Inelastic neutron scattering is used to study the magnetic excitations of the $S=1$ square-lattice antiferromagnet ${\mathrm{La}}_2{\mathrm{NiO}}_4$. We find that the spin waves cannot be described by a simple classical (harmonic) Heisenberg model with only nearest-neighbor interactions. The spin-wave dispersion measured along the antiferromagnetic Brillouin-zone boundary shows a minimum energy at the $(1/2,0)$ position as is observed in some $S=1/2$ square-lattice antiferromagnets. Thus, our results suggest that the quantum dispersion renormalization effects or longer-range exchange interactions observed in cuprates and other $S=1/2$ square-lattice antiferromagnets are also present in ${\mathrm{La}}_2{\mathrm{NiO}}_4$. We also find that the overall intensity of the spin-wave excitations is suppressed relative to linear spin-wave theory, indicating that covalency is important. Two-magnon scattering is also observed.

In recent years, hydrodynamic optical-field-ionized (HOFI) channels have emerged as a promising technique to create laser waveguides suitable for guiding tightly focused laser pulses in a plasma, as needed for laser-plasma accelerators. While experimental advances in HOFI channels continue to be made, the underlying mechanisms and the roles of the main parameters remain largely unexplored. In this paper, we propose a start-to-end simulation pipeline of the HOFI channel formation and the resulting laser guiding and use it to explore the underlying physics and the tunability of HOFI channels. This approach is benchmarked against experimental measurements. HOFI channels are shown to feature excellent guiding properties over a wide range of parameters, making them a promising and tunable waveguide option for laser-plasma accelerators.

We report the direct observation of a commensurate-ordered antiferromagnetic (AFM) state but incommensurate helical spin dynamics in the natural mineral brochantite ${\mathrm{Cu}}_4{\mathrm{SO}}_4{\left(\mathrm{OH}\right)}_6$ through neutron diffraction and neutron spectroscopy measurements. Inelastic neutron scattering measurements reveal magnonlike excitations with considerable dispersion along the $c$ axis and almost flat branches in other principal directions, indicating the strong one-dimensional character of the magnetic correlations. We experimentally observe the effect of the uniform Dzyaloshinskii-Moriya (DM) interaction, which elevates the degeneracy of the spin-wave modes, shifting them in opposite directions in reciprocal space. The system has a commensurate AFM ground state, stabilized by the anisotropic symmetric Heisenberg exchange interactions, and quasi-one-dimensional chiral spin dynamics due to the antisymmetric DM interaction. Employing linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian. We quantify both the symmetric exchange parameters and the DM vector components in ${\mathrm{Cu}}_4{\mathrm{SO}}_4{\left(\mathrm{OH}\right)}_6$ and determine the mechanism of the magnetic frustration. Our work provides detailed insights into the complex dynamics of the spin chain in the presence of uniform DM interaction.

While optics and mechanics are two distinct branches of physics, they are connected. It is well known that the geometrical/ray treatment of light has direct analogies to mechanical descriptions of particle motion. However, connections between coherence wave optics and classical mechanics are rarely reported. Here we report links of the two through a systematic quantitative analysis of polarization and entanglement, two optical coherence properties under the wave description of light pioneered by Huygens and Fresnel. A generic complementary identity relation is obtained for arbitrary light fields. More surprisingly, through the barycentric coordinate system, optical polarization, entanglement, and their identity relation are shown to be quantitatively associated with the mechanical concepts of center of mass and moment of inertia via the Huygens-Steiner theorem for rigid body rotation. The obtained result bridges coherence wave optics and classical mechanics through the two theories of Huygens.

After a decade of intense theoretical and experimental efforts, demonstrating braiding of Majorana modes remains an unsolved problem in condensed matter physics due to platform-specific challenges. In this work, we propose topological superconductor–magnetic multilayer heterostructures with on-chip microwave cavity readout as a platform for initializing, braiding, and reading out Majorana modes. Stray fields from a skyrmion in the magnetic layers can nucleate a vortex in the topological superconducting layer. Such a vortex is known to host Majorana bound states at its core. Through analytical calculations within London and Thiele formalisms, and through micromagnetic simulations, we show that our nucleation and braiding scheme can be effectively realized with a variety of existing options for magnetic and superconducting layers. Furthermore, we show that the coupling of the Majorana bound states to the electric field of a resonator leads to an experimentally observable parity-dependent dispersive shift of the resonator frequency. Our work paves the way for realizing Majorana braiding in the near future.

Ordered lattices of emitters with subwavelength periodicities support unconventional forms of light-matter interactions arising from collective effects. Here, we propose the realization and control of subradiant optical states within the radiation continuum in two-dimensional lattices. We show how bound states in the continuum (BICs) which are completely decoupled from radiative states emerge in non-Bravais lattices of emitters. Symmetry breaking results in quasi-BICs with greatly extended lifetimes, which can be exploited for quantum information storage. The analytical derivation of a generalized effective polarizability tensor allows us to study the optical response of these arrays. We discuss how thanks to the quasi-BICs, a rich phenomenology takes place in the reflectivity spectrum, with asymmetric Fano resonances and an electromagnetically induced transparency window. Finally, we exploit these lattices as quantum metasurfaces acting as efficient light polarizers.

The connections between standard theoretical tools used to study open quantum systems can sometimes seem opaque. Whether it is a Lindblad master equation, the equation of motion for the Wigner function, or a dissipative Keldysh action, features evident in one formalism are often masked in another. Here, we reformulate the technique of third quantization in a way that explicitly connects all three methods. We first show that our formulation reveals a fundamental dissipative symmetry present in all quadratic bosonic or fermionic Lindbladians. This symmetry can then be used to easily diagonalize these models, and it provides an intuitive way to demonstrate the separation of dissipation and fluctuations in linear systems. For bosons, we then show that the Wigner function and the characteristic function can be thought of as “wave functions” of the density matrix in the eigenbasis of the third-quantized superoperators we introduce. The field-theory representation of the time-evolution operator in this basis is then the Keldysh path integral. To highlight the utility of our approach, we apply our version of third quantization to a dissipative nonlinear oscillator, and we use it to obtain new exact results.

The dynamics triggered by the impact of an ion onto a solid surface has been explored mainly by theoretical modeling or computer simulation to date. Results indicate that the microscopic nonequilibrium relaxation processes triggered by the interaction of the ion with the solid occur on (sub)picosecond time scales. A suitable experimental approach to these dynamics therefore requires a pump-probe method with an appropriate time resolution. Recent experiments have successfully used laser photoionization of noble gas atoms in combination with a Wiley-MacLaren ion buncher to obtain arrival time distributions as narrow as $t_{\mathrm{ion}}=180$ ps. Here, we show that this setup can be significantly improved by replacing the gas at a temperature of $T_{\mathrm{atoms}}\simeq 300$ K with a supersonic beam of cooled noble gas atoms at $T_{\mathrm{atoms}}\simeq 4$ K. The detailed analysis of measured arrival times of individual ${\mathrm{Ne}}^{+}$ ions with a kinetic energy of 4 keV reveals that the arrival time jitter can be reduced by this technique down to $(18\pm 4)$ ps. This opens the door to pump-probe experiments with keV ions with a time resolution in the picosecond range.

We report the effects of off-diagonal coupling on eigenstates and eigenvalues of a realistic quantum Hamiltonian, a ${}^{121}\mathrm{Sb}$ (spin $I=5/2$) nuclear spin Hamiltonian in an antimony single crystal. In our nuclear resonance experiment, off-diagonal matrix elements are generated and controlled by the angle $\theta$ between the directions of the electric field gradient and an external magnetic field. With a very small $\theta$, a two-state-like avoided level crossing is directly probed as a function of an external magnetic field. On the other hand, by increasing $\theta$ under a weak magnetic field, we find that the nuclear spin transitions undergo dramatic changes as $\theta$ approaches ${90}^{\circ }$. All our experimental observations are in excellent agreement with exact diagonalization calculations for the Hamiltonian, demonstrating how superposition states of a multistate quantum system evolve as a function of a well-defined control parameter.

Black holes are considered among the most fascinating objects that exist in our universe since in the classical formalism nothing, not even light, can escape from their vicinity due to gravity. The gravitational potential causes the light to bend toward the hole, which is known as gravitational lensing. Here, we present a synthetic realization of this phenomenon in a laboratory-scale two-dimensional network of mechanical circuits, based on analogous condensed matter formalism of Weyl semimetals with inhomogeneous nodal tilt profiles. Some of the underlying network couplings turn out as unstable and nonreciprocal and are implemented by embedded active feedback interactions in an overall stabilized structure. We demonstrate the lensing by propagating mechanical wave packets through the network with a programed funnel-like potential, achieving wave bending toward the circle center. We then demonstrate the versatility of our platform by reprogramming it to mimic quantum tunneling of particles through the event horizon, known as Hawking radiation, achieving an exceptional correspondence to the original mass loss rate within the hole. The network couplings and the potential can be further reprogrammed to realize other curvatures and associated relativistic phenomena.

The interaction of magnetic dopants with delocalized electron states can result in interesting many-body physics. Here, the magnetic properties of neutral and charged finite silver metal host clusters with a magnetic cobalt atom impurity were investigated experimentally by exploiting the complementary methods of Stern-Gerlach molecular beam deflection and x-ray magnetic circular dichroism spectroscopy and are accompanied by density functional theory calculations and charge transfer multiplet simulations. The influence of the number of valence electrons and the consequences of impurity encapsulation were addressed in free size-selected, singly cobalt-doped silver clusters ${\mathrm{CoAg}}_n^{0,+}$ ($n=2–15$). Encapsulation of the dopant facilitates the formation of delocalized electronic shells with complete hybridization of the impurity $3d$- and the host $5s$-derived orbitals, which results in impurity valence electron delocalization, effective spin relaxation, and a low-spin ground state. In the exohedral size regime, spin pairing in the free electron gas formed by the silver $5s$ electrons is the dominating driving force determining the local $3d$ occupation of the impurity and therefore, adjusting the spin magnetic moment accordingly.

The emergence of quasiparticles is a universal property in integrable systems. String-type quasiparticles, which are characterized by the string solutions of Bethe equations, play fundamental roles in the analysis of their physics. Through an investigation of the Bethe equations in the asymmetric simple exclusion process, we reveal the existence of string solutions in the presence of non-Hermiticity resulting from asymmetrical hopping. Because of the non-Hermiticity, the string solutions exhibit exotic properties such as the complexification of the center of string solutions and the delocalization of Bethe quantum numbers. In addition, we find the picture of string-type quasiparticles collapses in the strong-asymmetry regime. The collapse of string solutions characterizes the transition of eigenstates from bound states to scattering states.

Nonequilibrium thermodynamics provides a general framework for understanding nonequilibrium processes, particularly in small systems that are typically far from equilibrium and dominated by fluctuations. However, the experimental investigation of nonequilibrium thermodynamics remains challenging due to the lack of approaches to precisely manipulate nonequilibrium states and dynamics. Here, by shaping the effective potential of energy, we propose a general method to construct a nonequilibrium steady state (NESS) with arbitrary energy distribution. Using a well-designed energy-dependent feedback damping, the dynamics of an optically levitated nanoparticle in vacuum is manipulated and driven into a NESS with the desired energy distribution. Based on this approach, a phonon laser state is constructed with an ultra-narrow linewidth of $6.40\mu \text{Hz}$. Such an arbitrary NESS construction method provides an approach to manipulating the dynamics processes of micromechanical systems and paves the way for the systematic study of nonequilibrium dynamics in interdisciplinary research fields.

We report a phase diagram mapping for solid hydrogen based on several thousands of Raman spectroscopic data points obtained at simultaneous high-pressure and -temperature (PT) experiments. Information about phase behavior in the range of 300 K up to the melting line and 0–300 GPa for hydrogen has remained almost unknown to date; therefore, revealing it is very desirable. A network analysis of the isothermal and isobaric dependencies for various Raman modes has been used; discontinuities in these dependencies, plus the mode appearance/disappearance provide evidence for phase transitions. These transition data show self-consistency and have been used to outline possible phase boundaries for hydrogen in this PT region. Evidence we found shows hydrogen may experience a gradual molecular dissociation process before its metallization.

${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ is a pyroelectric honeycomb antiferromagnet exhibiting peculiar changes in its electric polarization at magnetic transitions. ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ stands out from isostructural magnetic compounds, showing an anomalously low magnetic transition temperature and unique magnetic anisotropy. We determine the magnetic structure of ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ utilizing high-resolution powder and single-crystal neutron diffraction. A noncollinear stripy antiferromagnetic order is found in the honeycomb planes. The magnetic space group is $P_{C}na{2}_1$. The in-plane magnetic connection is of the stripy type for both the $ab$-plane and $c$-axis spin components. This is a simpler connection than the one proposed previously. The ferromagnetic interlayer order of the $c$-axis spin components in our model is also distinct. The magnetic anisotropy of ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ is characterized by orientation-dependent magnetic susceptibility measurements on a single crystal, consistent with neutron diffraction analysis. The local magnetoelectric tensor analysis using our magnetic models provides insights into its magnetoelectric coupling and polarization. Thus, our results deliver essential information for understanding both the unusual magnetoelectric properties of ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ and the prospects for observing exotic nonreciprocal, Hall, and magnonic effects characteristic of this compound family.

Dissipation-free current transport in type II superconductors requires vortices, the topological defects of the superfluid, to be pinned by defects in the underlying material. The pinning capacity of a defect is quantified by the Labusch parameter $\kappa \sim f_p/\xi \overline{C}$, measuring the pinning force $f_p$ relative to the elasticity $\overline{C}$ of the vortex lattice, with $\xi$ denoting the coherence length (or vortex core size) of the superconductor. The critical value $\kappa =1$ separates weak from strong pinning, with a strong defect at $\kappa >1$ able to pin a vortex on its own. So far, this weak-to-strong pinning transition has been studied for isotropic defect potentials, resulting in a critical exponent $\mu =2$ for the onset of the strong pinning force density $F_{\mathrm{pin}}\sim n_p{f}_p{(\xi /a_0)}^2{(\kappa -1)}^{\mu }$, with $n_p$ denoting the density of defects and $a_0$ the intervortex distance. This result is owed to the special rotational symmetry of the defect producing a finite two-dimensional trapping area $S_{\mathrm{trap}}\sim {\xi }^2$ at the strong pinning onset. The behavior changes dramatically when studying anisotropic defects with no special symmetries: the strong pinning then originates out of isolated points with length scales growing as $\xi {(\kappa -1)}^{1/2}$, resulting in a different force exponent $\mu =5/2$. The strong pinning onset is characterized by the appearance of unstable areas ${\mathcal{U}}_{\tilde{\mathbf{R}}}$ of elliptical shape whose boundaries mark the locations where vortices jump. The associated locations of asymptotic vortex positions define areas ${\mathcal{B}}_{\overline{\mathbf{R}}}$ of bistable vortex states and assume the shape of a crescent. The geometries of unstable and bistable regions are associated with the local differential properties of the Hessian determinant $D\left(\mathbf{R}\right)$ of the pinning potential $e_p\left(\mathbf{R}\right)$, specifically, its minima, maxima, and saddle points. Extending our analysis to the case of a random two-dimensional pinning landscape, a situation describing strong pinning in a thin superconducting film, we discuss the topological properties of unstable and bistable regions as expressed through the Euler characteristic, with the latter related to the local differential properties of $D\left(\mathbf{R}\right)$ through Morse theory.