Graphene features topological edge bands that connect a pair of Dirac points through either sector of the one-dimensional Brillouin zone depending on edge configurations (zigzag or bearded). Because of their flat dispersion, spontaneous edge magnetization can arise from Coulomb interaction in graphene nanoribbons, which has caught a remarkable amount of interest in graphene spintronics. We find an anomalous Bloch oscillation in such edge bands, in which the flat dispersion freezes electron motion along the field direction, while the topological connection of the bands through the bulk leads to electron oscillation in the transverse direction between opposite sides or layers of a bilayer ribbon. Our Hubbard-model mean-field calculation shows that this phenomenon can be exploited for electrical switching of edge magnetization configurations.

Recent $\mu \mathrm{SR}$ measurements revealed remarkable signatures of spontaneous magnetism coexisting with superconductivity in elemental rhenium. Thus, pure rhenium could be the first elemental crystal where unconventional superconductivity is realized in nature. Here we provide a quantitative theory that uncovers the nature of the superconducting instability by incorporating every details of the electronic structure together with spin-orbit coupling and multiorbital physics. We show that conventional $s$-wave superconductivity combined with strong spin-orbit coupling is inducing even-parity odd-orbital spin triplet Cooper pairs, and in presence of a screw-axis Cooper pairs' migration between the induced equal-spin triplet component leads to an exotic magnetic state with atomic-scale texture. Our first-principles-based model contains two phenomenological parameters that characterizes the pairing interaction fixed by the experimental value of the superconducting transition temperature and the slope of the specific heat, and allows quantitative prediction of the magnetic structure.

${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ is a van der Waals ferromagnet with a Curie temperature at 66 K. Here we report a swift change in the magnetic ground state upon application of small DC electric current; a giant yet anisotropic magnetoelectric effect; and a sharp, lattice-driven quantum switching manifested in the $I\text{−}V$ characteristic of the bulk single-crystal ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. At the heart of these observed phenomena is a strongly anisotropic magnetoelastic coupling that enables strongly anisotropic responses of the lattice to application of electric current and/or magnetic field, which explains the exotic phenomena in ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. Such a rare mechanical tunability in the magnetic semiconductors promises tantalizing prospects for unique functional materials and devices.

In narrow-band systems, correlated contribution to effective hopping becomes important, and one needs to carefully consider three types of hopping processes: between doubly and singly occupied sites, between singly occupied and empty sites, and between doubly occupied and empty or two singly occupied sites. All three hopping parameters cannot vanish simultaneously, and one should specify for which of these processes the band becomes flat. By means of exact diagonalization of finite systems we demonstrate that three hoppings play qualitatively different roles in many-body effects, in particular, in the formation of half-metallic ferromagnetic (Nagaoka) states.

The transition metal monopnictide family of Weyl semimetals recently has been shown to exhibit anomalously strong second-order optical nonlinearity, which is theoretically attributed to a highly asymmetric polarization distribution induced by their polar structure. We experimentally test this hypothesis by measuring optical second harmonic generation (SHG) from TaAs across a pressure-tuned polar-to-nonpolar structural phase transition. Despite the high-pressure structure remaining noncentrosymmetric, the SHG yield is reduced by more than 60% by 20 GPa as compared to the ambient pressure value. By examining the pressure dependence of distinct groups of SHG susceptibility tensor elements, we find that the yield is primarily controlled by a single element that governs the response along the polar axis. Our results confirm a connection between the polar axis and the giant optical nonlinearity of Weyl semimetals and demonstrate pressure as a means to tune this effect in situ.

Defects in solid-state materials provide an ideal, robust platform for quantum sensing. To deliver maximum sensitivity, a large ensemble of noninteracting defects hosting quantum states with long coherence is required. Control of such an ensemble is challenging due to the spatial variation in both the defect energy levels and in any control field across a macroscopic sample. In this work, we experimentally demonstrate that we can overcome these challenges using Floquet theory and optimal control optimization methods to efficiently and coherently control a large defect ensemble, suitable for sensing. We apply our methods experimentally to a spin ensemble of up to $4\times {10}^9$ nitrogen-vacancy centers in diamond. By explicitly including the hyperfine interaction to the intrinsic ${}^{14}\mathrm{N}$ nuclear spin in the optimization, we design shaped microwave control pulses that can outperform conventional ($\pi$) pulses when applied to sensing schemes, with an improvement in the strength of ensemble response of between 11% and 78%. Through simulation of the ensemble dynamics, we shed light on the bandwidth limitations of large-ensemble reinitialization and propose alternative routes for further improvement.

We consider one-dimensional quantum circuits of the brickwork type, where the fundamental quantum gate is dual unitary. Such models are solvable: the dynamical correlation functions of the infinite temperature ensemble can be computed exactly. We review various existing constructions for dual unitary gates and we supplement them with new ideas in a number of cases. We discuss connections with various topics in physics and mathematics, including quantum information theory, tensor networks for the AdS/CFT correspondence (holographic error correcting codes), classical combinatorial designs (orthogonal Latin squares), planar algebras, and Yang-Baxter maps. Afterwards we consider the ergodicity properties of a special class of dual unitary models, where the local gate is a permutation matrix. We find an unexpected phenomenon: nonergodic behavior can manifest itself in multisite correlations, even in those cases when the one-site correlation functions are fully chaotic (completely thermalizing). We also discuss the circuits built out of perfect tensors. They appear locally as the most chaotic and most scrambling circuits, nevertheless they can show global signs of nonergodicity: if the perfect tensor is constructed from a linear map over finite fields, then the resulting circuit can show exact quantum revivals at unexpectedly short times. A brief mathematical treatment of the recurrence time in such models is presented in Appendix (written by Roland Bacher and Denis Serre).

The role played by interfaces in metallic multilayers is not only to change the momenta of incident electrons; their symmetry lowering also results in an enhancement of the effects of spin-orbit coupling, in particular, the flipping of the spins of conduction electrons. This leads to a significant reduction of a spin current through a metallic interface that is quantitatively characterized by a dimensionless parameter $\delta$ called the spin memory loss (SML) parameter, the interface counterpart of the spin-flip diffusion length for bulk metals. In this paper, we use first-principles scattering calculations that include temperature-induced lattice and spin disorder to systematically study three parameters that govern spin transport through metallic interfaces of Cu with Pt, Pd, Py (permalloy), and Co: the interface resistance, spin polarization, and the SML. The value of $\delta$ for a $\mathrm{Cu}|\mathrm{Pt}$ interface is found to be comparable to what we recently reported for a $\mathrm{Au}|\mathrm{Pt}$ interface [Gupta et al., Phys. Rev. Lett. 124, 087702 (2020)]. For $\mathrm{Cu}|\mathrm{Py}$ and $\mathrm{Cu}|\mathrm{Co}$ interfaces, $\delta$ decreases monotonically with increasing temperature to become negligibly small at room temperature. The calculated results are in good agreement with currently available experimental values in the literature. Inserting a Cu layer between Pt and the Py or Co layers slightly increases the total spin current dissipation at these compound interfaces.

A lot of attention has been drawn to superconductivity in ${\mathrm{UTe}}_2$, with suggestions of time-reversal symmetry-breaking triplet chiral superconducting order parameter. The chirality of the order parameter has been attributed to an accidental near degeneracy of two superconducting components belonging to one-dimensional (1D) irreducible representations (irreps) $B_{2u}$ and $B_{3u}$ of the relevant $D_{2h}$ point group, and it has been argued that the chiral $B_{2u}+i{B}_{3u}$ combination is selected by ferromagnetic fluctuations. In this work we present a possible explanation of the near degeneracy as a result of an accidental $C_4$ symmetry of the band structure, with the superconducting order parameter belonging to the 2D $E_u$ irrep of $D_{4h}$ that uniquely descends to the sought-after $B_{2u}+i{B}_{3u}$ combination. We show that the $C_4$ symmetry is emergent at the level of the interactions using a renormalization group calculation and argue that the chiral combination of the order parameter is favored when spin-orbit coupling is added to the model.

We theoretically investigate the droplets formation in a tightly trapped one-dimensional dipolar gas of bosonic atoms. When the strength of the dipolar interaction becomes sufficiently attractive compared to the contact one, we show how a solitoniclike density profile evolves into a liquidlike droplet on increasing the number of particles in the trap. The incipient gas-liquid transition is also signaled by a steep increase of the breathing mode and a change in sign of the chemical potential. Upon a sudden release of the trap, varying the number of trapped atoms and the scattering length, the numerical solution of a time-dependent generalized Gross-Pitaevskii equation shows either an evaporation of the cloud, the formation of a single self-bound droplet, or a fragmentation in multiple droplets. These results can be probed with lanthanide atoms and help in characterizing the effect of the dipolar interaction in a quasi-one-dimensional geometry.

A time Floquet transfer-matrix method for the description of acoustic wave propagation through dynamic stratified structures, modulated by another, low-frequency, pump acoustic wave, is reported. The method is applied to a specific example of a GaAs/AlAs periodic superlattice, subject to a spatiotemporal modulation induced by an evanescent pump wave with frequency in the lowest acoustic band gap of the structure. By means of systematic numerical calculations, we provide compelling evidence for the occurrence of significant nonreciprocal transmission of an acoustic signal with frequency in a high-order acoustic band gap, through inelastic multiple-scattering processes. Our results indicate a promising route to design nonreciprocal acoustic devices.

The non-Hermitian skin effect (NHSE) is a phenomenon whereby certain non-Hermitian lattice Hamiltonians host an extensive number of eigenmodes condensed to the boundary, called skin modes. Although the NHSE has mostly been studied in the classical or single-particle regime, it can also manifest in interacting quantum systems with boson number nonconserving processes. We show that lattices of coupled nonlinear resonators can function as reciprocal quantum amplifiers. A one-dimensional chain exhibiting the NHSE can perform strong photon amplification aided by the skin modes, which scales exponentially with the chain length and outperforms alternative lattice configurations lacking the NHSE. Moreover, two-dimensional lattices can perform directional photon amplification between different lattice corners, due to the two-dimensional NHSE. These quantum amplifiers are based on experimentally feasible lattice configurations with uniform parametric driving schemes.

Interacting central spin systems, in which a central spin is coupled to a strongly correlated spin bath with intrabath interaction, consist of an important class of spin systems beyond the usual Gaudin magnet. These systems are relevant to several realistic setups and serve as an interesting platform to study interaction controlled decoherence and frustration induced instability of magnetic order. Using an equations-of-motion method based on analytical representations of spin-operator matrix elements in the XX chain, we obtain exact long-time dynamics of a generalized Heisenberg star consisting of a spin-$S$ central spin and an inhomogeneously coupled XXZ chain of $N\le 16$ bath spins. In contrast to previous studies where the central spin dynamics is mainly concerned, we focus on the influence of the central spin on the dynamics of magnetic orders within the spin bath. By preparing the XXZ bath in a Néel state, we find that in the gapless phase of the bath even weak system-bath coupling could lead to nearly perfect relaxation of the antiferromagnetic order. In the gapped phase, the staggered magnetization decays rapidly and approaches a steady value that increases with increasing anisotropy parameter. These findings suggest the possibility of controlling internal dynamics of strongly correlated many-spin systems by certain coupled auxiliary systems of even few degrees of freedom. We also study the dynamics of the Rényi entanglement entropy of the central spin when the bath is prepared in the ground state. Both the overall profile and initial growth rate of the Rényi entropy are found to exhibit minima at the critical point of the XXZ bath.

We present a numerical macrospin model for harmonic voltage detection in multilayer spintronic devices. The core of the computational backend is based on the Landau-Lifshitz-Gilbert-Slonczewski equation, which combines high performance and satisfactory agreement with the experimental results in large-scale applications. We compare the simulations with the experimental findings in a Ta/CoFeB bilayer system for angular- and magnetic-field-dependent resistance measurements, electrically detected magnetization dynamics, and harmonic Hall voltage detection. Using simulated scans of the selected system parameters such as the polar angle $\theta$, magnetization saturation $\left({\mu }_{\text{0}}{M}_{\text{s}}\right)$, or uniaxial magnetic anisotropy $\left(K_{\text{u}}\right)$, we show the resultant changes in the harmonic Hall voltage, demonstrating the dominating influence of the ${\mu }_{\text{0}}{M}_{\text{s}}$ on the first and second harmonics. In the spin-diode ferromagnetic resonance method, the $({\mu }_{\text{0}}{M}_{\text{s}},$ $K_{\text{u}})$ parameter space may be optimized numerically to obtain a set of viable curves that fit the experimental data.

Using Keldysh field theory, we find conditions for nonequilibrium condensation in the open Tavis-Cummings model under a direct finite-bandwidth incoherent cavity drive. Experimentally, we expect the condensation transition to be easily accessible to hybrid superconducting systems coupled to microscopic spins, as well as to many other incoherently driven light-matter systems. In our theoretical analysis, we explicitly incorporate the drive's spectral distribution into the saddle-point description. We show that the injected incoherent photons create a drive-dependent effective coupling between spin-1/2 particles. The condensation transition arises at a critical regime of driving which we can now accurately predict. Our results also provide important guidelines for future quantum simulation experiments of nonequilibrium phases with hybrid devices.

The recent discovery of superconductivity in heavy the fermion compound ${\mathrm{UTe}}_2$, a candidate topological and triplet-paired superconductor, has aroused widespread interest. However, to date, there is no consensus on whether the stoichiometric sample of ${\mathrm{UTe}}_2$ is superconducting or not due to the lack of reliable evidence to distinguish the difference between the nominal and real compositions of samples. Here, we are the first to clarify that the stoichiometric ${\mathrm{UTe}}_2$ is nonsuperconducting at ambient pressure and under hydrostatic pressure up to 6 GPa; however, we find that it can be compressed into superconductivity by application of quasi-uniaxial pressure. Measurements of resistivity, magnetoresistance, and susceptibility reveal that the quasi-uniaxial pressure results in a suppression of the Kondo coherent state seen at ambient pressure, which then leads to a superconductivity initially emerged on the ab-plane at 1.5 GPa. At 4.8 GPa, the superconductivity is developed in three crystallographic directions. The superconducting state coexists with an exotic magnetic ordered state that develops just below the onset temperature of the superconducting transition. The discovery of the quasi-uniaxial pressure-induced superconductivity with an exotic magnetic state in the stoichiometric ${\mathrm{UTe}}_2$ not only provides new understandings of this compound, but also highlights the vital role of Te deficiency in developing the superconductivity at ambient pressures.

High-entropy alloy (HEA) superconductors characterized as having a high mixing entropy have attracted substantial interest because of their remarkable potential for technological applications. However, the superconducting (SC) gap of HEA superconductors, which is one of the important factors for designing SC devices, has scarcely been studied. Herein, we report the direct observation of the fully gapped pairing symmetry of the HEA superconductor ${\mathrm{Ta}}_{1/6}{\mathrm{Nb}}_{2/6}{\mathrm{Hf}}_{1/6}{\mathrm{Zr}}_{1/6}{\mathrm{Ti}}_{1/6}$, probed using Andreev reflection spectroscopy. The Andreev reflection, a pronounced signature of superconductivity, is observed in the differential conductance ($\mathrm{d}I/\mathrm{d}V$) spectra below the SC transition temperature $T_{\mathrm{c}}$ of 7.9 K, which is well explained by the modified Blonder-Tinkham-Klapwijk model. The SC energy gap (Δ) as a function of temperature and magnetic field follows the Bardeen-Cooper-Schrieffer (BCS) theory with $\mathrm{\Delta }\left(0\right)=1.37\mathrm{meV}$. However, the gap to $T_{\mathrm{c}}$ ratio, $2\mathrm{\Delta }\left(0\right)/k_B{T}_c=4.03$, is larger than the value of 3.54 predicted by the weak-coupling BCS theory, indicating that ${\mathrm{Ta}}_{1/6}{\mathrm{Nb}}_{2/6}{\mathrm{Hf}}_{1/6}{\mathrm{Zr}}_{1/6}{\mathrm{Ti}}_{1/6}$ belongs to the class of moderately coupled nodeless superconductors.

The electronic and magnetic excitations of bulk NiO have been determined using the ${}^3{A}_{2g}$ to ${}^3{T}_{2g}$ crystal-field transition at the Ni $M_{2,3}$ edges with resonant inelastic x-ray scattering at 66.3- and 67.9-eV photon energies and 33-meV spectral resolution. Unambiguous assignment of the high-energy side of this state to a spin-flip satellite is achieved. We extract an effective exchange field of $89\pm 4$ meV in the ${}^3{T}_{2g}$ excited final state from empirical two-peak spin-flip model. The experimental data is found consistent with crystal-field model calculations using exchange fields of 60–100 meV. Full agreement with crystal-field multiplet calculations is achieved for the incident photon energy dependence of line shapes. The lower exchange parameter in the excited state as compared to the ground-state value of 120 meV is discussed in terms of the modification of the orbital occupancy (electronic effects) and of the structural dynamics: (A) With pure electronic effects, the lower exchange energy is attributed to the reduction in effective hopping integral. (B) With no electronic effects, we use the $S=1$ Heisenberg model of antiferromagnetism to derive a second-nearest-neighbor exchange constant $J_2$ = $14.8\pm 0.6$ meV. Based on the linear correlation between $J_2$ and the lattice parameter from pressure-dependent experiments, an upper limit of 2% local Ni-O bond elongation during the femtosecond scattering duration is derived.

Using a systematic relation between topological gapless phases in three dimensions and topological gapped phases in two dimensions, we identify four types of higher-order topological semimetals or nodal superconductors (HOTS), hosting (i) flat zero-energy “Fermi arcs” at crystal hinges, (ii) flat zero-energy hinge arcs coexisting with surface Dirac cones, (iii) chiral or helical hinge modes, or (iv) flat zero-energy hinge arcs connecting nodes only at finite momentum. Bulk-boundary correspondence relates the hinge states to the bulk topology protecting the nodal point or loop. We classify all HOTS for all tenfold-way classes with an order-two crystalline (anti)symmetry, such as mirror, twofold rotation, or inversion.

Koopmans-compliant functionals provide an orbital-density-dependent framework for an accurate evaluation of spectral properties; they are obtained by imposing a generalized piecewise-linearity condition on the total energy of the system with respect to the occupation of any orbital. In crystalline materials, due to the orbital-density-dependent nature of the functionals, minimization of the total energy to a ground state provides a set of minimizing variational orbitals that are localized and thus break the periodicity of the underlying lattice. Despite this, we show that Bloch symmetry can be preserved and it is possible to describe the electronic states with a band-structure picture, thanks to the Wannier-like character of the variational orbitals. We also present a method to unfold and interpolate the electronic bands from supercell ($\mathrm{\Gamma }$-point) calculations, which enables us to calculate full band structures with Koopmans-compliant functionals. The results obtained for a set of benchmark semiconductors and insulators show very good agreement with state-of-the-art many-body perturbation theory and experiments, underscoring the reliability of these spectral functionals in predicting band structures.