We study a generic class of fermionic two-band models under synchronized periodic driving, i.e., with the different terms in a Hamiltonian subject to periodic drives with the same frequency and phase. With all modes initially in a maximally mixed state, the synchronized drive is found to produce nonperiodic patterns of dynamical quantum phase transitions, with their appearance determined by an interplay of the band structure and the frequency of the drive. A case study of the anisotropic XY chain in a transverse magnetic field, transcribed to an effective two-band model, shows that the modes come with quantized geometric phases, allowing for the construction of an effective dynamical order parameter. Numerical studies in the limit of a strong magnetic field reveal distinct signals of precursors of dynamical quantum phase transitions also when the initial state of the XY chain is perturbed slightly away from maximal mixing, suggesting that the transitions may be accessible experimentally. A blueprint for an experiment built around laser-trapped circular Rydberg atoms is proposed.

${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here, we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ below $T_{\text{C}}=177$ K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal $c$ axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged $(\mathbf{k}=\mathbf{0})$ in-plane ${120}^{\circ }$ antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between $\sim$90 K and $T_{\text{C}}$. A discontinuous change in bulk properties and neutron polarization observed at $T=125$ K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ is a magnetic Weyl semimetal.

Nonreciprocal propagation of spin waves provides the possibility to design novel functional magnonic devices. In the present paper, based on micromagnetic simulation, we investigate the properties of the spin waves propagating within magnetic domain walls in a synthetic antiferromagnetic bilayer structure and demonstrate a giant frequency nonreciprocity, of which not only the magnitude but also the sign can be efficiently controlled by an external magnetic field. This results from the field-induced modification of the hybridization between the domain-wall spin wave channels from different magnetic layers. The unidirectional excitation of the gigahertz hybridized domain-wall spin waves by a nanostripline antenna is also demonstrated. Our finding suggests the nanoscale antiferromagnetic domain walls should be a promising platform for programmable nonreciprocal spin-wave devices.

Isolated islands in two-dimensional strongly disordered and strongly coupled superconductors become optically active, inducing subgap collective excitations in the ac conductivity. Here, we investigate the fate of these excitations as a function of the disorder strength in the experimentally relevant case of weak electron-phonon coupling. An explicit calculation of the ac conductivity, that includes vertex corrections to restore gauge symmetry, reveals the existence of collective subgap excitations, related to phase fluctuations and therefore identified as the Goldstone modes, for intermediate to strong disorder. As disorder increases, the shape of the subgap excitation transits from peaked close to the spectral gap to a broader distribution reaching much smaller frequencies. Phase coherence still holds in part of this disorder regime. The requirement to observe subgap excitations is not the existence of isolated islands acting as nanoantennas but rather the combination of a sufficiently inhomogeneous order parameter with a phase fluctuation correlation length smaller than the system size. Our results indicate that, by tuning disorder, the Goldstone mode may be observed experimentally in metallic superconductors based, for instance, on Al, Sn, Pb, or Nb.

The Josephson vortex formed in a superconductor-insulator-superconductor (SIS) junction can affect the quantum mechanics of quasiparticles by creating an effective adiabatic potential determined by the inhomogeneous distribution of the phase difference of the order parameter along the junction. Starting from the quasiclassical version of the Bogoliubov–de Gennes theory, we found the quasiparticle spectrum and the local density of states (DOS) both for the isolated Josephson vortex and the vortex chain. The spatially resolved DOS reveals a peculiar two-peak structure for each Josephson vortex, which can be detected experimentally using scanning tunneling microscopy and spectroscopy techniques.

We analyze theoretically the influence of domain walls (DWs) on the DC Josephson current in magnetic superconducting ${\mathrm{S}}_m/\mathrm{Fl}/\mathrm{F}/\mathrm{Fl}/{\mathrm{S}}_m$ junctions. The Josephson junction consists of two “magnetic” superconductors ${\mathrm{S}}_m$ (superconducting film covered by a thin ferromagnetic layer), spin filters Fl, and a ferromagnetic layer F with or without DW(s). The spin filters Fl allow electrons to pass with one specific spin orientation, such that the Josephson coupling is governed by a fully polarized long-range triplet component. In the absence of DW(s), the Josephson and spin currents are nonzero when the right and left filters, ${\mathrm{Fl}}_{r,l}$, pass electrons with equal spin orientation and differ only by a temperature-independent factor. They become zero when the spin orientation of the triplet Cooper pairs passing through the ${\mathrm{Fl}}_{r,l}$ have opposite directions. Furthermore, for the different chiralities of the injected triplet Cooper pairs, the spontaneous currents arise in the junction yielding a diode effect. Once a DW is introduced, it reduces the critical Josephson current $I_c$ in the case of equal spin polarization and makes it finite in the case of opposite spin orientation. The critical current $I_c$ is maximal when the DW is in the center of the F film. A deviation of the DW from the center generates a force that pushes the DW to the center of the F film. In addition, we consider the case of an arbitrary number $N$ of DWs, with the case $N=2$ resembling a magnetic skyrmion.

Topological crystalline superconductors are known to have possible higher-order topology, which results in Majorana modes on $d-2$ or lower-dimensional boundaries. Given the rich possibilities of boundary signatures, it is desirable to have topological invariants that can predict the type of Majorana modes from band structures. Although symmetry indicators—a type of invariant that depends only on the band data at high-symmetry points—have been proposed for certain crystalline superconductors, there exist symmetry classes in which symmetry indicators fail to distinguish superconductors with different Majorana boundaries. Here, we systematically obtain topological invariants for an example of this kind, two-dimensional time-reversal symmetric superconductors with twofold rotational symmetry $C_2$. First, we show that the nontrivial topology is independent of band data on the high-symmetry points by conducting a momentum-space classification study. Then, from the resulting K groups, we derive calculable expressions for four ${\mathbb{Z}}_2$ invariants defined on high-symmetry lines or general points in the Brillouin zone. Finally, together with a real-space classification study, we establish the bulk-boundary correspondence and show that the four ${\mathbb{Z}}_2$ invariants can predict Majorana boundary types from band structures. Our proposed invariants can fuel practical material searches for $C_2$-symmetric topological superconductors featuring Majorana edge and corner modes.

Hard x rays induce atomic dynamics in oxide glasses at doses low enough that the average structure is unchanged. X-ray photon correlation spectroscopy is used here to study this effect in a series of alkali borate glasses characterized by different network connectivity. The product of the x-ray dose rate per atom and the decay time of the density correlation function is a sample-dependent parameter with the dimensions of an energy: It varies between a fraction of eV and few tens of eV and is sensitive to the network connectivity of the glass. It can then be used to access some bonding information also in cases where the x rays perturb the material, a regime of great interest in modern x-ray-based experiments.

We consider a mechanical lattice inspired by the Su-Schrieffer-Heeger model along with cubic Klein-Gordon–type nonlinearity. We investigate the long-time dynamics of the nonlinear edge states, which are obtained by nonlinear continuation of topological edge states of the linearized model. Linearly unstable edge states delocalize and lead to chaos and thermalization of the lattice. Linearly stable edge states also reach the same fate, but after a critical strength of perturbation is added to the initial edge state. We show that the thermalized lattice in all these cases shows an effective renormalization of the dispersion relation. Intriguingly, this renormalized dispersion relation displays a unique symmetry, i.e., its square is symmetric about a finite squared frequency, akin to the chiral symmetry of the linearized model.

We theoretically investigate the stability of a square skyrmion crystal (SkX) in a centrosymmetric tetragonal lattice structure with the emphasis on the role of the magnetic anisotropy arising from the absence of vertical mirror symmetry. Our analysis is based on an effective bilinear and biquadratic model in momentum space, which is a canonical model for itinerant magnets in a weak-coupling regime. By performing the simulated annealing for the model on the two-dimensional square lattice, we find that the off-diagonal spin component in the interaction, which becomes nonzero when the vertical mirror symmetry is broken, gives rise to the square SkX with a definite helicity in an external magnetic field. We show that the helicity of the square SkXs is determined by the competition between the off-diagonal and diagonal anisotropic interactions, the latter of which appears in the discrete fourfold-rotational lattice structure. Furthermore, we discuss helicity-dependent physical phenomena by introducing odd-parity multipoles, where electric (magnetic) and electric (magnetic) toroidal multipoles are sources of an antisymmetric spin polarization and an Edelstein effect (a magnetoelectric effect). We also discuss the stability of the SkXs with different helicities in a magnetic field rotation. Our results provide a way of engineering the helicity-locked SkXs by the symmetric anisotropic interaction in centrosymmetric magnets, which is distinct from that by the antisymmetric Dzyaloshinskii-Moriya interaction in noncentrosymmetric magnets.

We have investigated the magnetic, structural, and thermodynamic properties of ${\mathrm{Ho}}_5{\mathrm{Co}}_6{\mathrm{Sn}}_{18}$ through x-ray and neutron diffraction, magnetization, and specific heat measurements. ${\mathrm{Ho}}_5{\mathrm{Co}}_6{\mathrm{Sn}}_{18}$ displays a magnetic transition at $T_M=3.4\mathrm{K}$ and can be described with the magnetic space group $I{4}_1/a{c}^{\prime }{d}^{\prime }$. The two holmium sublattices Ho(1) and Ho(2) exhibit different magnetic behaviors. Ferromagnetic order on the Ho(1) site develops into a net magnetization along the $c$ axis below $T_M$. In contrast, the ordering of the Ho(2) site is determined by geometric magnetic frustrations. The Ho(2) spins order into the noncollinear, two-in-two-out antiferromagnetic pattern on the $\mathrm{Ho}{\left(2\right)}_4$ tetrahedron. At 60 mK, the ordered moment of Ho(2) reaches $4.01\left(8\right){\mu }_{\mathrm{B}}$, which is about half of the Ho(1) moment. Upon the application of a magnetic field, ferromagnetic components along the $c$ axis are induced on Ho(2), along with the in-plane antiferromagnetic components, indicating the XY-like spin anisotropy of the Ho(2) spins. A magnetic quasielastic neutron scattering signal is observed above $T_M$ and significantly weakens with the magnetic ordering.

The control of the magnetic properties of shapeable devices and the manipulation of flexible structures by external magnetic fields is a keystone of future magnetoelectronics-based devices. In this work we study the elastic properties of a flexible magnetic nanodisk that hosts a meron as the magnetic state and can be deformed from structures with positive to negative Gaussian curvature. We show that the winding number of the hosted meron is crucial to determine the curvature sign of the stable obtained shape. Additionally, we show that the curvature that minimizes the total energy of the nanodisk depends on geometrical and mechanical parameters. It is shown that an increase in the external radius and Young's modulus leads to a decrease in the curvature absolute value. Finally, it is shown that the nanodisk's shape also depends on the connection between the polarity and chirality of the meronlike state.

In the presence of strong atomic spin-orbit coupling (SOC), tending to the $j\text{−}j$ coupling limit, $5d^4$ iridates are speculated to possess a nonmagnetic $J_{\mathrm{eff}}=0$ singlet ground state from atomic consideration, which invariably gets masked due to different solid-state effects (e.g., hopping). Here, we try to probe the trueness of the atomic SOC-based proposal in an apparently one-dimensional system, ${\mathrm{Sr}}_3{\mathrm{NaIrO}}_6$, with well-separated ${\mathrm{Ir}}^{5+}$ $\left(5d^4\right)$ ions. But all the detailed experimental as well as theoretical characterizations reveal that the ground state of ${\mathrm{Sr}}_3{\mathrm{NaIrO}}_6$ is not nonmagnetic. However, our combined dc susceptibility $\chi$, ${}^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR), muon spin relaxation/rotation $\left(\mu \mathrm{SR}\right)$, and heat capacity $C_p$ measurements clearly refute any sign of spin freezing or ordered magnetism among the ${\mathrm{Ir}}^{5+}$ moments due to geometrical exchange frustration, while in-depth zero-field and longitudinal field $\mu \mathrm{SR}$ investigations strongly point towards an inhomogeneous quantum spin liquid (QSL)-like ground state. In addition, the linear temperature dependence of both the NMR spin-lattice relaxation rate and the magnetic heat capacity at low temperatures suggest low-lying gapless spin excitations in the QSL phase of this material. Finally, we conclude that the effective SOC realized in $d^4$ iridates is unlikely to offer a ground state which will be consistent with a purely atomic $j\text{−}j$ coupling description.

Both helicity-independent and helicity-dependent all-optical switching processes driven by single ultrashort laser pulse have been experimentally demonstrated in ferrimagnetic alloys as GdFeCo. Although the switching has been previously reproduced by atomistic simulations, the lack of a robust micromagnetic framework for ferrimagnets limits the predictions to small nanosystems, whereas the experiments are usually performed with lasers and samples of tens of micrometers. Here we develop a micromagnetic model based on the extended Landau-Lifshitz-Bloch equation, which is firstly validated by directly reproducing atomistic results for small samples and uniform laser heating. After that, the model is used to study ultrafast single shot all-optical switching in ferrimagnetic alloys under realistic conditions. We find that the helicity-independent switching under a linearly polarized laser pulse is a pure thermal phenomenon, in which the size of inverted area directly correlates with the maximum electron temperature in the sample. On the other hand, the analysis of the helicity-dependent processes under circular polarized pulses in ferrimagnetic alloys with different composition indicates qualitative differences between the results predicted by the magnetic circular dichroism and the ones from inverse Faraday effect. Based on these predictions, we propose experiments that would allow one to resolve the controversy over the physical phenomenon that underlies these helicity-dependent all optical processes.

To explore the high-temperature superconductivity of hydrogen-rich compounds at low pressures, we have investigated the crystal structures, electronic and dynamical properties, electron-phonon interactions, and possible superconductivity of the ternary hydride $M{\mathrm{C}}_2{\mathrm{H}}_8$ ($M=$ Na, K, Mg, Al, and Ga) in the low-pressure range of $0–100\mathrm{GPa}$ based on the first-principles calculations. The results show that there is no imaginary frequency in phonon spectra for $M{\mathrm{C}}_2{\mathrm{H}}_8$ at selected pressures which indicates that $M{\mathrm{C}}_2{\mathrm{H}}_8$ is dynamically stable. Furthermore, according to the Eliashberg spectral function under pressures, $M{\mathrm{C}}_2{\mathrm{H}}_8$ is predicted to be superconducting at low pressure. Especially, the superconducting critical temperature ($T_c$) of ${\mathrm{MgC}}_2{\mathrm{H}}_8$ is higher than 55 K at 40 GPa and the $T_c$ in ${\mathrm{AlC}}_2{\mathrm{H}}_8$ reaches 67 K at 80 GPa. Electronic and phonon states and the electron-phonon interactions show that H has a considerable contribution to this ternary hydride superconductor and suggest that increasing the contribution of H to total electron-phonon coupling is a way to design materials with high $T_c$. Our study shows that it is one of the feasible routes to explore the low-pressure and high-temperature superconductivity in ternary carbon-based hydrides.

As a new type of iron pnictide superconductors, ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$, featured by its unique FeAs bilayer, presents a quasi-two-dimensional characteristic which distinguishes itself from other iron pnictides. Here we systematically investigated its transport properties and thermoelectric effects, including the Seebeck and Nernst effect, and NMR. An unusual hump in the Nernst coefficient is observed as temperature decreases below 90 K, far above its $T_c$ of 28.5 K. Simultaneously there is a suppression in the Hall coefficient as well as the nuclear spin-lattice relaxation rate in the same temperature range. To clarify this phenomenon, we comparatively investigated two other similar iron-based pnictide superconductors, ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ and ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. Our work indicates there could exist an incoherence-coherence crossover or a kind of Cooper pairing precursor in ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ before achieving its superconductivity, which is different from other iron-based high-$T_c$ superconductors. Such a comparative study may help us to understand the effect of dimensional evolution from three dimensional to two dimensional on high-$T_c$ superconductivity.

We investigate theoretically the excitation of Higgs oscillations of the order parameter in superconductors by incoherent short pulses of light with frequency much larger than the superconducting gap. The excitation amplitude of the Higgs mode is controlled by a single parameter which is determined by the total number of quasiparticles excited by the pulse. This fact can be traced back to the universality of the shape of the light-induced quasiparticle cascade at energy below the Debye frequency and above the gap.

The manipulation of electronic properties in layered materials is of great importance for both device design and fundamental research. Here, we report the pressure engineering on $\mathrm{Sn}{\mathrm{S}}_2$, a typical van der Waals crystal with promising application in the field of photonic and electronic devices, which is a semiconductor with a large band gap at ambient condition. The band gap was tuned by pressure from $\sim 2.2\mathrm{eV}$ at ambient condition to $\sim 0.6\mathrm{eV}$ at 30.7 GPa, and then $\mathrm{Sn}{\mathrm{S}}_2$ became a metal near 35 GPa. Upon further compression, superconducting (SC) transition was observed above $\sim 50\mathrm{GPa}$, and the SC critical temperature $T_c$ increased with pressure, reaching $\sim 6.0\mathrm{K}$ at $\sim 105\mathrm{GPa}$. Since there is much similarity with ${\mathrm{MoS}}_2$ and/or other metal dichalcogenides, $\mathrm{Sn}{\mathrm{S}}_2$ provides an extra platform to investigate the pressure-driven superconductivity in the large metal dichalcogenide family, which benefits the exploration of new superconductors and underlying mechanisms as well.

Compared with conventional atomic crystals, superatomic solids are more attractive for providing opportunities to generate unexpected emergent states associated with their unique hierarchical structure. Using first-principles calculations, herein, we identify two cluster- and energy-separated extreme states in a synthesized binary fulleride. An emergent superatomic Dirac state and a flat band with extreme effective mass are found on different sides of the Fermi level, which are separately contributed by different constituent clusters. The coexistence of Dirac and flat states in a semiconductor rather than a metal/semimetal may open a door for constructing Dirac field-effect devices, giving rise to ultrafast Dirac carrier transport.

We study how the nonlinear propagation dynamics of bulk states may be used to distinguish topological phases of slowly driven Floquet lattices. First, we show how instabilities of nonlinear Bloch waves may be used to populate Floquet bands and measure their Chern number via the emergence of nontrivial polarization textures in a similar manner to static (undriven) lattices. Second, we show how the nonlinear dynamics of nonstationary superposition states may be used to identify dynamical symmetry inversion points in the intracycle dynamics, thereby allowing anomalous Floquet phases to be distinguished from the trivial phase. The approaches may be readily implemented using light propagation in nonlinear waveguide arrays.