We numerically compute current-induced spin-transfer torques for antiferromagnetic domain walls, based on a linear response theory in a tight-binding model. We find that, unlike for ferromagnetic domain-wall motion, the contribution of adiabatic spin torque to antiferromagnetic domain-wall motion is negligible, consistent with previous theories. As a result, the nonadiabatic spin-transfer torque is a main driving torque for antiferromagnetic domain-wall motion. Moreover, the nonadiabatic spin-transfer torque for narrower antiferromagnetic domain walls increases more rapidly than that for ferromagnetic domain walls, which is attributed to the enhanced spin mistracking process for antiferromagnetic domain walls.

The search for superconducting hydrides has, so far, largely focused on finding materials exhibiting the highest possible critical temperatures ($T_c$). This has led to a bias toward materials stabilized at very high pressures, which introduces a number of technical difficulties in experiment. Here we apply machine learning methods in an effort to identify superconducting hydrides that can operate closer to ambient conditions. The output of these models informs subsequent crystal structure searches, from which we identify stable metallic candidates prior to performing electron-phonon calculations to obtain $T_c$. Hydrides of alkali and alkaline earth metals are identified as especially promising; of particular note, a $T_c$ of up to 115 K is calculated for ${\mathrm{RbH}}_{12}$ at 50 GPa, which extends the operational pressure-temperature range occupied by hydride superconductors toward ambient conditions.

Optically generated electron-hole pairs can probe strongly correlated electronic matter, or, by forming exciton-polaritons within an optical cavity, give rise to photonic nonlinearities. The present paper theoretically studies the properties of electron-hole pairs in a two-dimensional electron liquid in the fractional quantum Hall regime. In particular, we quantify the effective interactions between optical excitations by numerically evaluating the system's energy spectrum under the assumption of full spin and Landau level polarization. Optically most active are those pair excitations which do not modify the correlations of the electron liquid, also known as multiplicative states. In the case of spatial separation of electrons and holes, these excitations interact repulsively with each other. However, when the electron liquid is compressible, other nonmultiplicative configurations occur at lower energies. The interactions of such dark excitations strongly depend on the liquid, and can also become attractive. For the case of a single excitation, we also study the effect of Landau level mixing in the valence band which can dramatically change the effective mass of an exciton.

We present a powerful concept in the field of photonic crystals and metamaterials, applying the resonant-state expansion (RSE) to planar photonic crystal structures. The RSE allows us to understand and quantify optical resonances in photonic crystal structures in terms of the analytic resonant states of a homogeneous planar waveguide. The RSE provides an efficient and reliable tool for accurate calculation of a complete set of the resonant states of a photonic crystal slab, which is required for the correct description and a better understanding of its optical spectra. For the proof of principle, numerical verification of the RSE, and demonstration of its unprecedented accuracy and convergence, an infinite planar photonic crystal slab periodic in one dimension is taken as an example. To illustrate the power of this approach, we consider the mode evolution with the amplitude of the periodic modulation, revealing the role of the guided modes in the formation of bound states in the continuum.

We present a general quantum kinetic theory that accounts for the interplay between a temperature gradient, momentum-space Berry curvatures of Bloch electrons, and Bloch-state scattering. Using a theory that incorporates the presence of a temperature gradient by introducing a “thermal vector potential,” we derive a quantum kinetic equation for Bloch electrons in the presence of disorder and a temperature gradient. In contrast to the semiclassical Boltzmann formalism in which a temperature gradient is introduced by setting $\dot{\mathit{r}}·\frac{\partial }{\partial \mathit{r}}\to \dot{\mathit{r}}·\frac{\partial T}{\partial \mathit{r}}\frac{\partial }{\partial T}$ in the Boltzmann equation, the presence of a temperature gradient in our formalism is described as a driving force just as in the case of an electric field (i.e., comes from $\dot{\mathit{k}}$ in the language of the semiclassical Boltzmann formalism). Taking also into account the presence of electric and magnetic fields, the quantum kinetic equation we derive makes it possible to compute transport coefficients at arbitrary orders of electric-field $\mathit{E}$, magnetic-field $\mathit{B}$, and temperature-gradient $\mathbf{\nabla }T$ strengths ${\left|\mathit{E}\right|}^a{\left|\mathit{B}\right|}^b{\left|\mathbf{\nabla }T\right|}^c$. Our theory enables a systematic calculation of magnetothermoelectric and magnetothermal conductivities of systems with momentum-space Berry curvatures. As an illustration, we derive from a general microscopic electron model a general expression for the rate of pumping of electrons between valleys in parallel temperature gradient and magnetic field. From this expression we find a relation, which is analogous to the Mott relation, between the rate of pumping due to a temperature gradient and that due to an electric field. We also apply our theory to a two-band model for Weyl semimetals to study thermoelectric and thermal transport in a magnetic field. We show that the Mott relation is satisfied in the chiral-anomaly induced thermoelectric conductivity, and that the Wiedemann-Franz law is violated in the chiral-anomaly induced thermal conductivity, which are both consistent with the results obtained by invoking semiclassical wave-packet dynamics.

In this paper we investigate the impact of gating potential and magnetic field on phonon induced spin relaxation rate and the speed of the electrically driven single-qubit operations inside the InSb nanowire spin qubit. We show that a strong $g$ factor and high magnetic field strength lead to the prevailing influence of electron-phonon scattering due to deformation potential, considered irrelevant for materials with a weak $g$ factor, like GaAs or Si/SiGe. In this regime we find that spin relaxation between qubit states is significantly suppressed due to the confinement perpendicular to the nanowire axis. We also find that maximization of the number of single-qubit operations that can be performed during the lifetime of the spin qubit requres single quantum dot gating potential.

Inherent symmetries of a system lead to multiple degeneracies of its energy spectra. Introducing individual atomic impurities can locally break these symmetries, which is expected to lift the degenerate degrees of freedom around the impurities. Although central to our understanding of the fundamental properties of solids, the broken-symmetry states induced by individual atomic impurities have so far eluded observation. Here, we report nanoscale probing of the broken-symmetry states in graphene induced by two types of individual atomic impurities, i.e., isolated nitrogen dopants and isolated hydrogen atoms chemisorbed on graphene. Our experiments demonstrate that both types of atomic impurities can locally break sublattice symmetry of graphene and generate valley-polarized states, which extends several nanometers around the impurities. For the isolated hydrogen atom chemisorbed on graphene, the enhanced spin-orbit coupling, which arises from the $s{p}^3$ distortion of graphene due to the hydrogen chemisorption, further lifts the spin degeneracy, resulting in a fully spin- and valley-polarized state within about 1 nm around the hydrogen atom. Our result paves the way to control various broken-symmetry states at the nanoscale by various atomic impurities.

We investigate topological phases of bilayer graphene subject to antiferromagnetic exchange fields, interlayer bias, and light irradiation. We discover that at finite bias and light intensity the system transitions into a previously unknown spin-valley polarized quantum anomalous Hall (SVP-QAH) insulator state, for which the subsystem of one spin is a valley Hall topological insulator (TI) and that of the other spin is a QAH insulator. We assess the TI phases occurring in the system by analytically calculating the spin-valley-dependent Chern number and characterize them by considering edge states in a nanoribbon. We demonstrate that the SVP-QAH edge states lead to a unique spin rectification effect in a domain wall. Along the phase boundary, we observe a bulk half-metal state with Berry's phase of $2\pi$.

We consider the radiation of angular momentum (AM) from current-carrying molecular junctions. Using the nonequilibrium Green's function method, we derive a convenient formula for the AM radiation and apply it to a prototypical benzene molecule junction. We discuss the selection rules for inelastic transitions between the molecular angular momentum eigenstates due to a sixfold rotational symmetry. Our study provides important insights into the generation of light with AM from DC-biased molecular junctions.

Monolayer ${\mathrm{AlB}}_2$ is composed of two atomic layers: honeycomb borophene and triangular aluminum. In contrast with the bulk phase, monolayer ${\mathrm{AlB}}_2$ is predicted to be a superconductor with a high critical temperature. Here, we demonstrate that monolayer ${\mathrm{AlB}}_2$ can be synthesized on Al(111) via molecular beam epitaxy. Our theoretical calculations revealed that the monolayer ${\mathrm{AlB}}_2$ hosts several Dirac cones along the $\mathrm{\Gamma }\text{−}M$ and $\mathrm{\Gamma }\text{−}K$ directions; these Dirac cones are protected by crystal symmetries and are thus resistant to external perturbations. The extraordinary electronic structure of the monolayer ${\mathrm{AlB}}_2$ was confirmed via angle-resolved photoemission spectroscopy measurements. These results are likely to stimulate further research interest to explore the exotic properties arising from the interplay of Dirac fermions and superconductivity in two-dimensional materials.

We theoretically investigate a relation between the crystalline symmetry and the transient modulation of optical properties of crystalline dielectrics in pump-probe measurements using intense pump and attosecond probe fields. When the photon energy of the pump field is much below the bandgap energy, the modulation of the optical conductivity is caused by the intraband electronic motion, that is, the dynamical Franz-Keldysh effect. We analytically investigate symmetry properties of the modulated optical conductivity utilizing the Houston function and derive a formula that relates the temporal oscillation in the absorption with the transformation properties of the modulated optical conductivity. To verify the validity of the formula, we perform real-time first-principles calculations based on the time-dependent density functional theory for a pump-probe process taking 4H-SiC crystal as an example.

It is demonstrated by explicit construction that three-dimensional bosonic crystalline symmetry protected topological (cSPT) phases are classified by $H_{\varphi }^5(G;\mathbb{Z})\oplus H_{\varphi }^1(G;\mathbb{Z})$ for all 230 space groups $G$, where $H_{\varphi }^n(G;\mathbb{Z})$ denotes the $n\mathrm{th}$ twisted group cohomology of $G$ with $\mathbb{Z}$ coefficients, and $\varphi$ indicates that $g\in G$ acts nontrivially on coefficients by sending them to their inverses if $g$ reverses space-time orientation and acts trivially otherwise. The previously known summand $H_{\varphi }^5(G;\mathbb{Z})$ corresponds only to crystalline phases built without the $E_8$ state or its multiples on 2-cells of space. It is the crystalline analog of the “group cohomology proposal” for classifying bosonic symmetry protected topological (SPT) phases, which takes the form $H_{\varphi }^{d+2}(G;\mathbb{Z})\cong H_{\varphi }^{d+1}(G;\text{U}\left(1\right))$ for finite internal symmetry groups in $d$ spatial dimensions. The new summand $H_{\varphi }^1(G;\mathbb{Z})$ classifies possible configurations of $E_8$ states on 2-cells that can be used to build crystalline phases beyond the group cohomology proposal. The completeness of our classification and the physical meaning of $H_{\varphi }^1(G;\mathbb{Z})$ are established through a combination of dimensional reduction, surface topological order, and explicit cellular construction. The value of $H_{\varphi }^1(G;\mathbb{Z})$ can be easily read off from the international symbol for $G$. Our classification agrees with the prediction of the “generalized cohomology hypothesis,” which concerns the general structure of the classification of SPT phases, and therefore provides strong evidence for the validity of the said hypothesis in the realm of crystalline symmetries.

We propose a single-site mean-field description, an analog of Weiss mean-field theory, suitable for narrow-band systems with correlation-induced hybridization at finite temperatures. Presently, this approach, based on the notion of a fluctuating onsite density matrix (OSDM), is developed for the case of extended Falicov-Kimball model (EFKM). In an EFKM, an excitonic insulator phase can be stabilized at zero temperature. With increasing temperature, the excitonic order parameter (interaction-induced hybridization onsite, characterized by the absolute value and phase) eventually becomes disordered, which involves fluctuations of both its phase and (at higher $T$) its absolute value. In order to build an adequate finite-temperature description, it is important to clarify the nature of degrees of freedom associated with the phase and absolute value of the induced hybridization, and correctly account for the corresponding phase-space volume. We show that the OSDM-based treatment of the local fluctuations indeed provides an intuitive and concise description (including the phase-space integration measure). This allows to describe both the lower-temperature regime where phase fluctuations destroy the long-range order, and the higher-temperature crossover corresponding to a decrease of the absolute value of hybridization. In spite of the rapid progress in the studies of excitonic insulators, a unified picture of this kind has not been available to date. We briefly discuss recent experiments on ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ and also address the amplitude mode of collective excitations in relation to the measurements reported for $1T\text{−}{\mathrm{TiSe}}_2$. Both the overall scenario and the theoretical framework are also expected to be relevant in other contexts, including the Kondo lattice model.

Symmetry fractionalization, generating a large amount of symmetry-protected topological phases, provides scenarios for continuous phase transitions different from spontaneous symmetry breaking. However, it is hard to detect these symmetry-protected topological phase transitions experimentally. Motivated by the recent development of highly tunable ultracold polar molecules, we show that the setup in a zigzag optical lattice of this system provides the perfect platform to realize symmetry-protected topological phase transitions. By using infinite time-evolving block decimation, we obtain the phase diagram in a large parameter region and find another scheme to realize the long-sought vector chiral phase, which is robust from quantum fluctuations. We discuss the existence of the chiral phase by an effective field analysis.

Analytical expressions for calculating the energy density and spatial correlation function of thermal emission by a homogeneous, isothermal sphere of arbitrary size and material are presented. The spectral distribution and the power law governing the distance-dependent energy density are investigated in the near-field and far-field regimes for silicon carbide (dielectric), silicon (semiconducting), and tungsten (metallic) spheres of various size parameters ranging from $X=0.002$ to 5. The spatial coherence of the thermal field emitted by spheres of different size and material is also studied in both radial and polar directions, and the effect of localized surface phonons (LSPhs) on the correlation length and angle is elucidated. It is shown that the energy density follows a power law of $d^{-2}$ ($d$ is the observation distance) in the far field independent of the size and material of the sphere. The power law in the near field is strongly dependent on the material, size parameter, and the ratio $\frac{d}{a}$ ($a$ is the sphere radius). In the near field, the energy density follows a power law of $d^{-6}$ when $X\ll 1$ and $\frac{d}{a}\gg 1$ (similar to an electric point dipole). With increasing $X$ or decreasing $\frac{d}{a}$, the contribution of multipoles to the energy density increases resulting in an increase in the power of $d$ until the power law converges to that for a semi-infinite medium ($d^{-2.5}$, $d^{-0.5}$, and $d^{-3.5}$ for silicon carbide, silicon, and tungsten, respectively, in the intermediate near field, and $d^{-3.5}$, $d^{-3.5}$, and $d^{-2.5}$ for silicon carbide, silicon, and tungsten, respectively, in the extreme near field.). It is also found that the spatial correlation length in the radial direction is approximately on the order of λ, 0.1λ, and 0.001λ in the far-field, intermediate near-field, and extreme near-field regimes, respectively, when the multipolar LSPhs are not supported. The correlation angle in the extreme near field is strongly dependent on the sphere size parameter, such that it decreases by three orders of magnitude (from 0.5π to 0.001π) when $X$ increases from 0.002 to 5. The dependence of the correlation angle on $X$ decreases significantly in the intermediate near-field and far-field regimes, and the correlation angle retains the same order of magnitude (0.15π − 0.7π) for all considered $X\mathrm{s}$ in these two regimes. While the excitation of dipolar LSPhs does not affect the correlation length and angle of the thermal field, the multipolar LSPhs reduce the spatial coherence in both directions.

We theoretically investigate coupled gyration modes of magnetic solitons whose distances to the nearest neighbors are staggered. In a one-dimensional bipartite lattice, we analytically and numerically find that there is a midgap gyration mode bounded at the domain wall connecting topologically distinct two phases which is analogous to the Su-Schrieffer-Heeger model. As a technological application, we show that a one-dimensional domain-wall string in a two-dimensional soliton lattice can serve as a waveguide of magnetic excitations, which offers functionalities of a signal localization and selective propagation of the frequency modes. Our result offers an alternative way to control the magnetic excitation modes by using a magnetic metamaterial for future spintronic devices.

Geometrically frustrated kagome-lattice antiferromagnets have received much attention because of their exotic magnetic behaviors. Here we report the synthesis, structure, and magnetism of an $S=5/2$ quasiclassical new spin kagome compound. Polycrystalline compounds of $\mathrm{M}{\mathrm{g}}_x\mathrm{M}{\mathrm{n}}_{4-x}{\left(\mathrm{OH}\right)}_6\mathrm{C}{\mathrm{l}}_2$, with a nominal ideal chemical formula of $\mathrm{MgM}{\mathrm{n}}_3{\left(\mathrm{OH}\right)}_6\mathrm{C}{\mathrm{l}}_2$, have been successfully synthesized by selectively substituting the triangular-lattice-plane $\mathrm{M}{\mathrm{n}}^{2+}$ with nonmagnetic $\mathrm{M}{\mathrm{g}}^{2+}$. The compounds studied crystallize in rhombohedral structure in space group $R\text{−}3m$, in a similar crystal structure to the much-researched quantum spin liquid candidates herbertsmithite $\mathrm{ZnC}{\mathrm{u}}_3{\left(\mathrm{OH}\right)}_6\mathrm{C}{\mathrm{l}}_2$ and tondiite $\mathrm{MgC}{\mathrm{u}}_3{\left(\mathrm{OH}\right)}_6\mathrm{C}{\mathrm{l}}_2$. Antiferromagnetic transition below $T_{\mathrm{N}}\sim 8\mathrm{K}$ and a 120° nearest-neighbor spin ordering confined in the kagome plane with positive spin chirality have been revealed by magnetic susceptibility measurements and neutron diffraction experiment. The obtained value of critical exponent $\beta =0.35$ agrees with a three-dimensional Heisenberg spin model. The present work suggests the intrinsic nature of long-range order in classical Heisenberg kagome aniferromagnet and provides a classical reference system to quantum kagome antiferromagnets.

Bardeen-Cooper-Schrieffer (BCS) theory describes a superconducting transition as a single critical point where the gap function or, equivalently, the order parameter vanishes uniformly in the entire system. We demonstrate that in superconductors described by standard BCS models, the superconducting gap survives near the sample boundaries at higher temperatures than superconductivity in the bulk. Therefore, conventional superconductors have multiple critical points associated with separate phase transitions at the boundary and in the bulk. We show this by revising the Caroli–De Gennes–Matricon theory of a superconductor-vacuum boundary and finding inhomogeneous solutions of the BCS gap equation near the boundary, which asymptotically decay in the bulk. This is demonstrated for a BCS model of almost free fermions and for lattice fermions in a tight-binding approximation. The analytical results are confirmed by numerical solutions of the microscopic model. The existence of these boundary states can manifest itself as discrepancies between the critical temperatures observed in calorimetry and transport probes.

By using mostly the muon-spin rotation/relaxation ($\mu \mathrm{SR}$) technique, we investigate the superconductivity (SC) of ${\mathrm{Nb}}_5{\mathrm{Ir}}_{3-x}{\mathrm{Pt}}_x\mathrm{O}$ ($x=0$ and 1.6) alloys, with $T_c=10.5$ and 9.1 K, respectively. At a macroscopic level, their superconductivity was studied by electrical resistivity, magnetization, and specific-heat measurements. In both compounds, the electronic specific heat and the low-temperature superfluid density data suggest a nodeless SC. The superconducting gap value and the specific heat discontinuity at $T_c$ are larger than that expected from BCS theory in the weak-coupling regime, indicating strong-coupling superconductivity in the ${\mathrm{Nb}}_5{\mathrm{Ir}}_{3-x}{\mathrm{Pt}}_x\mathrm{O}$ family. In ${\mathrm{Nb}}_5{\mathrm{Ir}}_3\mathrm{O}$, multigap SC is evidenced by the field dependence of the electronic specific heat coefficient and the superconducting Gaussian relaxation rate, as well as by the temperature dependence of the upper critical field. Pt substitution suppresses one of the gaps, and ${\mathrm{Nb}}_5{\mathrm{Ir}}_{1.4}{\mathrm{Pt}}_{1.6}\mathrm{O}$ becomes a single-gap superconductor. By combining our extensive experimental results, we provide evidence for a multiple- to single-gap SC crossover in the ${\mathrm{Nb}}_5{\mathrm{Ir}}_{3-x}{\mathrm{Pt}}_x\mathrm{O}$ family.