We present inelastic neutron scattering measurements of magnetic excitations in ${\mathrm{YbMnBi}}_2$, which reveal features consistent with a direct coupling of magnetic excitations to Dirac fermions. In contrast with the large broadening of magnetic spectra observed in antiferromagnetic metals such as the iron pnictides, here the spin waves exhibit a small but resolvable intrinsic width, consistent with our theoretical analysis. The subtle manifestation of spin-fermion coupling is a consequence of the Dirac nature of the conduction electrons, including the vanishing density of states near the Dirac points. Accounting for the Dirac fermion dispersion specific to ${\mathrm{YbMnBi}}_2$ leads to particular signatures, such as the nearly wave-vector-independent damping observed in the experiment.

The nonequilibrium (NEQ) excitonic-insulator (EI) phase is a dynamical state of matter for excited insulators or semiconductors, and it is characterized by self-sustained coherent oscillations of the excitonic condensate. In this Rapid Communication we highlight the main qualitative features of a NEQ-EI time- and angle-resolved photoemission spectroscopy (tr-ARPES) spectrum. We show that monochromatic probes yield a steady-state spectrum with excitonic structures originating from the dressing of conduction states with the coherent condensate. These structures are replicas of the NEQ valence bands but shifted upward in energy by the condensate frequency. Reducing the probe duration below the condensate period, the band structure gradually fades away and the tr-ARPES signal becomes proportional to the excitonic wave function. In addition, the signal amplitude becomes periodic in the impinging time of the probe, with the same period of the oscillating condensate.

Density functional theory investigations of effects arising from ordered structure of cation vacancies in ${\mathrm{KNi}}_2{\mathrm{Se}}_2$ are reported. The simulated cation-deficient ${\mathrm{K}}_x{\mathrm{Ni}}_{2-y}{\mathrm{Se}}_2$ phases with $x=0.8$, $y=0.0$, and $y=0.4$ lie within the stoichiometry range of experimental samples produced by the self-flux method or oxidative deintercalation of a vacancy-free system. Results of the present studies indicate pronounced impact of cation vacancy superstructure on the structural, electronic, and vibrational properties of ${\mathrm{KNi}}_2{\mathrm{Se}}_2$. Revealed modifications of the local structure, atomic bond lengths, electronic, and phonon bands, which are especially noticeable in the system with both potassium and nickel deficiencies, are reflected in the simulated neutron pair-distribution functions, the phonon and Raman spectra, which are provided to facilitate both experimental verification of the predicted effects and analysis of the phase composition of a multiphase K-Ni-Se material.

The structural relaxation dynamics in liquid and 20 K undercooled gallium has been studied by neutron spectroscopy. The intermediate scattering function at the structure factor maximum was measured over a wide temperature range up to twice the melting temperature. A stretched exponential decay was observed near the melting point which transforms to an exponential decay above $1.2{\text{T}}_{\text{melting}}$. The structural relaxation process beyond 3 ps can be described as a further slow exponential decay. Mode coupling theory is able to calculate the decay rate of this slow structural relaxation, which agrees well with the measurement. This additional structural relaxation, observed in the collective particle motions on next-neighbor distances, can be understood as a slow cage relaxation process, setting in around 370 K and indicating the slowing down of dynamics towards solidification. This process might be the microscopic origin for observed changes in the collective particle dynamics of liquid gallium with decreasing temperature. Liquid aluminium, lead, and rubidium demonstrate a similar behavior, which suggests a universal crossover in dynamics of the equilibrium liquid metal state.

We analyze the quantum dynamics of a two-level emitter in a resonant microcavity with optical feedback provided by a distant mirror (i.e., a half cavity) with a focus on stabilizing the emitter-microcavity subsystem. Our treatment is fully carried out in the framework of cavity quantum electrodynamics. Specifically, we focus on the dynamics of a perturbed subradiant state of the emitter-microcavity subsystem to ascertain its stability (existence of time oscillatory solutions around the candidate state) or lack thereof. In particular, we find conditions under which multiple feedback modes of the half cavity contribute to the stability, showing certain analogies with the Lang-Kobayashi equations, which describe a laser diode subject to classical optical feedback.

The deconfined quantum critical point, a prototype Landau-forbidden transition, could exist, in principle, in the phase transitions involving a symmetry-protected topological phase; however, examples of such kinds of transitions in physical systems are rare beyond one-dimensional systems. Here, using a density-matrix renormalization-group calculation, we unveil a bosonic integer quantum Hall phase in a two-dimensional correlated honeycomb lattice, by full identification of its internal structure from the topological $\mathbf{K}$ matrix. Moreover, we demonstrate that imbalanced periodic chemical potentials can destroy the bosonic integer quantum Hall state and drive it into a featureless trivial (Mott) insulator, where all physical observables evolve smoothly across the critical point. At the critical point, the entanglement entropy reveals a characteristic scaling behavior, which is consistent with the critical field theory as an emergent ${\mathrm{QED}}_3$ with two flavors of Dirac fermions.

The $GW$ method is a many-body electronic structure technique capable of generating accurate quasiparticle properties for realistic systems spanning physics, chemistry, and materials science. Despite its power, $GW$ is not routinely applied to study large complex assemblies due to the method's high computational overhead and quartic scaling with particle number. Here, the $GW$ equations are recast, exactly, as Fourier-Laplace time integrals over complex time propagators. The propagators are then “shredded” via energy partitioning and the time integrals approximated in a controlled manner using generalized Gaussian quadrature(s) while discrete variable methods are employed to represent the required propagators in real space. The resulting cubic scaling $GW$ method has a sufficiently small prefactor to outperform standard quartic scaling methods on small systems ($⪆10$ atoms) and offers 2–3 order of magnitude improvement in large systems ($\approx 200–300$ atoms). It also represents a substantial improvement over other cubic methods tested for all system sizes studied. The approach can be applied to any theoretical framework containing large sums of terms with energy differences in the denominator.

We provide new insights into the Abelian and non-Abelian chiral Kitaev spin liquids on the star lattice using the recently proposed loop gas (LG) and string gas states [Lee, Kaneko, Okubo, and Kawashima, Phys. Rev. Lett. 123, 087203 (2019)]. Those are compactly represented in the language of the tensor network. By optimizing only one or two variational parameters, accurate ansatze are found in the whole phase diagram of the Kitaev model on the star lattice. In particular, the variational energy of the LG state becomes exact (within machine precision) at two limits in the model, and the criticality at one of those is analytically derived from the LG feature. It reveals that the Abelian chiral spin liquids (CSLs) are well demonstrated by the short-ranged LG whereas the non-Abelian CSLs are adiabatically connected to the critical LG where the macroscopic loops appear. Furthermore, by constructing the minimally entangled states and exploiting their entanglement spectrum and entropy, we identify the nature of anyons and the chiral edge modes in the non-Abelian phase with the Ising conformal field theory.

Injecting a sufficiently large energy density into an isolated many-particle system prepared in a state with long-range order will lead to the melting of the order over time. Detailed information about this process can be derived from the quantum mechanical probability distribution of the order parameter. We study this process for the paradigmatic case of the spin-1/2 Heisenberg XXZ chain. We determine the full quantum mechanical distribution function of the staggered subsystem magnetization as a function of time after a quantum quench from the classical Néel state. We establish the existence of an interesting regime at intermediate times that is characterized by a very broad probability distribution. Based on our findings we propose a simple general physical picture of how long-range order melts.

The dispersive resonant-state expansion, developed for an accurate calculation of the resonant states in open optical systems with frequency dispersion, is applied here to realistic materials, such as metallic nanoparticles and semiconductor microspheres. The material permittivity is determined by fitting the measured indices of refraction and absorption with a generalized Drude-Lorentz model containing a number of poles in the complex frequency plane. Each Drude or Lorentz pole generates an infinite series of resonant states. Furthermore, for small nanoparticles, each of these poles produces a distinct surface plasmon polariton mode. The evolution of these multiple surface modes with increasing radius traces the transition from the electrostatic limit to significant retardation and radiation. Treating the optical phonon range in a semiconductor microsphere, a reststrahlen band separating the resonant states is found. Considering a small energy range around the semiconductor band gap, the transition from absorption to gain is described by inverting the Lorentz pole weight, which results in the formation of lasing resonant states. Interestingly, the series of resonant states converging towards the absorption pole from the lower frequency side reshapes for a gain pole into a clockwise loop approaching the pole from the higher frequency side, being separated from a series spanning from low to high frequencies and containing the lasing modes.

A density functional theory based semiempirical van der Waals (vdW) method with dielectric functions being incorporated is developed for two-dimensional materials. The coefficients of interatomic pairwise potentials are derived from atomic polarizabilities obtained via a Clausius-Mossotti relation dedicated for layered crystals. The method not only can efficiently describe the dispersion energy for a range of planar graphene-like materials at nearly the same accuracy as the adiabatic connection fluctuation-dissipation theorem, but also rationalizes experimentally measured relative interfacial strengths of heterostructures and interlayer registry of hexagonal boron nitride that have plagued other vdW methods.

First-principles electronic-structure calculations have been employed to investigate the Rashba-Dresselhaus spin splitting of bands in the recently predicted ferroelectric nitride perovskite ${\mathrm{LaWN}}_3$. Our first-principles results are supplemented with an effective $\mathbf{k}·\mathbf{p}$ model analysis. A systematic study of orthorhombic and rhombohedral phases of this system reveals the importance of symmetry in realizing the nature of the splitting of bands around the time-reversal invariant $k$ points. Orthorhombic ${\mathrm{LaWN}}_3$ shows linear Rashba-Dresselhaus splitting where nonsymmorphic symmetries play an important role in enhancing the band splitting in the $k_z=\pi /c$ plane, while the rhombohedral phase shows the existence of a unique higher order Rashba-Dresselhaus term which mixes with the linear Rashba-Dresselhaus term to produce an unusual pattern of out-of-plane spin components in the spin texture.

We consider the problem of electron transport across a quasi-one-dimensional disordered multiply scattering medium and study the statistical properties of the electron density inside the system. In the physical setup that we contemplate, electrons of a given energy feed the disordered conductor from one end. The physical quantity that is mainly considered is the logarithm of the electron density, $ln\mathcal{W}\left(x\right)$, since its statistical properties exhibit a self-averaging behavior. We also describe a gedanken experiment as a possible setup to measure the electron density. We study analytically and through computer simulations the ballistc, diffusive, and localized regimes. We generally find a good agreement between the two approaches. The extension of the techniques that were used in the past to find information outside the sample is done in terms of the scattering properties of the two segments that form the entire conductor on each side of the observation point. The problem is of interest in various other branches of physics, such as electrodynamics and elasticity.

We derive an exact renormalization group recursion relation for the Loschmidt amplitude of the quantum $Q$-state clock model and the quantum $Q$-state Potts model in one dimension. The renormalization group flow is discussed in detail. The fixed points of the renormalization group flow are found to be complex in general. These fixed points control the dynamical phases of the two models, giving rise to nonanalyticities in its Loschmidt rate function, for both the pure and the disordered system. For the quench protocols studied, dynamical quantum phase transitions are found to occur in the clock model for all $Q\mathrm{s}$ considered, while in the Potts model, they only occur when $Q<4$.

Majorana edge modes are candidate elements of topological quantum computing. In this paper, we propose a Floquet engineering approach to generate multiple non-Hermitian Majorana zero and $\pi$ modes at the edges of a one-dimensional topological superconductor. Focusing on a Kitaev chain with periodically kicked superconducting pairings and gain/losses in the chemical potential or nearest-neighbor hopping terms, we found rich non-Hermitian Floquet topological superconducting phases, which are originated from the interplay between drivings and non-Hermitian effects. Each of the phases is characterized by a pair of topological winding numbers, which can in principle take arbitrarily large integer values thanks to the applied driving fields. Under the open boundary condition, these winding numbers also predict the number of degenerate Majorana edge modes with quasienergies zero and $\pi$. Our findings thus expand the family of Floquet topological phases in non-Hermitian settings, with potential applications in realizing environmentally robust Floquet topological quantum computations.

We investigate the magnetic dynamics of the orthorhombic perovskite ${\mathrm{TmFeO}}_3$ at low temperatures, below the spin reorientation transition at $T_{\mathrm{SR}}\approx 80$ K, by means of time-of-flight neutron spectroscopy. We find that the magnetic excitation spectrum combines two emergent collective modes associated with different magnetic sublattices. The Fe subsystem orders below $T_{\mathrm{N}}\sim 632$ K into a canted antiferromagnetic structure and exhibits sharp, high-energy magnon excitations. We describe them using linear spin-wave theory, and reveal a pronounced anisotropy between in- and out-of-plane exchange interactions, which was mainly neglected in previous reports on the spin dynamics in orthoferrites. At lower energies, we find two crystalline electrical field (CEF) excitations of ${\mathrm{Tm}}^{3+}$ ions at energies of $\sim 2$ and 5 meV. In contrast to the sister compound ${\mathrm{YbFeO}}_3$, where the ${\mathrm{Yb}}^{3+}$ ions form quasi-one-dimensional chains along the $c$ axis, the Tm excitations show dispersion along both directions in the $\left(0KL\right)$ scattering plane. Analysis of the neutron scattering polarization factor reveals a longitudinal polarization of the 2 meV excitation. To evaluate the effect of the CEF on the ${\mathrm{Tm}}^{3+}$ ions, we perform point-charge model calculations, and their results quantitatively capture the main features of Tm single-ion physics, such as energies, intensities, and polarization of the CEF transitions, and the type of magnetic anisotropy.

The excitation of ultrafast dynamics in antiferromagnetic materials is an appealing feature for the realization of spintronic devices. Several experiments have shown that static and dynamic behaviors of the antiferromagnetic order are strictly related to the stabilization of multidomain states and the manipulation of their domain walls (DWs). Hence, a full micromagnetic framework should be used as a comprehensive theoretical tool for a quantitative understanding of those experimental findings. This model is used to perform numerical experiments to study the antiferromagnetic DW motion driven by the spin-orbit torque. We have derived simplified expressions for the DW width and velocity that exhibit a very good agreement with the numerical calculations in a wide range of parameters. Additionally, we have found that a mechanism limiting the maximum applicable current in an antiferromagnetic racetrack memory is the continuous domain nucleation from its edges, which is qualitatively different from what observed in the ferromagnetic case.

An extensive equilibrium Monte Carlo simulation is performed on the three-dimensional isotropic Heisenberg spin-glass (SG) model with the random nearest-neighbor Gaussian coupling, with particular interest in its chiral-glass (CG) and SG orderings. For this model, the possibility of the spin-chirality decoupling, i.e., the CG order setting in at a higher temperature than that of the SG order, was suggested earlier but still remains controversial. We simulate the model up to the maximum size (linear dimension) $L=48$ under both periodic and open boundary conditions (BCs). In locating the CG and SG transition temperatures $T_{\mathrm{CG}}$ and $T_{\mathrm{SG}}$ with the $L\to \infty$ extrapolation, a variety of independent physical quantities under both BCs are computed and utilized to get larger number of degrees of freedom (NDF). Because of the large NDF up to $\text{NDF}=43$, we succeed in obtaining stable and accurate estimates of the CG and SG transition temperatures, $T_{\mathrm{CG}}=0.142\pm 0.001$ and $T_{\mathrm{SG}}=0.{131}_{-0.006}^{+0.001}$. No sign of the size crossover is observed. For larger $L$, the CG correlation length progressively outgrows the SG correlation length at low temperatures. These results provide strong numerical support for the spin-chirality decoupling. The critical exponents associated with the CG and SG transitions are evaluated using the finite-size scaling with the scaling correction. For the CG transition, we get the CG exponents, ${\nu }_{\mathrm{CG}}=1.36\pm 0.10$ and ${\eta }_{\mathrm{CG}}=0.49\pm 0.10$, consistent with the corresponding experimental exponents of canonical SG. Implications for the chirality scenario of experimental SG ordering are discussed.

The spin- and energy-dependent interface reflectivity of a ferromagnetic (FM) film in contact with a nonmagnetic (NM) film is calculated using a first-principles transport method and incorporated into the superdiffusive spin transport model to study the femtosecond laser-induced ultrafast demagnetization of $\mathrm{Fe}|\mathrm{NM}$ and $\mathrm{Ni}|\mathrm{NM}$ (NM = Au, Al, and Pt) bilayers. By comparing the calculated demagnetization with transparent and real interfaces, we demonstrate that the spin-dependent reflection of hot electrons has a noticeable influence on the ultrafast demagnetization and the associated terahertz (THz) electromagnetic radiation. In particular, a spin filtering effect is found at the $\mathrm{Fe}|\mathrm{NM}$ interface that increases the spin current injected into the NM metal, which enhances both the resulting demagnetization and the resulting THz emission. This suggests that the THz radiation can be optimized by tailoring the interface, indicating a very large tunability.