes and which exhibit Yu-Shiba-Rusinov (YSR) states. We make use of a mean-field Anderson model with broken spin symmetry to compute the supercurrent in this system for an arbitrary range of parameters (coupling between the impurities, orientation of the impurity spins, etc.). We predict a variety of physical phenomena such as (i) the occurrence of multiple 0-$\pi$ transitions in the regime of weak coupling that can be induced by changing the energy of the YSR states or the temperature; (ii) the critical current strongly depends on the relative orientation of the impurity spins and it is maximized when the spins are either parallel or antiparallel, depending on the ground state of the impurities; and (iii) upon increasing the coupling between impurities, triplet superconductivity is generated in the system and it is manifested in a highly nonsinusoidal current-phase relation. In principle, these predictions can be tested experimentally with the existing realization of this system and the main lessons of this work are of great relevance for the field of superconducting spintronics.