We study the impact of quenched disorder (random exchange couplings or site dilution) on easy-plane pyrochlore antiferromagnets. In the clean system, order by disorder selects a magnetically ordered state from a classically degenerate manifold. In the presence of randomness, however, different orders can be chosen locally depending on details of the disorder configuration. Using a combination of analytical considerations and classical Monte Carlo simulations, we argue that any long-range-ordered magnetic state is destroyed beyond a critical level of randomness where the system breaks into magnetic domains due to random exchange anisotropies, becoming, therefore, a glass of spin clusters, in accordance with the available experimental data. These random anisotropies originate from off-diagonal exchange couplings in the microscopic Hamiltonian, establishing their relevance to other magnets with strong spin-orbit coupling.

We study exciton polaritons in a two-dimensional Lieb lattice of micropillars. The energy spectrum of the system features two flat bands formed from $S$ and $P_{x,y}$ photonic orbitals, into which we trigger bosonic condensation under high power excitation. The symmetry of the orbital wave functions combined with photonic spin-orbit coupling gives rise to emission patterns with pseudospin texture in the flat band condensates. Our Letter shows the potential of polariton lattices for emulating flat band Hamiltonians with spin-orbit coupling, orbital degrees of freedom, and interactions.

We investigate the effect of stress fluctuations on the stochastic dynamics of an inclusion embedded in a viscous gel. We show that, in nonequilibrium systems, stress fluctuations give rise to an effective attraction towards the boundaries of the confining domain, which is reminiscent of an active Casimir effect. We apply this generic result to the dynamics of deformations of the cell nucleus, and we demonstrate the appearance of a fluctuation maximum at a critical level of activity, in agreement with recent experiments [E. Makhija, D. S. Jokhun, and G. V. Shivashankar, Proc. Natl. Acad. Sci. U.S.A. 113, E32 (2016)].

We present an experimental demonstration of converting a microwave field to an optical field via frequency mixing in a cloud of cold ${}^{87}\mathrm{Rb}$ atoms, where the microwave field strongly couples to an electric dipole transition between Rydberg states. We show that the conversion allows the phase information of the microwave field to be coherently transferred to the optical field. With the current energy level scheme and experimental geometry, we achieve a photon-conversion efficiency of $\sim 0.3\%$ at low microwave intensities and a broad conversion bandwidth of more than 4 MHz. Theoretical simulations agree well with the experimental data, and they indicate that near-unit efficiency is possible in future experiments.

Robust spin Hall effects (SHE) have recently been observed in nonmagnetic heavy metal systems with strong spin-orbit interactions. These SHE are either attributed to an intrinsic band-structure effect or to extrinsic spin-dependent scattering from impurities, namely, side jump or skew scattering. Here we report on an extraordinarily strong spin Hall effect, attributable to spin fluctuations, in ferromagnetic ${\mathrm{Fe}}_x{\mathrm{Pt}}_{1-x}$ alloys near their Curie point, tunable with $x$. This results in a dampinglike spin-orbit torque being exerted on an adjacent ferromagnetic layer that is strongly temperature dependent in this transition region, with a peak value that indicates a lower bound $0.34\pm 0.02$ for the peak spin Hall ratio within the FePt. We also observe a pronounced peak in the effective spin-mixing conductance of the $\mathrm{FM}/\mathrm{FePt}$ interface, and determine the spin diffusion length in these ${\mathrm{Fe}}_x{\mathrm{Pt}}_{1-x}$ alloys. These results establish new opportunities for fundamental studies of spin dynamics and transport in ferromagnetic systems with strong spin fluctuations, and a new pathway for efficiently generating strong spin currents for applications.

The decay rate of a false vacuum is determined by the minimal action solution of the tunneling field: bounce. In this Letter, we focus on models with scalar fields which have a canonical kinetic term in $N(>2)$ dimensional Euclidean space, and derive an absolute lower bound on the bounce action. In the case of four-dimensional space, we show the bounce action is generically larger than $24/{\lambda }_{\mathrm{cr}}$, where ${\lambda }_{\mathrm{cr}}\equiv \max [-4V(\varphi )/|\varphi {|}^4]$ with the false vacuum being at $\varphi =0$ and $V(0)=0$. We derive this bound on the bounce action without solving the equation of motion explicitly. Our bound is derived by a quite simple discussion, and it provides useful information even if it is difficult to obtain the explicit form of the bounce solution. Our bound offers a sufficient condition for the stability of a false vacuum, and it is useful as a quick check on the vacuum stability for given models. Our bound can be applied to a broad class of scalar potential with any number of scalar fields. We also discuss a necessary condition for the bounce action taking a value close to this lower bound.

One of the interesting features of open quantum and wave systems is the non-Hermitian degeneracy called an exceptional point, where not only energy levels but also the corresponding eigenstates coalesce. We demonstrate that such a degeneracy can appear in optical microdisk cavities by deforming the boundary extremely weakly. This surprising finding is explained by a semiclassical theory of dynamical tunneling. It is shown that the exceptional points come in nearly degenerated pairs, originating from the different symmetry classes of modes. A spatially local chirality of modes at the exceptional point is related to vortex structures of the Poynting vector.

Transient chaos in spatially extended systems can be characterized by the length of the transient phase, which typically grows quickly with the system size (supertransients). For a large class of these systems, the chaotic phase terminates abruptly, without any obvious precursors in commonly used observables. Here we investigate transient spatiotemporal chaos in two different models of this class. By probing the state space using perturbed trajectories we show the existence of a “terminal transient phase,” which occurs prior to the abrupt collapse of chaotic dynamics. During this phase the impact of perturbations is significantly different from the earlier transient and particular patterns of (non)susceptible regions in state space occur close to the chaotic trajectories. We therefore hypothesize that even without perturbations proper precursors for the collapse of chaotic transients exist, which might be highly relevant for coping with spatiotemporal chaos in cardiac arrhythmias or brain functionality, for example.

Terahertz (THz) radiation promises breakthrough advances in compact advanced accelerators due to the gigavolts-per-meter fields achievable, but the challenge of maintaining overlap and synchronism between beams and short laser-generated THz pulses has so far limited interactions to the few-millimeter scale. We implement a novel scheme for simultaneous group and phase velocity matching of nearly single-cycle THz radiation with a relativistic electron beam for meter-scale inverse free-electron laser interaction in a magnetic undulator, resulting in energy modulations of up to 150 keV using modest THz pulse energies ($\le 1\mu \mathrm{J}$). Using this scheme, we demonstrate for the first time the use of a laser-based THz source for bunch-length compression and time-stamping of a relativistic electron beam.

We report the observation of shadowing between two optical transition radiation (OTR) sources from a 205 MeV electron beam. The total optical intensity is measured as a function of the distance $d$ between the sources, covering the range $0<d<4L_v$, where $L_v$ is the formation length of the particles. Data show that the total optical intensity starts decreasing due to shadowing when $d$ approaches $L_v$ until it becomes undetectable for very short distances $d/L_v\to 0$. A model based solely on interference between the two OTR sources is in good agreement with experimental data. To the knowledge of the authors this is the first systematic experimental observation of shadowing in OTR.

Using in situ synchrotron x-ray diffraction and Raman spectroscopy in concert with first principles calculations we demonstrate the synthesis of stable $\mathrm{Xe}(\mathrm{Fe},\mathrm{Fe}/\mathrm{Ni}{)}_3$ and ${\mathrm{XeNi}}_3$ compounds at thermodynamic conditions representative of Earth’s core. Surprisingly, in the case of both the Xe-Fe and Xe-Ni systems Fe and Ni become highly electronegative and can act as oxidants. The results indicate the changing chemical properties of elements under extreme conditions by documenting that electropositive at ambient pressure elements could gain electrons and form anions.

Understanding the interplay between the topological nature and the symmetry property of interacting systems has been a central matter of condensed matter physics in recent years. In this Letter, we establish nonperturbative constraints on the quantized Hall conductance of many-body systems with arbitrary interactions. Our results allow one to readily determine the many-body Chern number modulo a certain integer without performing any integrations, solely based on the rotation eigenvalues and the average particle density of the many-body ground state.

As the Fermi level and band structure of two-dimensional materials are readily tunable, they constitute an ideal platform for exploring the Lifshitz transition, a change in the topology of a material’s Fermi surface. Using tetralayer graphene that host two intersecting massive Dirac bands, we demonstrate multiple Lifshitz transitions and multiband transport, which manifest as a nonmonotonic dependence of conductivity on the charge density $n$ and out-of-plane electric field $D$, anomalous quantum Hall sequences and Landau level crossings that evolve with $n$, $D$, and $B$.

We report the discovery of a field driven transition from a single-$q$ to multi-$q$ spin density wave (SDW) in the tetragonal heavy fermion compound ${\mathrm{CeAuSb}}_2$. Polarized along $\mathbf{c}$, the sinusoidal SDW amplitude is $1.8(2){\mu }_B/\mathrm{Ce}$ for $T\ll T_N=6.25(10)\mathrm{K}$ with a wave vector ${\mathbf{q}}_1=(\eta ,\eta ,1/2)$ $[\eta =0.136(2)]$. For $\mathbf{H}\parallel \mathbf{c}$, harmonics appearing at $2{\mathbf{q}}_1$ evidence a striped magnetic texture below ${\mu }_{\circ }{H}_1=2.78(1)\mathrm{T}$. Above $H_1$, these are replaced by coupled harmonics at ${\mathbf{q}}_1+{\mathbf{q}}_2=(2\eta ,0,0)+{\mathbf{c}}^{*}$ until ${\mu }_{\circ }{H}_2=5.42(5)\mathrm{T}$, where satellites vanish and magnetization nonlinearly approaches saturation at $1.64(2){\mu }_B/\mathrm{Ce}$ for ${\mu }_{\circ }H\approx 7\mathrm{T}$.

Magnetocrystalline anisotropy, the microscopic origin of permanent magnetism, is often explained in terms of ferromagnets. However, the best performing permanent magnets based on rare earths and transition metals (RE-TM) are in fact ferrimagnets, consisting of a number of magnetic sublattices. Here we show how a naive calculation of the magnetocrystalline anisotropy of the classic RE-TM ferrimagnet ${\mathrm{GdCo}}_5$ gives numbers that are too large at 0 K and exhibit the wrong temperature dependence. We solve this problem by introducing a first-principles approach to calculate temperature-dependent magnetization versus field (FPMVB) curves, mirroring the experiments actually used to determine the anisotropy. We pair our calculations with measurements on a recently grown single crystal of ${\mathrm{GdCo}}_5$, and find excellent agreement. The FPMVB approach demonstrates a new level of sophistication in the use of first-principles calculations to understand RE-TM magnets.

We study a system of spins (qubits) coupled to a common noisy environment, each precessing at its own frequency. The correlated noise experienced by the spins implies long-lived correlations that relax only due to the differing frequencies. We use a mapping to a non-Hermitian integrable Richardson-Gaudin model to find the exact spectrum of the quantum master equation in the high-temperature limit and, hence, determine the decay rate. Our solution can be used to evaluate the effect of inhomogeneous splittings on a system of qubits coupled to a common bath.