An analytic method is proposed to compute the surface energy and elementary excitations of the XXZ spin chain with generic nondiagonal boundary fields. For the gapped case, in some boundary parameter regimes the contributions of the two boundary fields to the surface energy are nonadditive. Such a correlation effect between the two boundaries also depends on the parity of the site number $N$ even in the thermodynamic limit $N\to \infty$. For the gapless case, contributions of the two boundary fields to the surface energy are additive due to the absence of long-range correlation in the bulk. Although the $U\left(1\right)$ symmetry of the system is broken, exact spinonlike excitations, which obviously do not carry spin-$\frac{1}{2}$, are observed. The present method provides a universal procedure to deal with quantum integrable systems either with or without $U\left(1\right)$ symmetry.

The Dzyaloshinskii-Moriya interaction, i.e., antisymmetric exchange interaction, combined with a Zeeman magnetic field gives rise to various magnetic states such as chiral, helical, and skyrmionic states. This interaction conventionally originates from spin-orbit coupling and thus needs somewhat heavy elements. In contrast, we here show a Dzyaloshinskii-Moriya interaction which is driven by electric current vorticity. We also find that the vorticity acts on localized spins as a Zeeman field, which may explain a recent experiment on current-driven magnetic skyrmion creation and annihilation without an external magnetic field in a device with a notch structure in FeGe. theory explains the control of skyrmion creation and annihilation by current direction and opens different possibilities for studies of magnetic textures by using structural settings.

In this work, the thermodynamic properties of the organic superconductor $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ are investigated to study a high-field superconducting state known as the putative Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. We observed a small thermodynamic anomaly in the field $H_{\mathrm{FFLO}}\sim 10$ T, which corresponds to the Pauli limiting field $H_{\mathrm{P}}$. This anomaly probably originates from a transition from a uniform superconducting state to the FFLO state. $H_{\mathrm{FFLO}}$ does not show a strong-field angular dependence due to a quasi-isotropic paramagnetic effect in $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$. The thermodynamic anomaly at $H_{\mathrm{FFLO}}$ is smeared out. and the low-temperature upper critical field $H_{\mathrm{c}2}$ changes significantly if fields are not parallel to the conducting plane, even for a deviation of $\sim 0.5^{\circ }$. This behavior indicates that the high-field state is very unstable, as it is influenced by the strongly anisotropic orbital effect. Our results are consistent with the theoretical predictions on the FFLO state and show that the high-field superconductivity is probably an FFLO state in $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ from a thermodynamic point of view.

We study the attractive Hubbard model on the Lieb lattice to understand the normal state above the superconducting critical temperature in a flat band system. We use cluster dynamical mean-field theory to compute the two-particle susceptibilities with full quantum fluctuations included in the cluster. At interaction strengths lower than the hopping amplitude, we find that the normal state on the flat band shows insulating behavior stemming from the localization properties of the flat band. A flat-band enhanced pseudogap with a depleted spectral function arises at larger interactions.

Intermetallic silicide compounds, LaScSi and ${\mathrm{Y}}_5{\mathrm{Si}}_3$, known for being hydrogen (H) storage materials, are drawing attention as candidates for electrides in which anions are substituted by unbound electrons. It is inferred from a muon spin rotation experiment that the local field at the muon site (which is the same site as that for H) in these compounds exhibits a large negative shift under an external magnetic field, which is mostly independent of temperature. Such anomalous diamagnetism signals a unique property of electride electrons associated with transition metals. Moreover, the diamagnetic shift decreases with increasing H content, suggesting that the electride electrons existing coherently in the hollow interstitial positions are adsorbed by H to form hydride ions $\left({\mathrm{H}}^{-}\right)$.

Manipulating electron spins on ultrashort timescales shows promise in the field of data processing. Because of the direct electron-photon coupling, photons have been widely used for such studies, but not phonons. Here we tie coherent phonon detection to transient spin populations. We optically excite gigahertz picosecond phonon wave packets in a metal-coated GaAs slab containing $\mathrm{GaAs}\text{/}{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ multiple quantum wells on the opposite side. Before the phonon wave-packet arrival, circularly polarized light induces a transient spin polarization. Phonon-induced ultrafast polarization rotation and reflectivity changes contingent on the transient spin population are detected by a heterodyne modulation technique. With an analytical model, we posit the presence of significant second-order interactions, enabled by spins, in the coherent-phonon optical detection. Applications include transient spin population monitoring and novel spintronic nanodevices.

The quest for nonequilibrium quantum phase transitions is often hampered by the tendency of driving and dissipation to give rise to an effective temperature, resulting in classical behavior. Could this be different when the dissipation is engineered to drive the system into a nontrivial quantum coherent steady state? In this work we shed light on this issue by studying the effect of disorder on recently introduced dissipation-induced Chern topological states, and examining the eigenmodes of the Hermitian steady-state density matrix or entanglement Hamiltonian. We find that, similarly to equilibrium, each Landau band has a single delocalized level near its center. However, using three different finite-size scaling methods we show that the critical exponent $\nu$ describing the divergence of the localization length upon approaching the delocalized state is significantly different from equilibrium if disorder is introduced into the nondissipative part of the dynamics. This indicates a different type of nonequilibrium quantum critical universality class accessible in cold-atom experiments.

Magnetism of graphene can be created by atomic defects, either hydrogen adsorption or single-carbon vacancy formation, owing to the unpaired $\pi$ electrons around the defects. Here we explore, based on rigorous first principles calculations, the possibility of voltage manipulation of two such types of $\pi$ magnetism in graphene via a scanning tunneling microscope tip. We find a remarkably different behavior. For the hydrogen, the magnetic moment can be switched on and off with voltage-induced doping, whereas, for the carbon vacancy, the spin splitting of the $\pi$ bands persists, almost independent of the extent of doping, due to the coupling between the $\pi$ and the $\sigma$ bonds. Furthermore, the local atomic structures near the vacancy can be reversibly manipulated by a coordination mechanism between an intermediate tip-defect distance and a moderate tip voltage, consequently leading to the reversal of spin polarization of the $\pi$ bands. Voltage control of the local magnetic states may open a new avenue for potential applications in spintronics.

Determination of atomic structures is a key challenge in the fields of computational physics and materials science, as a large variety of mechanical, chemical, electronic, and optical properties depend sensitively on structure. Here, we present a global optimization scheme where energy and force information from density functional theory (DFT) calculations is transferred to a probabilistic surrogate model to estimate both the potential energy surface (PES) and the associated uncertainties. The local minima in the surrogate PES are then used to guide the search for the global minimum in the DFT potential. We find that adding the gradients in most cases improves the efficiency of the search significantly. The method is applied to global optimization of ${\left[{\mathrm{Ta}}_2{\mathrm{O}}_5\right]}_x$ clusters with $x=1,2,3$, and the surface structure of oxidized ZrN.

The Green-Kubo theory of atomic heat transport relies on the ambiguous decomposition of total energy into “atomic” energies of individual atoms. The challenge is to understand how the transport coefficient thermodynamically emerges from such ill-defined atomic energies. Here, we show that a simple symmetry principle for atomic energies (that is, “all possible ways of distributing energy among atoms are equivalent”) dictates the general theory of atomic heat transport, in which, by construction, the same heat conductivity κ is ensured regardless of the choice of atomic energies. This approach is analogous to the way in which general covariance (“all coordinate systems are equivalent”) is used to construct the general theory of relativity. To this end, we define atomic gauges that regulate the redundant degrees of freedom in atomic energies. The atomic gauge symmetry then uniquely determines the gauge-invariant form of macroscopic energy transfer, which is identified as heat. The gauge theory not only lays a firm foundation of the Green-Kubo formalism of heat transport, but also offers a variational method for calculating κ of nonsolid materials, as demonstrated using machine learning inferred atomic energies of ${\mathrm{Cu}}_2\mathrm{S}$, an intriguing solid-liquid hybrid material for thermoelectrics.

We report in this paper the dynamical properties of ${\mathrm{GdMn}}_2{\mathrm{O}}_5$ studied by inelastic neutron scattering and infrared spectroscopy assisted by ab initio calculations. Our work sheds light on the electromagnon, a magnetic mode that can be excited by an electric field. Combining spin-wave measurements, simulations, and ab initio calculations of the single-ion anisotropies and the superexchange interactions, we describe in detail the magnetic contribution to this mode. An exhaustive study of the temperature and polarization dependence of its electroactivity completes this comprehensive study of the complex ${\mathrm{GdMn}}_2{\mathrm{O}}_5$ system.

Inertia effects in magnetization dynamics are theoretically shown to result in a different type of spin waves, i.e., nutation surface spin waves, which propagate at terahertz frequencies in in-plane magnetized ferromagnetic thin films. Considering the magnetostatic limit, i.e., neglecting exchange coupling, we calculate dispersion relation and group velocity, which we find to be slower than the velocity of conventional (precession) spin waves. In addition, we find that the nutation surface spin waves are backward spin waves. Furthermore, we show that inertia causes a decrease of the frequency of the precession spin waves, namely magnetostatic surface spin waves and backward volume magnetostatic spin waves. The magnitude of the decrease depends on the magnetic properties of the film and its geometry.

Levitation of superconductors is becoming an important building block in quantum technologies, particularly, in the rising field of magnetomechanics. In most of the theoretical proposals and experiments, solid geometries, such as spheres are considered for the levitator. Here we demonstrate that replacing them by superconducting rings brings two important advantages: First, the forces acting on the ring remain comparable to those expected for solid objects, whereas the mass of the superconductor is greatly reduced. In turn, this reduction increases the achievable trap frequency. Second, the flux trapped in the ring by in-field cooling yields an additional degree of control for the system. We construct a general theoretical framework with which we obtain analytical formulations for a superconducting ring levitating in an anti-Helmholtz quadrupole field and a dipole field for both zero-field and field cooling. The positions and the trapping frequencies of the levitated rings are analytically found as a function of the parameters of the system and the field applied during the cooling process. Unlike what is commonly observed in bulk superconductors, lateral and rotational stabilities are not granted for this idealized geometry. We, therefore, discuss the requirements for simple superconducting structures to achieve stability in all degrees of freedom.

One of the hallmarks of light-spin interaction in solids is the appearance of photocurrent that depends on the light helicity. Recent studies have shown that helicity dependent photocurrent (HDP) emerges due to light induced spin current and the inverse spin Hall effect of semimetal thin films. We have studied HDP in metal/semimetal bilayers. Compared to Bi single layer films, we find the HDP is enhanced in metal/Bi bilayers. For the bilayers, the sign of HDP under back illumination reverses from that of front illumination. The back illumination photocurrent is the largest for Ag/Bi bilayers among the bilayers studied. Using a diffusive spin transport model, we show that the HDP sign reversal under back illumination is caused by spin absorption and spin to charge conversion at the interface. Such interfacial effects contribute to the HDP enhancement under front illumination for the bilayers when the Bi layer thickness is small. These results show that the HDP can be used to assess interface states with strong spin orbit coupling.

Superconductor-ferromagnet heterostructures hosting vortices and skyrmions are a new area of the interplay between superconductivity and magnetism. We study the interaction of a Néel-type skyrmion and a Pearl vortex in thin heterostructures due to stray fields. Surprisingly, we find that it can be energetically favorable for the Pearl vortex to be situated at some nonzero distance from the center of the Néel-type skyrmion. The presence of a vortex-antivortex pair is found to result in the increase of the skyrmion radius. Our theory predicts that a spontaneous generation of a vortex-antivortex pair is possible under some conditions in the presence of a Néel-type skyrmion.

The dissolving of hydrogen (H) into rhenium (Re) is non-negligible in high-pressure experiments involving dense hydrogen and hydrogenous materials, in which rhenium is often used as a gasket material in the high-pressure diamond-anvil cell devices. However, the evolution of hydrogen dissolution into Re and the pressure-induced effects of hydrogen coupling on the superconductivity of metal-based hydrides remain poorly understood experimentally, and theoretical investigations are currently lacking. The present work addresses these issues by applying ab initio calculations for systematically investigating the dissolution of H atoms in Re systems, as well as the electronic properties, lattice dynamics, and electron-phonon coupling (EPC) in the corresponding interstitial Re hydrides. A series of $\mathrm{Re}{\mathrm{H}}_x$ compounds with increasing $x$ in the range of 0 to 1 in increments of 0.125 are predicted to be stable at increasing pressures ranging from 1 to 50 GPa. It is found that H atoms generally play the role of impurities in the $\mathrm{Re}{\mathrm{H}}_x$ compounds except for $\mathrm{Re}{\mathrm{H}}_{0.5}$ and ReH, where H atoms are integrated into the crystal structure. Moreover, the atomic configuration of the Re host lattice is basically preserved with increasing $x$ and increasing pressure. Further analysis of the superconductive mechanism indicates that an increasing H concentration results in a decreased electronic density of states at the Fermi level and increasing phonon frequencies, which lead to a decrease of EPC λ and superconducting critical temperature. These findings quantitatively clarify the effects of hydrogen doping on superconductivity in metal-based hydrides, and thereby support the exploration of new types of superconductive hydrides.

We investigate magnon-photon couplings by employing a small magnet within an irregular resonant cavity, which leads to a desirable nonreciprocity with a big isolation ratio. Moreover, the higher-order couplings between the spin wave modes and the polarized photon modes also exhibit the nonreciprocity. These couplings between polarized photon and spin waves could be regarded as a multimode hybridization mediated by cavity photons. We also derive a coupling matrix to predict the characteristics of this kind of indirect coupling. The existence of indirect couplings broadens the field range of the nonreciprocity of the system. The achieved nonreciprocal multimode magnon-photon couplings in a single system offer a feasible method to improve the signal transmission quality.

Noncollinear spin structures, such as helimagnetic structures, have been gathering attention because of their nontrivial magnetic transport induced by topological effects. For a precise determination of spin structure, in addition to traditional neutron diffraction measurements, the time-reversal-odd rectification of electronic transport, which is referred to as nonreciprocal electronic transport, provides useful information because of the symmetry sensitivity. We observed nonreciprocal electronic transport below the Néel temperature in an itinerant antiferromagnet, $\mathrm{Pd}\mathrm{Cr}{\mathrm{O}}_2$, with a ${120}^{\circ }$ spin structure. This implies breaking of spatial inversion symmetry owing to the magnetic ordering. Possible spin structures are discussed based on the magnetic field dependence of nonreciprocal transport.

We report a comprehensive study of the centrosymmetric ${\mathrm{Re}}_3\mathrm{B}$ and noncentrosymmetric ${\mathrm{Re}}_7{\mathrm{B}}_3$ superconductors. At a macroscopic level, their bulk superconductivity (SC), with $T_c=5.1\mathrm{K}$ (${\mathrm{Re}}_3\mathrm{B}$) and 3.3 K (${\mathrm{Re}}_7{\mathrm{B}}_3$), was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were investigated by means of muon-spin rotation and relaxation ($\mu \mathrm{SR}$). In both ${\mathrm{Re}}_3\mathrm{B}$ and ${\mathrm{Re}}_7{\mathrm{B}}_3$ the low-$T$ zero-field electronic specific heat and the superfluid density (determined via transverse-field $\mu \mathrm{SR}$) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by the temperature-dependent upper critical fields $H_{\mathrm{c}2}\left(T\right)$, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below the onset of SC, as determined from zero-field $\mu \mathrm{SR}$ measurements, indicates a preserved time-reversal symmetry in the superconducting state of both ${\mathrm{Re}}_3\mathrm{B}$ and ${\mathrm{Re}}_7{\mathrm{B}}_3$. Our results suggest that a lack of inversion symmetry and the accompanying antisymmetric spin-orbit coupling effects are not essential for the occurrence of multigap SC in these rhenium-boron compounds.