We present a detailed x-ray diffraction study of the structural evolution of epitaxial FeRh films across the temperature-driven phase transition between antiferromagnetic and ferromagnetic order. FeRh films grown onto MgO, W/MgO, and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ substrates show qualitatively different lattice distortions (tetragonal vs rhombohedral), while keeping a sharp transition above room temperature. Temperature-dependent x-ray reciprocal space mapping reveals the phase-specific crystal structure, giving access to both in-plane and out-of-plane lattice parameters and crystalline coherence lengths across different stages of the phase transition. Diffuse x-ray scattering from relaxed films is treated via a mosaic block model, which provides a robust data fitting scheme. It is found that the ferromagnetic phase fraction can stand a larger amount of strain before completely annihilating and transitioning to the antiferromagnetic phase upon cooling, as compared to heating. This is related to the distinct magnetic exchange correlations in the antiferromagnetic and ferromagnetic parent phases.

We investigate the Andreev-bound-state (ABS) spectra of three-terminal Josephson junctions which consist of 1D topological superconductors (TSCs) harboring multiple zero-energy edge Majorana bound states (MBSs) protected by chiral symmetry. Our theoretical analysis relies on the exact numerical diagonalization of the Bogoliubov-de Gennes (BdG) Hamiltonian describing the three interfaced TSCs, complemented by an effective low-energy description solely based on the coupling of the interfacial MBSs arising before the leads get contacted. Considering the 2D synthetic space spanned by the two independent superconducting phase differences, we demonstrate that the ABS spectra may contain either point or line nodes and identify ${\mathbb{Z}}_2$ topological invariants to classify them. We show that the resulting type of nodes depends on the number of preexisting interfacial MBSs, with nodal lines necessarily appearing when two TSCs harbor an unequal number of MBSs. Specifically, the precise number of interfacial MBSs determines the periodicity of the spectrum under $2\pi$ slidings of the phase differences and, as a result, also controls the shape of the nodal lines in synthetic space. When chiral symmetry is preserved, the lines are open and coincide with high-symmetry lines of synthetic space, while when it is violated, the lines can also transform into loops and chains. The nodal spectra are robust by virtue of the inherent particle-hole symmetry of the BdG Hamiltonian, and give rise to distinctive experimental signatures that we identify.

We demonstrate that a temperature gradient can strongly stimulate the thermoelectric signal, as well as dc Josephson current, in multiterminal superconducting hybrid nanostructures. At temperatures $T$ sufficiently exceeding the Thouless energy of our device, both the supercurrent and the thermoinduced voltage are dominated by the contribution from nonequilibrium low-energy quasiparticles and are predicted to decay slowly (algebraically rather than exponentially) with increasing $T$. We also predict a nontrivial current-phase relation and a transition to a $\pi$-junction state controlled by both the temperature gradient and the system topology. All these features are simultaneously observable in the same experiment.

Shock compressed diamond response at multimegabar stresses—of fundamental interest to high pressure science and relevant for high energy density experiments related to inertial confinement fusion—is often assumed to be hydrodynamic. To examine this assumption, plate impact experiments were conducted to measure wave profiles in diamond single crystals shocked to $\sim 900\mathrm{GPa}$ elastic impact stress (EIS). For the [110] and [111] orientations, two-wave structures (elastic-inelastic response) were observed to ∼900 GPa EIS; in contrast, single (overdriven) waves were observed at 480 GPa EIS and above for the [100] orientation. The elastic wave velocities for the [110] and [111] orientations were significantly larger than those for the [100] orientation. Strong orientation dependence was also observed for the elastic wave amplitudes; for [110] and [111] diamond, the amplitudes increased significantly with increasing elastic impact stress. The observed two-wave structures and the strong orientation dependence (elastic wave speeds and amplitudes) demonstrate unequivocally that the shock response of diamond single crystals is not hydrodynamic at stresses below the melt transition. As such, appropriate elastic-inelastic material descriptions are needed to accurately model the high stress response of diamond and other strong solids.

Multilayer structures incorporating nonmagnetic and ferromagnetic materials play an essential role in spintronic devices. Manipulation of the magnetization orientation in ferromagnets using adjacent nonmagnetic materials enables phenomena such as spin-torque driven magnetization switching and auto-oscillation. In order to improve the performance of spintronic devices, understanding of interfacial spin interactions is critical. Here we demonstrate detailed spatially resolved spectroscopic measurements obtained using ferromagnetic resonance force microscopy that provide a route to detailed understanding of spin interactions at normal-metal/ferromagnet interfaces. Our study reveals an unexpected 32-G uniaxial anisotropy field generated in a 20-nm-thick ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ thin film as a consequence of such an interaction with a 5-nm Au overlayer. The use of the low-damping ferrimagnetic insulator ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ enables imaging with multiple localized standing spin-wave modes (multimode imaging) confined by the micromagnetic tip. Multimode imaging not only improves the spatial resolution to be as fine as ∼100 nm, but also potentially enables the contributions of distinct spin interactions to be disentangled. We further discuss in detail the evolution of the multiple localized modes as the probe scans across the internal field step at the edge of the Au film. These capabilities provide the foundation for high spectral and spatial resolution study of spin interactions at the interface between a ferromagnet and a small-volume non-magnetic material including atomically thin two-dimensional materials.

Understanding the low-energy excitation state in three-dimensional layered compound ${\mathrm{ZrTe}}_5$ remains a challenging problem in the study of novel topological materials. Recently, a two-dimensional conical model was proposed to explain the experimental optical spectroscopy in the 3D ${\mathrm{ZrTe}}_5$, Martino et al. [Phys. Rev. Lett. 122, 217402 (2019)]. Motivated by this work, in this paper, we perform a systematic theoretical study on the optical conductivity of this model in both cases without and with an external magnetic field to further demonstrate the validity of this model and to recover new physics. We find that there exist completely different characteristics for optical conductivity along different directions, due to anisotropic low-energy excitations in this two-dimensional conical model. Specifically, for the interband optical conductivity, we find asymptotic dependence on the optical frequency as $\mathrm{Re}\left({\sigma }_x\right)\sim {\omega }^{\frac{1}{2}}$ and $\mathrm{Re}\left({\sigma }_z\right)\sim {\omega }^{\frac{3}{2}}$, which are universal both in the gapped insulator phase and Weyl semimetal phase. For the magneto-optical conductivity, on the contrary, $\mathrm{Re}\left({\sigma }_{x/z}^B\right)$ shows distinct signatures in the gapped insulator phase and Weyl semimetal phase, which can help distinguish the two phases. Our results, to be verified in future experiments, could provide more insights into the understanding of the topological nature of ${\mathrm{ZrTe}}_5$.

When the period of unit cells constituting metamaterials is no longer much smaller than the wavelength but only smaller, local material laws fail to describe the propagation of light in such composite media when considered at the effective level. Instead, nonlocal material laws are required. They have to be derived by approximating a general response function of the electric field in the metamaterial at the effective level that is accurate but cannot be handled practically. But how to perform this approximation is not obvious at all. Indeed, many approximations can be perceived and one should be able to decide as quickly as possible which of these possible material laws are mathematically and physically meaningful at all. Here, at the example of a second order Padé approximation of the general response function of the electric field, we present a checklist of each possible constitutive relation that has to pass in order to be physically and mathematically liable. As will be shown, only two out of these nine Padé approximations pass the checklist. The work is meant to be a guideline applicable to decide which constitutive relation actually makes any sense at all. It is an essential ingredient for future research on composite media as any possible constitutive relation to be discussed should pass it.

The one-photon absorption cross section of nanocrystals (NCs) of the inorganic perovskite ${\mathrm{CsPbBr}}_3$ is studied theoretically using a multiband $\mathbf{k}·\mathbf{p}$ envelope-function model combined with a treatment of intercarrier correlation by many-body perturbation theory. A confined exciton is described first within the Hartree-Fock (HF) approximation and correlation between the electron and hole is then included in leading order by computing the first-order vertex correction to the electron-photon interaction. The vertex correction is found to give an enhancement of the near-threshold absorption cross section by a factor of up to 4 relative to the HF (mean-field) value of the cross section, for NCs with an edge length $L=9$–12 nm (regime of intermediate confinement). The vertex-correction enhancement factors are found to decrease with increasing exciton energy; the absorption cross section for photons of energy $\omega =3.1$ eV (about 0.7 eV above threshold) is enhanced by a factor of only 1.4–1.5 relative to the HF value. The $\mathbf{k}·\mathbf{p}$ corrections to the absorption cross section are also significant; they are found to increase the cross section at an energy $\omega =3.1$ eV by about 30% relative to the value found in the effective-mass approximation. The theoretical absorption cross section at $\omega =3.1$ eV, assuming a Kane parameter $E_P=20$ eV, is found to be intermediate among the set of measured values (which vary among themselves by nearly an order of magnitude) and to obey a power-law dependence ${\sigma }^{\left(1\right)}\left(\omega \right)\propto L^{2.9}$ on the NC edge length $L$, in good agreement with experiment. The dominant contribution to the theoretical exponent 2.9 is shown to be the density of final-state excitons. We also calculate the radiative lifetimes of the ground-state $1S_e–1S_h$ exciton of NCs of ${\mathrm{CsPbBr}}_3$ and ${\mathrm{CsPbI}}_3$, finding an overestimate by a factor of up to about two (for $E_P=20$ and 17 eV, respectively) compared to the available experimental data, which vary among themselves by about $\pm 40$%. The sources of theoretical uncertainty and the possible reasons for the discrepancies with experiment are discussed. The main theoretical uncertainty in these calculations is in the value of the Kane parameter $E_P$.

Vortices are topological objects carrying quantized orbital angular momentum and have been widely studied in many physical systems for their applicability in information storage and processing. In systems with spin degrees of freedom, the elementary excitations are so-called half vortices, carrying a quantum rotation only in one of the two spin components. We study the spontaneous formation and stability of such localized half vortices in semiconductor microcavity polariton condensates, nonresonantly excited by a linearly polarized ring-shaped pump. The TE-TM splitting of optical modes in the microcavity system leads to an effective spin-orbit coupling, resulting in solutions with discrete rotational symmetry. The cross-interaction between different spin components provides an efficient method to realize all-optical half-vortex core switching, inverting its circular polarization state. This switching can be directly measured in the polarization-resolved intensity in the vortex core region and it can also be applied to higher order half-vortex states.

A macroscopic Coulomb system of identical charged particles with or without a compensating background charge can evolve maintaining spatial homogeneity and isotropy that mimic the cosmological evolution of a universe with repulsive gravity. Here we study dynamics of small perturbations on the background of the corresponding Hubble flow by analyzing its normal modes of vibrations. Arbitrary disturbance of the flow can be resolved into two electroacoustic, two vortical, and one entropic modes whose dynamics is investigated. Specifically, in the zero pressure or long-wavelength limits perturbations of density and velocity evolve in a manner that is independent of the form of the initial disturbance. The same conclusion applies to vortical perturbations of the velocity for arbitrary pressure while entropic perturbations are advected by the Hubble flow. Without the background charge the underlying Hubble flow describes a Coulomb explosion whose stability with respect to small disturbances is also demonstrated.

The presence of nonanalyticity in observables is a manifestation of phase transitions. Through the study of two paradigmatic topological models in one and two dimensions, in this work we show that the circuit complexity based on our specific quantification can reveal the occurrence of topological phase transitions, both in and out of equilibrium, by the presence of nonanalyticity. By quenching the system out of equilibrium, we find that the circuit complexity grows linearly or quadratically in the short-time regime if the quench is finished instantaneously or in a finite time, respectively. Notably, we find that for both the sudden quench and the finite-time quench, a topological phase transition in the pre-quench Hamiltonian will be manifested by the presence of nonanalyticity in the first-order or second-order derivative of circuit complexity with respect to time in the short-time regime, and a topological phase transition in the post-quench Hamiltonian will be manifested by the presence of nonanalyticity in the steady value of circuit complexity in the long-time regime. We also show that the increase of dimension does not remove, but only weakens the nonanalyticity of circuit complexity. Our findings can be tested in quantum simulators and cold-atom systems.

We present an $S=2$ antiferromagnet composed of a Mn-verdazyl complex $\left[\mathrm{Mn}{\left(\mathrm{hfac}\right)}_2\right]\left[o\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}{\left(4\text{−}\mathrm{F}\right)}_2\right]$. An analysis of the magnetic behavior reveals that the strong antiferromagnetic intramolecular interaction between spins on ${\mathrm{Mn}}^{2+}$ and the verdazyl radical forms an $S=2$ hybrid spin below approximately 100 K. In the low-temperature regions, the magnetic susceptibility and the specific heat indicate a phase transition to the three-dimensional order of the $S$ = 2 system. Below the transition temperature, the magnetization curve exhibits a metamagnetic phase transition. From the analysis of electron spin resonance modes, we evaluate the on-site biaxial anisotropy responsible for the anisotropic magnetic behavior.

Josephson radiation is a powerful method to probe Majorana zero modes in topological superconductors. Recently, Josephson radiation with half the Josephson frequency has been experimentally observed in a HgTe-based junction, possibly from Majorana zero modes. However, this radiation vanishes above a critical voltage, sharply contradicting previous theoretical results. In this Rapid Communication, we theoretically obtain a radiation spectrum quantitatively in agreement with the experiment after including the nonlinear dynamics of the Majorana states into a standard resistively shunted junction model. We further predict two alternative structures of the radiation spectrum for future experimental verification: an interrupted emission line and a chaotic regime. We develop a fixed-point analysis to understand all these features. Our results resolve an apparent discrepancy between theory and experiments, and will inspire the reexamination of structures in radiation spectra of various topological Josephson junctions.

We report the control of tunnel ionization in lithium niobate (${\mathrm{LiNbO}}_3$) using phase-controlled two-color laser fields. Through a macroscopic observable of high contrast, we disclose the crucial contribution of the microscopic spontaneous polarization of the ferroelectric material to the ionization rate: as the relative two-color phase is varied, the ablated area of ${\mathrm{LiNbO}}_3$ is modulated by 35% when the laser and crystal polarization directions are parallel. Rotating the sample by ${180}^{\circ }$ around the laser propagation axis leads to an out-of-phase modulation. We use a two-band model to highlight the key contribution of the material's spontaneous polarization for the symmetry breaking of the ionization rate. Our results open new perspectives for the direct control of ionization dynamics in solids by tailoring the electric field of femtosecond laser pulses.

We study a mobile quantum impurity, possessing internal rotational degrees of freedom, confined to a ring in the presence of a many-particle bosonic bath. By considering the recently introduced rotating polaron problem, we define the Hamiltonian and examine the energy spectrum. The weak-coupling regime is studied by means of a variational ansatz in the truncated Fock space. The corresponding spectrum indicates that there emerges a coupling between the internal and orbital angular momenta of the impurity as a consequence of the phonon exchange. We interpret the coupling as a phonon-mediated spin-orbit coupling and quantify it by using a correlation function between the internal and the orbital angular momentum operators. The strong-coupling regime is investigated within the Pekar approach, and it is shown that the correlation function of the ground state shows a kink at a critical coupling, that is explained by a sharp transition from the noninteracting state to the states that exhibit strong interaction with the surroundings. The results might find applications in such fields as spintronics or topological insulators where spin-orbit coupling is of crucial importance.

We use ab initio self-consistent phonon theory combined with compressive sensing techniques to investigate the role of quartic anharmonicity in the lattice dynamics and thermal transport properties of the cubic antiperovskites $A_{3}B\text{O}$ ($A=\text{K}$, Rb; $B=\text{Br}$, Au). Our findings indicate that the strong quartic anharmonicity of alkali-metal atoms plays a crucial role in the phonon quasiparticles free from imaginary frequencies in ${\mathrm{K}}_3\mathrm{BrO}$, ${\mathrm{Rb}}_3\mathrm{BrO}$, and ${\mathrm{Rb}}_3\mathrm{AuO}$, and gives rise to an evident hardening of vibrational frequencies of the low-lying modes in ${\mathrm{K}}_3\mathrm{AuO}$. Based on these results, the lattice thermal transport properties are predicted through the Boltzmann transport equation within the relaxation time approximation. The results exhibit that the four cubic antiperovskites have remarkably low thermal conductivities ${\kappa }_L$, e.g., 0.73–1.70 W/mK at 300 K, and an anomalously weak temperature dependence of the ${\kappa }_L$, e.g., ${\kappa }_L\sim T^{-0.3}$, of which the latter has not been reported in normal perovskites and other conventional semiconductors to date. We demonstrate that the strong three-phonon scattering along with strong quartic anharmonic renormalization of the soft modes and the presence of high-frequency dispersive optical modes are responsible for the thermal conductivity spectrum ${\kappa }_L\left(\omega \right)$ and thus the low ${\kappa }_L$ and its anomalously weak temperature dependence.

Strong quantum fluctuations in magnetic systems can create disordered quantum spin liquid phases of matter which are not predicted by classical physics. The complexity of the exotic phenomena on display in spin liquids has led to a great deal of theoretical and experimental interest. However, understanding the fundamental nature of the excitations in these systems remains challenging. In this work, we consider the Lifshitz quantum critical point in a two-dimensional frustrated $XY$ antiferromagnet. At this point, quantum fluctuations destroy long-range order, leading to the formation of an algebraic Lifshitz spin liquid. We demonstrate that the bosonic magnon excitations are long-lived and well-defined in the Lifshitz spin liquid phase, though paradoxically, the dynamic structure factor has a broad non-Lorentzian frequency distribution with no single-particle weight. We resolve this apparent contradiction by showing that the Lifshitz spin liquid suffers from an infrared catastrophe: An external physical probe always excites an infinite number of arbitrarily low-energy quasiparticles, which leads to significant radiative broadening of the spectrum.