Magnetic bimerons are topologically nontrivial spin textures in in-plane easy-axis magnets, which can be used as particle-like information carriers. Here, we report a theoretical study on the nonreciprocal dynamics of asymmetrical ferrimagnetic (FiM) bimerons induced by spin currents. The FiM bimerons have the ability to move at a speed of kilometers per second and do not show the skyrmion Hall effect at the angular momentum compensation point. Our micromagnetic simulations and analytical results demonstrate that spin currents are able to induce the nonreciprocal transport and a drift motion of the FiM bimeron even if the system is at the angular momentum compensation point. By analyzing the current-induced effective fields, we find that the nonreciprocal transport is attributed to the asymmetry of the bimeron structure. Our results are useful for understanding the physics of bimerons in ferrimagnets and may provide guidelines for building bimeron-based spintronic devices.

${\mathrm{EuAl}}_4$ and ${\mathrm{EuGa}}_4$ are two candidate materials for studying the interplay between correlated-electron phenomena, topological spin textures, and topologically nontrivial bands. Both compounds crystallize in a centrosymmetric tetragonal ${\mathrm{BaAl}}_4$-type structure (space group $I4/mmm$) and show antiferromagnetic (AFM) order below $T_{\mathrm{N}}=15.6$ and 16.4 K, respectively. Here, we report on systematic muon-spin rotation and relaxation ($\mu \mathrm{SR}$) studies of the magnetic properties of ${\mathrm{EuAl}}_4$ and ${\mathrm{EuGa}}_4$ single crystals at a microscopic level. In both cases, transverse-field $\mu \mathrm{SR}$ measurements, spanning a wide temperature range (from 1.5 to 50 K), show clear bulk AFM transitions, with an almost 100% magnetic volume fraction. Zero-field $\mu \mathrm{SR}$ measurements, covering both the AFM and the paramagnetic (PM) states, reveal internal magnetic fields $B_{\mathrm{int}}\left(0\right)=0.33$ T and 0.89 T in ${\mathrm{EuAl}}_4$ and ${\mathrm{EuGa}}_4$, respectively. The transverse muon-spin relaxation rate ${\lambda }_{\mathrm{T}}$, a measure of the internal field distribution at the muon-stopping site, shows a contrasting behavior. In ${\mathrm{EuGa}}_4$, it decreases with lowering the temperature, reaching its minimum at zero temperature, ${\lambda }_{\mathrm{T}}\left(0\right)=0.71 \mu {\mathrm{s}}^{-1}$. In ${\mathrm{EuAl}}_4$, it increases significantly below $T_{\mathrm{N}}$, to reach 58 $\mu {\mathrm{s}}^{-1}$ at 1.5 K, most likely reflecting the complex magnetic structure and the competing interactions in the AFM state of ${\mathrm{EuAl}}_4$. In both compounds, the temperature-dependent longitudinal muon-spin relaxation ${\lambda }_{\mathrm{L}}\left(T\right)$, an indication of the rate of spin fluctuations, diverges near the onset of AFM order, followed by a significant drop at $T<T_{\mathrm{N}}$. In the AFM state, spin fluctuations are much stronger in ${\mathrm{EuAl}}_4$ than in ${\mathrm{EuGa}}_4$, while being comparable in the PM state. The evidence of robust spin fluctuations against external magnetic fields provided by $\mu \mathrm{SR}$ may offer valuable insights into the origin of the topological Hall effect and the possible magnetic skyrmions in the ${\mathrm{EuAl}}_4$ and ${\mathrm{EuGa}}_4$ compounds.

Ising spin-orbital coupling is usually easy to identify in the Ising superconductors via an in-plane critical field enhancement, but we show that the Ising spin-orbital coupling also manifests in the vortex physics for perpendicular magnetic fields. By self-consistently solving the Bogoliubov–de Gennes equations of a model Hamiltonian built on the honeycomb lattice with the Ising spin-orbital coupling pertinent to the transition metal dichalcogenides, we numerically investigate the local breaking of the spin and sublattice degeneracies in the presence of a perpendicular magnetic field. It is revealed that the ferromagnetic orders are induced inside the vortex core region by the Ising spin-orbital coupling. The induced magnetic orders are antiphase in terms of their opposite polarizations inside the two nearest-neighbor vortices with one of the two polarizations coming predominantly from one sublattice site, implying the local breaking of the spin and sublattice degeneracies. The finite-energy peaks of the local density of states for spin-up and spin-down in-gap states are split and shifted oppositely by the Ising spin-orbital coupling, and the relative shifts of them on sublattices $A$ and $B$ are also of opposite algebraic sign. The calculated results and the proposed scenario may not only serve as experimental signatures for identifying the Ising spin-orbital coupling in the Ising superconductors, but also be prospective in the manipulation of electron spins in motion through the orbital effect in the superconducting vortex states.

A bipartite lattice with chiral symmetry is known to host zero-energy flat bands if the numbers of the two sublattices are different. We demonstrate that this mechanism of producing flat bands can be realized on graphene by introducing periodic vacancies. Using first-principles calculations, we elaborate that even though pristine graphene does not exactly preserve chiral symmetry, this mechanism applied to holey graphene still produces single or multiple bands as narrow as $\sim 0.5\mathrm{eV}$ near the Fermi surface throughout the entire Brillouin zone. Moreover, this mechanism can combine with vacancy-engineered nonsymmorphic symmetry to produce band structures with coexisting flat bands and nodal lines. A weak coupling mean-field treatment suggests the stabilization of superconductivity by these vacancy-engineered narrow bands. In addition, superconductivity occurs predominantly on the majority sublattices, with an amplitude that increases with the number of narrow bands.

Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is, however, only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content by only a tiny amount but has a dramatic impact on the electronic properties of the sample. Here we report the occurrence of superconductivity in oxygen-deficient ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ thin films grown in an oxygen-free environment after annealing in pure argon flow. As verified by x-ray diffraction, annealing induces an increase of the interlayer distance between ${\mathrm{CuO}}_2$ planes in the crystal structure. Since this distance is correlated to the concentration of oxygens in apical positions, and since oxygen content cannot substantially increase during annealing, our experiments indicate that the superconducting phase transition has to be ascribed to a migration of oxygen ions to apical positions during annealing. Moreover, as we confirm via first-principles density functional theory calculations, the changes in the structural and transport properties of the films can be theoretically described by a specific redistribution of the existing oxygen ions at apical positions with respect to ${\mathrm{CuO}}_2$ planes, which remodulates the electronic band structure and suppresses the antiferromagnetic order, allowing the emergence of hole superconductivity.

We propose a new concept of a two-dimensional (2D) Dirac semiconductor which is characterized by the emergence of four-fold degenerate band crossings near the band edge, and provide a generic approach to realize this novel semiconductor in the community of material science. Based on the first-principle calculations and symmetry analysis, we discover recently synthesized triple-layer (TL) ${\mathrm{Bi}}_2{\mathrm{OS}}_2$ is such a 2D Dirac semiconductor that features a Dirac cone at the $X/Y$ point, protected by a centrosymmetric nonsymmorphic space group. Due to the sandwich-like structure, each Dirac fermion in TL-${\mathrm{Bi}}_2{\mathrm{OS}}_2$ can be regarded as a combination of two Weyl fermions with opposite chiralities, degenerate in momentum-energy space but separated in real space. Such a 2D Dirac semiconductor carries hidden layer-dependent helical spin textures. Moreover, novel topological phase transitions are flexibly achieved in TL-${\mathrm{Bi}}_2{\mathrm{OS}}_2$: (i) a vertical electric field can drive it into the Weyl semiconductor with switchable spin polarization direction; (ii) an extensive strain is able to generate ferroelectric polarization and actuate it into the Weyl nodal ring around the $X$ point and into another type of four-fold degenerate point at the $Y$ point. Our work extends the Dirac fermion into semiconductor systems and provides a promising avenue to integrate spintronics and optoelectronics in topological materials.

With the recent experimental verification that ferroelectric lattice distortions survive in the metallic phase of some materials, there is a desire to create devices that are both switchable and take advantage of the novel functionalities afforded by polar interfaces. In this work, we explore a simple model for such an interface and demonstrate a mechanism by which a metallic ferroelectric substrate may be switched by a bias voltage. This finding is in contrast to the reasonable expectation that hysteresis is prevented by screening of external fields in ferroelectric metals. Instead, the electron gas binds to polarization gradients to form a compensated state. Uncompensated electrons, which may screen external fields, are generated either when the electron density exceeds the ferroelectric polarization or when the bias voltage exceeds a “spillover” threshold. We propose that switchable thin films may be optimized by choosing an electron density that is slightly less than the lattice polarization. In addition to the high-polarization states, we find that thin metallic films also have a low-polarization state with average polarization near zero. Unlike in insulating films, where the polarization is small everywhere in this state, the low-polarization state in the metallic films consists of two head-to-head domains of opposite polarization. This domain formation is enabled by the screening of depolarizing fields by the electron gas.

The optical absorptance of a single graphene layer over a wide range of wavelengths is known to be remarkably constant at the universal value $\pi \alpha$ where $\alpha$ is the fine structure constant. Using atomistic tight-binding calculations, we show that the absorptance spectra of nanometer-thin layers (quantum wells) of group-IV, III-V, II-VI, or IV-VI semiconductors are characterized by marked plateaus at integer values of $\pi \alpha$, in the absence of excitonic effects. In the case of InAs, the results obtained are in excellent agreement with the currently available experimental data. By revisiting experimental data on semiconductor superlattices, we show that $\pi \alpha$ is also a metric of their absorption when normalized to a single period. We conclude that the $\pi \alpha$ quantization is universal in semiconductor quantum wells provided that excitonic effects are weak and is therefore not specific to the zero-gap graphene case. The physical origin of this universality and its limits are discussed using analytical models that capture the main underlying physics of the lowest optical transitions in III-V and II-VI semiconductor quantum wells. These models show that the absorptance is ruled by $\pi \alpha$ independent of the material characteristics because of the presence of a dominant term in the Hamiltonian, linear in the wave vector $\mathbf{k}(\sim \mathbf{V}·\mathbf{k})$, which couples the conduction band to the valence bands. However, the prefactor in front of $\pi \alpha$ is not unity as in graphene due to the different nature of the electronic states. In particular, the spin-orbit coupling plays an important role in bringing the absorptance plateaus closer to integer values of $\pi \alpha$. The case of IV-VI semiconductor (PbSe) quantum wells characterized by a rocksalt lattice and multivalley physics is very similar to that of graphene, with the specification that a “massful gap” is formed around the Dirac points.

Using seven semiconductors and insulators with band gaps covering the range from 1 to 10 eV we systematically explore the performance of two different variants of self-consistency associated with the famous Hedin system of equations: the full self-consistency and the so-called quasiparticle approximation to it. The pros and cons of these two variants of self-consistency are sufficiently well documented in the literature for the simplest GW approximation to the Hedin equations. Our study, therefore, aims primarily at the level of theory beyond GW approximation, i.e., at the level of theory which includes vertex corrections. Whereas quasiparticle self-consistency has certain advantages at the GW level (a well-known fact), the situation becomes quite different when vertex corrections are included. In the variant with full self-consistency, vertex corrections (both for polarizability and for self-energy) systematically reduce the calculated band gaps making them closer to the experimental values. In the variant with quasiparticle self-consistency, however, an inclusion of the same diagrams has a considerably larger effect and calculated band gaps become severely underestimated. Different effects of vertex corrections in two variants of self-consistency can be related to the $Z$-factor cancellation which plays a positive role in quasiparticle self-consistency at the GW level of theory but appears to be destructive for the quasiparticle approximation when higher-order diagrams are included. The second result of our study is that we were able to reproduce the results obtained with the questaal code using our flapwmbpt code when the same variant of self-consistency (quasiparticle) and the same level of vertex corrections (for polarizability only, static approximation for screened interaction, and Tamm-Dancoff approximation for the Bethe-Salpeter equation) are used.

Coupled cluster (CC) has established itself as a powerful theory to study correlated quantum many-body systems. Finite-temperature generalizations of CC theory have attracted considerable interest and have been shown to work as nicely as the ground-state theory. However, most of these recent developments address only fermionic or bosonic systems. The distinct structure of the $su\left(2\right)$ algebra requires the development of a similar thermal CC theory for spin degrees of freedom. In this paper, we provide a formulation of our thermofield-inspired thermal CC for SU(2) systems. We apply the thermal CC to the Lipkin-Meshkov-Glick system as well as the one-dimensional transverse field Ising model as benchmark applications to highlight the accuracy of thermal CC in the study of finite-temperature phase diagrams in SU(2) systems.

Higher-order topological insulators host gapless states on hinges or corners of three-dimensional crystals. Recent studies suggested that even topologically trivial insulators may exhibit fractionally quantized charges localized at hinges or corners. Although most of the previous studies focused on two-dimensional systems, in this work, we take the initial step toward the systematic understanding of hinge and corner charges in three-dimensional insulators. We consider five crystal shapes of vertex-transitive polyhedra with the cubic symmetry such as a cube, an octahedron, and a cuboctahedron. We derive real-space formulas for the hinge and corner charges in terms of the electric charges associated with bulk Wyckoff positions. We find that both the hinge and corner charges can be predicted from the bulk perspective only modulo certain fractions depending on the crystal shape, because the relaxation near boundaries of the crystal may affect the fractional parts. In particular, we show that a fractionally quantized charge $1/24$ mod $1/12$ in the unit of elementary charge can appear in a crystal with a shape of a truncated cube or a truncated octahedron. We also investigate momentum-space formulas for the hinge and corner charges. It turns out that the irreducible representations of filled bands at high-symmetry momenta are not sufficient to determine the corner charge. We introduce an additional Wilson-loop invariant to resolve this issue.

We study the out-of-equilibrium dynamics of the quantum Ising model with power-law interactions and positional disorder. For arbitrary dimension $d$ and interaction range $\alpha \ge d$ we analytically find a stretched-exponential decay of the global magnetization and ensemble-averaged single-spin purity with a stretch power $\beta =d/\alpha$ in the thermodynamic limit. Numerically, we confirm that glassy behavior persists for finite system sizes and sufficiently strong disorder. We identify dephasing between disordered coherent pairs as the main mechanism leading to a relaxation of global magnetization, whereas genuine many-body interactions lead to a loss of single-spin purity which signifies the buildup of entanglement. The emergence of glassy dynamics in the quantum Ising model extends prior findings in classical and open quantum systems, where the stretched-exponential law is explained by a scale-invariant distribution of timescales, to both integrable and nonintegrable quantum systems.

We theoretically study the nonequilibrium dynamics of the order parameter of a superconducting ring inhomogeneously quenched through its transition temperature. Numerical simulations based on spectral decomposition of the time-dependent Ginzburg-Landau equation reveal that current-carrying superconducting states can be generated in the ring under certain fast local temperature quench conditions. We also show that illumination of the ring with a circularly polarized electromagnetic radiation during rapid cooling strongly promotes the generation of current-carrying states with rotation directions controlled by the helicity of the radiation field.

Pressure of 4 GPa applied on liquid ${\mathrm{B}}_2{\mathrm{O}}_3$ leads to the formation of fourfold coordinated boron atoms and the resulting pressure-quenched glasses reflect the morphology of a “two-species” liquid mainly formed from triangular $\mathrm{B}{\mathrm{O}}_3$ and tetrahedral $\mathrm{B}{\mathrm{O}}_4$ groups. Raman spectra of compacted glasses show that pressure quenching of the liquid preserves the two species, also favoring the formation of two superstructural units: boroxol rings $\left({\mathrm{B}}_3{\mathrm{O}}_6\right)$ involving only $\mathrm{B}{\mathrm{O}}_3$ units and pentaborate groups (two boroxol rings linked by a fourfold coordinated boron atom). Calorimetric analysis up to the liquid state shows that these polyamorphic glasses are single-phase systems characterized by a single glass transition with a much higher $T_g$ and a lower thermodynamic fragility than those of normal v-${\mathrm{B}}_2{\mathrm{O}}_3$. Above $T_g$, a sharp endothermic process due to the inverse liquid-liquid phase transition converting the coordination of boron atoms from 4 to 3 is also revealed. It leads to recovering the classical structure (at ambient pressure) of the “single-species” liquid ${\mathrm{B}}_2{\mathrm{O}}_3$.

We present a study of the soft transverse optic phonon mode in SnTe in comparison to the corresponding mode in PbTe using inelastic neutron scattering and ab initio lattice dynamical calculations. In contrast to previous reports our calculations predict that the soft mode in SnTe features a strongly asymmetric spectral weight distribution qualitatively similar to that found in PbTe. Experimentally, we find that the overall width in energy of the phonon peaks is comparable in our neutron scattering spectra for SnTe and PbTe. We observe the well-known double-peaklike signature of the TO mode in PbTe even down to $T=5\mathrm{K}$, questioning its proposed origin purely based on phonon-phonon scattering. The proximity to the incipient ferroelectric transition in PbTe likely plays an important role not included in current models.

In this study, we clarify the selection rules of optical transitions from the ground state to the magnetic excited states in the quasi-one-dimensional quantum Ising-like antiferromagnet ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ with spin $S=\frac{1}{2}$ using the high-field electron spin resonance measurement with illuminating polarized electromagnetic waves. We demonstrate that the unconventional magnetic excitation via the pair creation of quasipaticles at wave numbers $q_z$ = $\pi /2$ and $\pi$ in the field-induced quantum critical state can couple with both oscillating magnetic and electric fields. Our density matrix renormalization group calculations indicate that the observed selection rules can be explained by two- and four-fold periodical spin interactions, originating from the screw chain structure in ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, in addition to the magnetoelectric coupling between spins and electric dipoles.

We study the dynamical properties of both bulk and edge spins of a two-dimensional Affleck-Kennedy-Lieb-Tasaki (AKLT) model mainly by using the stochastic series expansion quantum Monte Carlo method with stochastic analytic continuation. In the deep AKLT phase, we obtain a spin spectrum with the flat band, which is a strong evidence for a localized state. Through the spectrum analysis, we see a clear continuous phase transition from the AKLT phase to the Néel phase in the model, and the energy gap becomes closed at the corresponding momentum point. In comparison with linear spin-wave theory, we find that there are strong interactions among magnons at high energies. With an open boundary condition, the gap of edge spins in the AKLT phase closes at both the $\mathrm{\Gamma }$ point and the $\pi$ point interestingly to emerge into a flat-band-like Luttinger liquid phase, which can be explained by symmetry and perturbation approximation. This paper helps us to better understand the completely different dynamical behaviors of bulk and edge spins in the symmetry protected topological phase.

We have established an analyzing method to determine eigenstates of $4f$ electrons under the crystal electric field (CEF) at low-symmetry sites. This analysis requires macroscopic physical properties only, namely the saturation moments along several symmetrical directions of the crystal in addition to the magnetic susceptibility and specific heat, instead of the spectrum of the inelastic neutron scattering frequently used for such a complicated circumstance. As a successful case, the eigenstates of a CEF Hamiltonian under an orthogonal point group $C_{2v}$ are determined for the $J=\frac{5}{2}$ state of the cerium ion in a tetragonal compound ${\mathrm{Ce}}_2{\mathrm{Pd}}_2\mathrm{Pb}$. During the analysis, an angular momentum operator along the arbitrary direction is deduced from the space rotation operator. A matrix representation of the projection operator that transforms the basis of wave functions from $|J_z\rangle$ to $|J_x\rangle$ or $|J_y\rangle$ is also shown for $J=\frac{5}{2}$. This analysis reveals the anisotropic moments within highly symmetrical structures, and will contribute to understanding anisotropic field responses in rare-earth compounds.

By means of the ferromagnetic resonance (FMR) technique (at the $X$ band) we investigated the magnetic properties of ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$ (ZFO) films grown on different substrates. Angular dependence of the FMR field showed the existence of twofold and fourfold anisotropies for the same material, grown on Si(111), MgO(100), and STO(100), respectively. By using a model that takes into consideration the relevant contributions to the energy, we were able to extract the following parameters: effective magnetization $M_{\mathrm{eff}}$, cubic magnetocrystalline anisotropy field, in-plane and out of plane uniaxial anisotropy fields, and the $g$ factor. The samples deposited on Si showed a twofold in-plane anisotropy while a fourfold magnetic anisotropy was observed for the ZFO/MgO and ZFO/${\mathrm{SrTiO}}_3$ samples. All samples showed a strong perpendicular uniaxial anisotropy above 1 kOe and low in-plane uniaxial and cubic anisotropy field, on the order of tens of oersteds. The FMR linewidth ($\mathrm{\Delta }H$) fitted from FMR data shows $\mathrm{\Delta }H$ values above 190 Oe for all samples, indicating high damping of this material to be $\sim {10}^{-2}$ for the ZFO/MgO. The present analysis compared the FMR data obtained for the in-plane and out of plane measurements. Likewise, the effective magnetization and perpendicular uniaxial anisotropy field by FMR were compared with the magnetic response obtained by a vibrating sample magnetometer.

We study a device composed of an optical interferometer integrated with a ferrimagnetic sphere resonator (FSR). Magneto-optic coupling can be employed in such a device to manipulate entanglement between optical pulses that are injected into the interferometer and the FSR. The device is designed to allow measuring the lifetime of such macroscopic entangled states in the region where environmental decoherence is negligibly small. This is achieved by recycling the photons interacting with the FSR in order to eliminate the entanglement before a pulse exits the interferometer. The proposed experiment may provide some insight into the quantum to classical transition associated with a measurement process.