The kagome lattice can host exotic magnetic phases arising from frustrated and competing magnetic interactions. However, relatively few insulating kagome materials exhibit incommensurate magnetic ordering. Here, we present a study of the magnetic structures and interactions of antiferromagnetic ${\mathrm{Na}}_2{\mathrm{Mn}}_3{\mathrm{Cl}}_8$ with an undistorted ${\mathrm{Mn}}^{2+}$ kagome network. Using neutron-diffraction and bulk magnetic measurements, we show that ${\mathrm{Na}}_2{\mathrm{Mn}}_3{\mathrm{Cl}}_8$ hosts two different incommensurate magnetic states, which develop at $T_{N1}=1.6\mathrm{K}$ and $T_{N2}=0.6\mathrm{K}$. Magnetic Rietveld refinements indicate magnetic propagation vectors of the form $\mathbf{q}=(q_x,q_y,\frac{3}{2})$, and our neutron-diffraction data can be well described by cycloidal magnetic structures. By optimizing exchange parameters against magnetic diffuse-scattering data, we show that the spin Hamiltonian contains ferromagnetic nearest-neighbor and antiferromagnetic third-neighbor Heisenberg interactions, with a significant contribution from long-ranged dipolar coupling. This experimentally determined interaction model is compared with density-functional-theory simulations. Using classical Monte Carlo simulations, we show that these competing interactions explain the experimental observation of multiple incommensurate magnetic phases and may stabilize multi-q states. Our results expand the known range of magnetic behavior on the kagome lattice.

The newly discovered kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ exhibits numerous exotic phenomena and offers a rare platform to study the interplay of topological state, charge-density wave order, and superconductivity. Here we systematically investigated the twofold symmetry in the superconducting state through the measurements of off-plane resistivity under in-plane rotating magnetic field. It is found that the distinct twofold symmetry exists in the superconducting phase. Furthermore, this symmetry breaking is very fragile and sensitive to the surface degradation. When the sample is exposed to the atmosphere for several days, the twofold symmetry tends to be weakened and eventually disappears with increasing exposing time. However, it can be recovered after sample surface exfoliation. Combined with other measurements, we propose that this fragile twofold symmetry should be related to the surface condition. The surface degradation may be caused by Cs atom vacancies on the surface, which could destroy the topological surface state. We discussed the possible relationship between the fragile twofold symmetry and the topological surface state discovered by recent angle-resolved photoemission spectroscopy experiments.

We investigate superconducting hybrid nanowires defined by patterned gates on a two-dimensional heterostructure of InAs and Al, with lateral quantum dots operating as single-level spectrometers along the side of the nanowire. Applying magnetic field along the wire axis spin splits dot levels, providing spin-resolved spectroscopy. We investigate spin and charge polarizations of subgap states in the nanowire and their evolution with magnetic field and gate voltage.

Complexity plays a very important part in quantum computing and simulation where it acts as a measure of the minimal number of gates that are required to implement a unitary circuit. We study the lower bound of the complexity [Eisert, Phys. Rev. Lett. 127, 020501 (2021)] for the unitary dynamics of the one-dimensional lattice models of noninteracting fermions. We find analytically using quasiparticle formalism, the bound grows linearly in time and is followed by a saturation for short-ranged tight-binding Hamiltonians. We show numerical evidence that for an initial Neel state the bound is maximum for tight-binding Hamiltonians as well as for the long-range hopping models. However, the increase of the bound is sublinear in time for the later, in contrast to the linear growth observed for short-range models. The upper bound of the complexity in noninteracting fermionic lattice models is calculated, which grows linearly in time even beyond the saturation time of the lower bound, and finally, it also saturates.

In recent years, transition metal dichalcogenides (TMDs) have been extensively studied for their efficient spin-orbit torque generation in TMD/ferromagnetic bilayers, owing to their large spin-orbit coupling, large variety of crystal symmetries, and pristine interfaces. Although the TMD layer was considered essential for the generation of the observed spin-orbit torques (SOTs), recent reports show the presence of a self-torque in single-layer ferromagnetic devices with magnitudes comparable to TMD/ferromagnetic devices. Here, we perform second-harmonic Hall SOT measurements on metal-organic chemical vapor deposition (MOCVD) grown ${\text{MoS}}_2$/permalloy/${\text{Al}}_2{\text{O}}_3$ devices and compare them to a single-layer permalloy/${\text{Al}}_2{\text{O}}_3$ device to accurately disentangle the role of self-torques, arising from the ferromagnetic layer, from contributions from the TMD layer in these bilayers. We report a fieldlike spin-torque conductivity of ${\sigma }_{FL}=(-2.8\pm 0.3)\times {10}^3\frac{\hslash }{2e}{\left(\mathrm{\Omega }\mathrm{m}\right)}^{-1}$ in a single-layer permalloy/${\text{Al}}_2{\text{O}}_3$ device, which is comparable to our ${\text{MoS}}_2$/permalloy/${\text{Al}}_2{\text{O}}_3$ devices and previous reports on similar TMD/ferromagnetic bilayers, indicating only a minor role of the ${\text{MoS}}_2$ layer. In addition, we observe a comparatively weak dampinglike torque in our devices, with a strong device-to-device variation. Finally, we find a linear dependence of the SOT conductivity on the Hall bar arm/channel width ratio of our devices, indicating that the Hall bar dimensions are of significant importance for the reported SOT strength. Our results accentuate the importance of delicate details, like device asymmetry, Hall bar dimensions, and self-torque generation, for the correct disentanglement of the microscopic origins underlying the SOTs, essential for future energy-efficient spintronic applications.

In ab initio calculations of superconducting properties, the Coulomb repulsion is accounted for at the GW level and is usually computed in the random phase approximation (RPA), which amounts to neglecting vertex corrections both at the polarizability level and in the self-energy. Although this approach is unjustified, the brute force inclusion of higher-order corrections to the self-energy is computationally prohibitive. We propose to use a generalized GW self-energy, where vertex corrections are incorporated into W by employing the Kukkonen and Overhauser (KO) ansatz for the effective interaction between two electrons in the electron gas. By computing the KO interaction in the adiabatic local density approximation for a diverse set of conventional superconductors, and using it in the Eliashberg equations, we find that vertex corrections lead to a systematic decrease of the critical temperature ($T_c$), ranging from a few percent in bulk lead to more than 40% in compressed lithium. We propose a set of simple rules to identify those systems where large $T_c$ corrections are to be expected and hence the use of the KO interaction is recommended. Our approach offers a rigorous extension of the RPA and GW methods for the prediction of superconducting properties at a negligible extra computational cost.

Single flux quantum (SFQ) circuitry is a promising candidate for a scalable and integratable cryogenic quantum control system. However, the operation of SFQ circuits introduces nonequilibrium quasiparticles (QPs), which are a significant source of qubit decoherence. In this study, we investigate QP behavior in a superconducting quantum-classical hybrid chip that comprises an SFQ circuit and a qubit circuit. By monitoring qubit relaxation time, we explore the dynamics of SFQ-circuit-induced QPs. Our findings reveal that the QP density near the qubit reaches its peak after several microseconds of SFQ circuit operation, which corresponds to the phonon-mediated propagation time of QPs in the hybrid circuits. This suggests that phonon-mediated propagation dominates the spreading of QPs in the hybrid circuits. Our results lay the foundation to suppress QP poisoning in quantum-classical hybrid systems.

We study the effects of static nonlocal Coulomb interactions in layered conventional superconductors and show that they generically suppress superconductivity and reduce the critical temperature. Although the nonlocal Coulomb interaction leads to a significant structure in the superconducting gap function, we find that most properties can be effectively described by means of an appropriately revised local Coulomb pseudopotential ${\tilde{\mu }}_C^{*}$, which is larger than the commonly adopted retarded Tolmachev-Morel-Anderson pseudopotential ${\mu }_C^{*}$. To understand this, we analyze the Bethe-Salpeter equation describing the screening of Coulomb interaction in the superconducting state and obtain an expression for ${\tilde{\mu }}_C^{*}$, which is valid in the presence of nonlocal Coulomb interactions in two dimensions. This analysis also reveals how the structure of the nonlocal Coulomb interaction weakens the screening effects from high-energy pair fluctuations and therefore yields larger values of the pseudopotential. Our findings are especially important for layered conventional superconductors with small Fermi energies and can be readily taken into account within ab initio studies.

The concept of the energy gap is a fundamental characteristic of the band structure of a material and it determines its physical properties. Formally the energy gap appears in the dispersion relation $E_k$, where the vector $k$ is determined on the whole momentum space. However, today the gapped momentum materials are in the focus of research in which the so-called momentum or $k$ gap can emerge; i.e., some lacunae of momentum space are excluded from the domain of the function $E_k$. One such example presents the non-Hermitian systems. Within the random phase approximation we study the dielectric properties of the momentum gapped materials in one, two, and three dimensions for cases of both zero and finite temperatures. We find the corresponding plasmon modes and determine the unusual behavior of the appropriate dispersion relations for each dimensionality. Based on these findings we evaluate the absorption coefficient of gapped momentum media and provide some numerical estimations of its value for the practical applications.

Motivated by recent cold-atom experiments, we study the relaxation of spin helices in quantum XXZ spin chains. The experimentally observed relaxation of spin helices follows scaling laws that are qualitatively different from linear-response transport. We construct a theory of the relaxation of such spin helices, combining generalized hydrodynamics with diffusive corrections and a generalized form of the local density approximation. Although helices are far from local equilibrium, our hydrodynamic approach reproduces the experimentally observed relaxational dynamics and also predicts the late-time relaxation, which is outside the experimentally accessible timescales. In particular, our theory explains the existence of temporal regimes with apparent anomalous diffusion, as well as the asymmetry between positive and negative anisotropy regimes at short and intermediate times.

We compute the spin-Hall conductance in a multiband model describing the two-dimensional electron gas formed at a ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface in the presence of a finite concentration of impurities. Combining linear response theory with a systematic calculation of the impurity contributions to the self-energy, as well as to the vertex corrections of the relevant diagrams, we recover the full spin-Hall vs sheet-conductance dependence of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ as reported in Trier et al. [Nano Lett. 20, 395 (2020)], finding a very good agreement with the experimental data below and above the Lifshitz transition. In particular, we demonstrate that the multiband electronic structure leads to only a partial, instead of a complete, screening of the spin-Hall conductance, which decreases with increasing the carrier density. Our method can be generalized to other two-dimensional systems characterized by a broken inversion symmetry and multiband physics.

A nonmagnetic phase transition at $T_0\sim 12$ K in the tetragonal system CeCoSi with a Kramers doublet ground state is reminiscent of an electric quadrupole ordering, even though its well-separated crystal-electric-field (CEF) levels are unlikely to acquire higher-order multipole degrees of freedom. Here, we report ${}^{59}\mathrm{Co}$ nuclear magnetic resonance (NMR) studies that are highly compatible with a ferroic quadrupole ordering below $T_0$. Changes in the NMR spectra below $T_0$ suggest that an external magnetic field induces ferroic Ce dipole moments orthogonal to the field, enabling domain selection in the nonmagnetic phase. Our findings suggest the presence of a ferroic $O_{zx}$-type quadrupole component in CeCoSi and demonstrate that quadrupole ordering may occur under well-separated CEF levels in tetragonal systems.

The electronic structure of ${\mathrm{FeWO}}_4$ is studied by photoelectron spectroscopy at x-ray and hard x-ray photon energies on high-quality single crystals. Photoionization cross-section effects together with full atomic multiplet configuration interaction and band structure calculations allow us to identify the contributions of iron and tungsten to the valence band. The analysis shows that the correlations in ${\mathrm{FeWO}}_4$ necessitate theoretical approaches beyond standard band structure models even for the description of the tungsten with a formal $5d^0$ configuration.

We report calculations of terahertz ellipticities in large-angle, $21.{79}^{\circ }$ and $38.{21}^{\circ }$, commensurate twisted bilayer graphene and predict values as high as 1.5 mdeg in the terahertz region for this nonmagnetic material. This terahertz circular dichroism is surprisingly large and has a magnitude comparable to that of chiral materials in the visible region. At low frequencies, the dichroic response arises from the strong interlayer hybridization, which allows us to probe the symmetry and strength of these couplings. Crucially, lateral interlayer translation tunes this response, in striking contrast to the near invariance of small twist angle bilayer to interlayer translation. We examine the magnitude and phase of the interlayer coupling for all structures containing fewer than 400 atoms per unit cell. Finally, we find that the dichroism can be manipulated by applying an electric field or with doping.

The recent application of topological quantum chemistry to rhombohedral bismuth established the nontrivial band structure of this material. This is a second-order topological insulator characterized by the presence of topology-imposed hinge states. The spatial distribution of hinge states and the possible presence of additional symmetry-protected surface states is expected to depend on the crystal shape and symmetries. To explore this issue, we have grown bismuth nanocrystals in the tens of nanometers on the (110) surface of InAs. By scanning tunneling spectroscopy, we mapped the local density of states on all facets and identified the presence of the hinge states at the intersection of all facets. Our paper confirms the classification of bulk bismuth as a second-order topological insulator. We propose that the ubiquitous presence of the hinge states result from their tunnel coupling across the nanometer-sized facets.

We consider Lifshitz criticality (LC) with the dynamical critical exponent $z=2$ in one-dimensional interacting fermion systems with a filled Dirac sea. We report that interactions have crucial effects on Lifshitz criticality. Single-particle excitations are destabilized by an interaction and decay into the particle-hole continuum, which is reflected in the logarithmic divergence in the imaginary part of one-loop self-energy. We show that the system is sensitive to the sign of the interaction. The random-phase approximation shows that the collective particle-hole excitations emerge only when the interaction is repulsive. The dispersion of collective modes is gapless and linear. If the interaction is attractive, the one-loop renormalization group (RG) shows that there may exist a stable RG fixed point described by two coupling constants. We also show that the on-site interaction (without any other perturbations at the UV scale) would always turn on the relevant velocity perturbation to the quadratic Lagrangian in the RG flow, driving the system flow to the conformal-invariant criticality. In the numerical simulations of the lattice model at half filling, we find that, for either on-site positive or negative interactions, the dynamical critical exponent becomes $z=1$ in the infrared limit and the entanglement entropy is a logarithmic function of the system size $L$. The work paves the way to study one-dimensional interacting LCs.

We present here a statistical study of the transit time required for a magnetic domain wall to go through a small laser spot focused on two-dimensional magnetic thin film. The domain wall velocity deduced this way is in good agreement with the other ways used to measure this parameter, but the main fact is that the transit time is not a reproducible parameter; we have observed a quite large distribution of this parameter. This distribution can be explained assuming the movement to occur through jumps, whose probabilities are given by Poisson's process. The fitting of this distribution has enabled us to get the required number of jumps to reverse the magnetization of the small area under the laser spot. This important parameter should lead to a better understanding of the creep regime.

The laser intensity dependence of high-order harmonic generation (HHG) in ZnO is investigated theoretically. We find that the intensities of lower-order harmonics mainly contributed by the intraband electron oscillation increase with light intensity monotonously. However, the intensities of higher-order harmonics where the interband electron-hole recollision dominates oscillate with light intensity. We build models of intraband and interband electronic dynamics to study the effect of laser intensity on every single-order harmonic that is far from the band gap. The model results are in good agreement with the numerical solution of semiconductor Bloch equations. The nonzero initial $\vec{k}$ positions play an important role in HHG, especially in higher-order harmonics. With the saddle-point method, the critical distance for electron-hole recombination in the current material and laser parameters is found to be one lattice constant in the dephasing time of a quarter of laser cycle. A unified description of channel closing effects is proposed to explain the laser intensity dependence of above threshold ionization and HHG in gases and solids including HHG not far from the band gap.

The electron-phonon interaction can induce the superconducting state in the low-dimensional mono- and bilayer systems. However, the calculated value of the temperature of the transition from the metallic to the superconducting state is greatly affected by the accuracy of the applied model. If taken into account, the vertex corrections to the electron-phonon interaction are usually of significant value due to the low Fermi level of such materials and were found to contribute to the reduction of the calculated value of the critical temperature of the transition. This fact weakens the prospects for the possibility of induction of the nonadiabatic superconducting state at the relatively high critical temperature since an increase either in the electron-phonon coupling constant or in the Debye frequency leads to the simultaneous increase in the corrected Migdal ratio, while influencing the critical temperature to a greater degree.