To a significant extent, the rich physical properties of photonic crystals (PCs) are determined by the underlying geometry, in which the composed symmetry operator and their combinations contribute to their unique topological invariant to characterize the topological phases. In this work, we use geometric anisotropy in the 2D square lattice composed of four rectangle blocks. Due to the presence of glide symmetry, we show a variety of topological phase transitions in designed nonsymmorphic PCs and these transitions shall be understood in terms of the Zak phase and Chern number in synthetic space, as well as the pseudospin-2 concept, combinationally. Furthermore, Zak phase winding in the periodic synthetic parameter space yields high-order Chern number and double interface states. In terms of the extended Zak phase and pseudospin Hall effect, a higher-order topological insulator is constructed in the PC system. Based on the combination of different lattices, this represents a unique system that requires multiple topological concepts and indices for theoretical examination and understanding. The intriguing and abundant topological features are also sustained in the corresponding three-dimensional PC slab, which makes it a very interesting platform to control the flow of optical signals.

We present a theoretical study, based on density functional theory and tight-binding modeling, of the electronic structure and spin transport properties of silicon nanoparticles with adsorbed bismuth atoms. We find the bismuth atoms to form clusters separated by quantum tunnel barriers. We predict strong spin filtering by these nanostructures in the high conductance regime when the source and drain leads are connected to the same bismuth cluster and in the low conductance regime when the source and drain leads are connected to different bismuth clusters. We relate the spin filtering to a symmetry obeyed by the spin transmission probability matrix of the system. We also predict that for such systems the direction of the spin polarization in the drain lead can be tuned through large angles and even reversed electrostatically simply by varying the voltage applied to a gate. Realization of these silicon-bismuth nanostructures in the laboratory is feasible. We expect the predicted spin filtering to be experimentally accessible and potentially relevant for device applications.

Spin-gapless semiconductors (SGSs), with intrinsic magnetism, $100\%$ spin polarization, and zero-gap band crossing points, have attracted great scientific interest owing to their potential applications in spintronics. In this Letter, using first-principles calculations and symmetry analysis, we demonstrate that the realistic material ${\mathrm{Sr}}_2{\mathrm{CuF}}_6$ is a spintronic material with multiple dimensions of spin-gapless semiconducting states. Tetragonal ${\mathrm{Sr}}_2{\mathrm{CuF}}_6$ has a zero-dimensional zero-gap nodal point, a one-dimensional zero-gap nodal line, and a two-dimensional nearly zero-gap nodal surface in one spin direction. Moreover, it hosts a wide band gap in the other spin direction. Our results extend the SGS members from nodal point SGSs and nodal line SGSs to nodal surface SGSs. Furthermore, we report a SGS candidate in an experimentally realized material exhibiting different dimensions of zero-gap points in momentum space. It is hoped that spintronic materials with multiple dimensions of spin-gapless semiconducting states may have significant applications in new-generation spintronics.

We show theoretically that an intense femtosecond optical pulse incident normally on a metal surface tends to decay into a pair of counterpropagating surface plasmon polaritons (SPPs). The interference field heats the medium periodically, which causes a periodic permittivity perturbation and, in turn, resonantly amplifies the magnitudes of the SPPs. The instability growth time is 10–50 fs for typical metals at laser fluences of about $\sim 1\mathrm{J}/{\mathrm{cm}}^2$, so this mechanism is important for understanding the dynamics of the optical pulse absorption under damaging conditions, and it is promising for the interpretation of laser-induced periodic surface structure formation in single- and few-pulse pumping regimes.

The Landau-Lifshitz equation describes the time evolution of magnetic dipoles and can be derived by taking the classical limit of a quantum mechanical spin Hamiltonian. To take this limit, one constrains the many-body quantum state to a tensor product of coherent states, thereby neglecting entanglement between sites. Expectation values of the quantum spin operators produce the usual classical spin dipoles. One may also consider expectation values of polynomials of the spin operators, leading to quadrupole and higher-order spin moments, which satisfy a dynamical equation of motion that generalizes the Landau-Lifshitz dynamics [Zhang and Batista, Phys. Rev. B 104, 104409 (2021)]. Here we reformulate the dynamics of these $N^2-1$ generalized spin components as a mean-field Schrödinger equation on the $N$-dimensional coherent state. This viewpoint suggests efficient integration methods that respect the local symplectic structure of the classical spin dynamics.

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy (spin-ARPES) and density functional theory. EuO exhibits unique properties of hosting both weakly dispersive nearly fully polarized Eu $4f$ bands, as well as O $2p$ levels indirectly exchange-split by the interaction with Eu nearest neighbors. Our temperature-dependent spin-ARPES data directly demonstrate the exchange splitting in O $2p$ and its vanishing at the Curie temperature. Our calculations with a Hubbard $U$ term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands. We discuss the mechanisms of indirect exchange in the O $2p$ levels by analyzing the orbital resolved band characters in ferromagnetic and antiferromagnetic phases. The directional effects due to spin-orbit coupling are predicted theoretically to be significant in particular in the Eu $4f$ band manifold. The analysis of the shape of spin-resolved spectra in the Eu $4f$ spectral region reveals signatures of hybridization with O $2p$ states, in agreement with the theoretical predictions. We also analyze spectral changes in the spin-integrated spectra throughout the Curie temperature, and demonstrate that they derive from both the magnetic phase transition and effects due to sample aging, unavoidable for this highly reactive material.

We study the Josephson effect in planar $\mathrm{S}{\mathrm{F}}_1{\mathrm{F}}_2\mathrm{S}$ junctions that consist of conventional $s$-wave superconductors (S) connected by two metallic monodomain ferromagnets (${\mathrm{F}}_1$ and ${\mathrm{F}}_2$) with an arbitrary transparency of interfaces. We solve the scattering problem in the clean limit based on the Bogoliubov–de Gennes equation for both spin-singlet and odd in frequency spin-triplet pairing correlations. We calculate numerically the Josephson current-phase relation $I\left(\varphi \right)$. While the first harmonic of $I\left(\varphi \right)$ is completely generated by spin-singlet and short-range spin-triplet superconducting correlations, for noncollinear magnetizations of ferromagnetic layers the second harmonic has an additional long-range spin-triplet component. Therefore, for a strong ferromagnetic influence, the long-range spin-triplet contribution to the second harmonic dominates. We find an exception due to the geometric resonance for equal ferromagnetic layers when the first harmonic is strongly enhanced. Both first and second harmonic amplitudes oscillate with ferromagnetic layer thicknesses due to $0\text{−}\pi$ transitions. We study the influence of interface transparencies and find additional resonances for finite transparency of the interface between ferromagnetic layers.

Searching for low-power-consuming and highly efficient methods for well-controllable driving of skyrmion motion is one of the most concerning issues for future spintronic applications, raising high concern with the choice of magnetic media and driving scenario. In this work, we propose a scenario of spin-wave-driven skyrmion motion in a ferrimagnetic (FiM) lattice with the net angular momentum ${\delta }_s$. We investigate theoretically the effect of both ${\delta }_s$ and the circular polarization of the spin wave on the skyrmion dynamics. It is revealed that the momentum onto the skyrmion imposed by the excited spin wave can be partitioned into a ferromagnetic term plus an antiferromagnetic term. The ratio of these two terms and consequently the Hall angle of skyrmion motion can be formulated as the functions of ${\delta }_s$, demonstrating the key role of ${\delta }_s$ as an effective control parameter for the skyrmion motion. Moreover, the spin wave frequency and chirality dependent skyrmion motion are discussed, predicting the frequency enhanced skyrmion Hall motion. This work thus represents an essential contribution to understanding the skyrmion dynamics in a FiM lattice.

We study high-order harmonics generation (HHG) in crystalline silicon and diamond subjected to near- and mid-infrared laser pulses. We employ time-dependent density functional theory and solve the time-dependent Kohn-Sham equation in the single-cell geometry. We demonstrate that clear and clean HHG spectra can be generated with careful selection of the pulse duration. In addition, we simulate dephasing effects in a large silicon supercell through a displacement of atomic positions prepared by a molecular dynamics simulation. We compare our results with the previous calculations by Floss et al. [Phys. Rev. A 97, 011401(R) (2018)] on diamond at 800 nm and by Tancogne-Dejean et al. [Phys. Rev. Lett. 118, 087403 (2017)] on Si at 3000 nm.

Andreev bound states (ABSs) are localized quantum states that contain both electron and hole components. They ubiquitously reside in inhomogeneous superconducting systems. Following theoretical analysis, we propose to probe the electron-hole weights of an ABS via a local tunneling measurement that detects the transient current under a steplike pulse bias. With our protocol, the ABS energy level can also be obtained from peaks of the Fourier spectrum of the transient current. Our protocol can be applied to detect robust zero-energy Majorana bound states (MBSs), which have equal electron-hole weights, in candidate platforms where local tunneling spectroscopy measurement is possible. In the one-dimensional Majorana nanowire model, we numerically calculate the electron-hole weights for different types of low-energy bound states, including ABSs, quasi-MBSs, and MBSs.

The search for a sweet spot, the locus in qubit parameters where quantum control is first-order insensitive to noises, is key to achieve high-fidelity quantum gates. Efforts to search for such a sweet spot in conventional double-quantum-dot singlet-triplet qubits where each dot hosts one electron (“two-electron singlet-triplet qubit”), especially for two-qubit operations, have been unsuccessful. Here we consider singlet-triplet qubits allowing each dot to host more than one electron, with a total of four electrons in the double quantum dots (“four-electron singlet-triplet qubit”). We theoretically demonstrate, using configuration interaction calculations, that sweet spots appear in this coupled qubit system. We further demonstrate that, under realistic charge noise and hyperfine noise, a two-qubit operation at the proposed sweet spot could offer gate fidelities ($\sim 99\%$) that are higher than the conventional two-electron singlet-triplet qubit system ($\sim 90\%$). Our results should facilitate realization of high-fidelity two-qubit gates in singlet-triplet qubit systems.

We extend a microscopic theory of polarization and magnetization to include the spin degree of freedom of the electrons, introducing a general spin-orbit coupling and Zeeman interaction term in the Hamiltonian. At finite frequencies and including spin, the magnetoelectric polarizability tensor is replaced by two separate tensors, one that relates the polarization P to the magnetic field B and a separate tensor that relates the magnetization M to the electric field E. When combined with other relevant response tensors a third-rank tensor that relates the induced current density to gradients in the electric field can be introduced; it is gauge invariant, in a form suitable for numerical calculations, and describes optical activity—including spin effects—even in materials that may lack time-reversal symmetry.

Nonorthogonality of eigenstates is a fundamental characteristic of non-Hermitian systems. It is expected that nonorthogonality-induced effects could be enhanced in the vicinity of the exceptional points (EPs) where the multiple eigenstates coalesce into one. Here, focusing on the wave-packet motions in non-Hermitian systems, we find that a wave packet near the EPs could exhibit new types of Zitterbewegung- (ZB-) like motion arising from the nonorthogonality of eigenstates. The observed ZB-like motion of a wave packet centered around a single eigenstate contradicts the notion that ZB motion only occurs for a wave packet centered around a superposed state. We further show the existence of a ZB-like motion in the form of an interesting helical trajectory of the wave packet on the incident plane, stemming from the imaginary part of group velocities. These exotic ZB-like motions may provide great insight into nonorthogonal eigenstates and complex eigenfrequencies of non-Hermitian systems.

Motivated by the pseudogap–Fermi liquid transition in doped Mott insulators, we examine the excitations of a $t\text{−}J$ model with random and all-to-all hopping and exchange. The stability of quasiparticles such as spin-$1/2$ fermions (Landau quasiparticles), spin-1 magnons, and emergent Jordan-Wigner (JW) fermions is cast as a problem of localization in the many-body Hilbert space, which is studied by the FEAST eigensolver algorithm. At low dopings, magnons and JW fermions are stable and better defined than Landau quasiparticles, which are unstable. Upon crossing a critical value of doping around $p_c=1/3$, the stabilities of Landau quasiparticles and the other two are interchanged. Near the critical doping, these quasiparticles are all found to be ill defined. The critical point is thus associated with a localization transition in the many-body Hilbert space. We discuss the possible role of the JW fermions in the anomalously large thermal Hall response of the cuprates.

We use scanning tunneling microscopy to study the domain structure of the nearly commensurate charge density wave (NC-CDW) state of $1T\text{−}{\mathrm{TaS}}_2$. In our subangstrom characterization of the state, we find a continual evolution of the CDW lattice from domain wall to domain center, instead of a fixed CDW arrangement within a domain. Further, we uncover an intradomain chirality characterizing the NC-CDW state. Unlike the orbital-driven chirality previously observed in related transition-metal dichalcogenides, the chiral nature of the NC-CDW state in $1T\text{−}{\mathrm{TaS}}_2$ appears driven by a strong coupling of the NC-CDW state to the lattice.

The impurity is inherently manifest in cuprate superconductors, as cation substitution or intercalation is necessary for the introduction of charge carriers, and its influence on the electronic state is at the heart of a great debate in physics. Here based on the microscopic octet scattering model, the influence of the impurity scattering on the electronic structure of cuprate superconductors is investigated in terms of the self-consistent $T$-matrix approach. The impurity scattering self-energy is evaluated firstly in the Fermi-arc-tip approximation of the quasiparticle excitations and scattering processes, and the obtained results show that the decisive role played by the impurity scattering self-energy in the particle-hole channel is the further renormalization of the quasiparticle band structure with a reduced quasiparticle lifetime, while the impurity scattering self-energy in the particle-particle channel induces a strong deviation from the $d$-wave behavior of the superconducting gap, leading to the existence of a finite gap over the entire electron Fermi surface. Moreover, these impurity scattering self-energies are employed to study the exotic features of the line shape in the quasiparticle excitation spectrum and the autocorrelation of the quasiparticle excitation spectra, and the obtained results are then compared with the corresponding experimental data. The theory therefore also indicates that the unconventional features of the electronic structure in cuprate superconductors is generated by both the strong electron correlation and impurity scattering.

First-principles calculations are performed to predict ferroelectricity in the binary rocksalt oxide family ($A\mathrm{O}$ with $A=\mathrm{Cd}$, Ba, Sr, Ca, Mg, etc.) via the application of biaxial strain. The origins of such ferroelectricity are discussed from the lattice dynamics perspective and electronic structure properties. Under compressive strain, the strikingly large longitudinal piezoelectric effect is induced due to the deformation along the $c$ direction as a response to the stress. These findings have the potential to motivate the search of new simple binary oxides possessing ferroelectricity that may lead to novel devices.

High-harmonic spectroscopy of solids is a powerful tool, which provides access to both electronic structure and ultrafast electronic response of solids, from their band structure and density of states to phase transitions, including the emergence of the topological edge states, to the PetaHertz electronic response. However, in spite of these successes, high-harmonic spectroscopy has hardly been applied to analyze the role of coherent femtosecond lattice vibrations in the attosecond electronic response. Here we study coherent phonon excitations in monolayer graphene to show how high-harmonic spectroscopy can be used to detect the influence of coherent lattice dynamics, particularly longitudinal and transverse optical phonon modes, on the electronic response. Coherent excitation of the in-plane phonon modes results in the appearance of sidebands in the spectrum of the emitted harmonic radiation. We show that the spectral positions and the polarization of the sideband emission offer a sensitive probe of the dynamical symmetries associated with the excited phonon modes. Our work brings the key advantage of high-harmonic spectroscopy—the combination of subfemtosecond to tens of femtoseconds temporal resolution—to the problem of probing phonon-driven electronic response and its dependence on the dynamical symmetries in solids.

We study the ground state and thermodynamic properties of the spin-half XXZ model, with an Ising interaction $J_z$ and a transverse exchange interaction $J_x$, on a pyrochlore tube obtained by joining together elementary cubes in a one-dimensional array. Periodic boundary conditions in the transverse directions ensure that the bulk of the system consists of corner-sharing tetrahedra, with the same local geometry as the pyrochlore lattice. We use exact diagonalization, the density matrix renormalization group (DMRG), and minimally entangled typical thermal states (METTS) methods to study the system. When $J_z$ is antiferromagnetic ($J_z>0$) and $J_x$ is ferromagnetic ($J_x<0$), we find a transition from a spin liquid to an XY ferromagnet, which has power-law correlations at $T=0$. For $J_z<0$ and $J_x>0$, spin-two excitations are found to have lower energy than spin-one at the transition away from the fully polarized state, showing evidence for incipient spin-nematic order. When both interactions are antiferromagnetic, we find a nondegenerate ground state with no broken symmetries and a robust energy gap. The low-energy spectra evolve smoothly from predominantly Ising to predominantly XY interactions. In the spin-liquid regime of small $|J_x|$, we study the confinement of monopole-antimonopole pairs and find that the confinement length scale is larger for $J_x<0$ than for $J_x>0$, although both length scales are very short. These results are consistent with a local spin-liquid phase for the Heisenberg antiferromagnet with no broken symmetries.

The real space formalism of orbital magnetization (OM) is an average of the local OM over some appropriate region of the system. Previous studies prefer a bulk average (i.e., without including boundaries). Based on a bilayer model with an adjustable Chern number at half filling, we numerically investigate the effects of boundaries on the real space expressions of OM. The size convergence processes of its three constituent terms $M_{\mathrm{LC}}$, $M_{\mathrm{IC}}$, and $M_{\mathrm{BC}}$ are analyzed. The topological term $M_{\mathrm{BC}}$ makes a nonnegligible contribution from boundaries as a manifestation of edge states, especially in the case of nonzero Chern numbers. However, we show that the influence of the boundary on $M_{\mathrm{LC}}$ and $M_{\mathrm{IC}}$ exactly compensates that on $M_{\mathrm{BC}}$. This compensation effect leads to the conclusion that the whole sample average is also a correct algorithm in the thermodynamic limit, which gives the same value as those from the bulk average and the $k$-space formula. This clarification will be beneficial to further studies on orbitronics, as well as the orbital magnetoelectric effects in higher dimensions.