We report on the magnetocaloric effect and ultrasound studies of the frustrated quasi-one-dimensional spin-1/2 compound ${\mathrm{LiCuVO}}_4$, evidencing a spin-nematic state. The magnetic Grüneisen parameter diverges at the transition to the spin-nematic phase, ${\mu }_0{H}_{\mathrm{c}3}\approx 40$ T, showing quantum criticality accompanied by entropy accumulation. The observed high-field anomalies in the acoustic properties clearly evidence a strong involvement of the lattice in the spin dynamics. The theoretical approach, based on exchange-striction coupling with dipolar and quadrupolar contributions, suggests that the spin-dipole-strain and quadrupole-strain interactions govern the spin-nematicity in ${\mathrm{LiCuVO}}_4$.

We examine theoretically that an electric dipole close to a subwavelength sphere can yield an anapole (the dipole-sphere system is a nonradiating source). Physically, the anapole arises when the dipole induced by the source inside the sphere cancels out with the dipole contribution of the source with respect to the center of the sphere. The condition for the anapole is established and this state can be achieved with either a dielectric sphere with a high refractive index or a metallic sphere. Since there is a residual radiation (much smaller than the radiated power of the dipole in the absence of the scatterer) originating from high-order multipoles, the anapole is almost ideal. However, we show that the residual radiated power can be reduced even more by placing two electric dipoles on opposite sides of the spherical scatterer. This paper might have implications for controlling radiative properties of emitters and implementing applications related to molecular localization and sensing.

We introduce a general method for building neural networks on quantum computers. The quantum neural network is a variational quantum circuit built in the continuous-variable (CV) architecture, which encodes quantum information in continuous degrees of freedom such as the amplitudes of the electromagnetic field. This circuit contains a layered structure of continuously parameterized gates which is universal for CV quantum computation. Affine transformations and nonlinear activation functions, two key elements in neural networks, are enacted in the quantum network using Gaussian and non-Gaussian gates, respectively. The non-Gaussian gates provide both the nonlinearity and the universality of the model. Due to the structure of the CV model, the CV quantum neural network can encode highly nonlinear transformations while remaining completely unitary. We show how a classical network can be embedded into the quantum formalism and propose quantum versions of various specialized models such as convolutional, recurrent, and residual networks. Finally, we present numerous modeling experiments built with the strawberry fields software library. These experiments, including a classifier for fraud detection, a network which generates tetris images, and a hybrid classical-quantum autoencoder, demonstrate the capability and adaptability of CV quantum neural networks.

The variational quantum eigensolver (VQE), a variational algorithm to obtain an approximated ground state of a given Hamiltonian, is an appealing application of near-term quantum computers. To extend the framework to excited states, we here propose an algorithm, the subspace-search variational quantum eigensolver (SSVQE). This algorithm searches a low-energy subspace by supplying orthogonal input states to the variational ansatz and relies on the unitarity of transformations to ensure the orthogonality of the output states. The $k\mathrm{th}$ excited state is obtained as the highest-energy state in the low-energy subspace. The proposed algorithm consists only of two parameter optimization procedures and does not employ any ancilla qubits. The avoidance of the estimation of the inner product and the small number of procedures required are considerable improvements from the existing proposals for excited states, making our proposal an improved near-term quantum algorithm. We further generalize the SSVQE to obtain all excited states up to the $k\mathrm{th}$ by only a single optimization procedure. From numerical simulations, we verify the proposed algorithms. This work extends the applicable domain of the VQE to excited states and their related properties as a transition amplitude without sacrificing any of its feasibility. Moreover, the proposed variational subspace search, which generalizes the state search problem to the search of a unitary mapping to a specific subspace, itself would be useful for various quantum information processing methods such as finding a protected subspace or a good variational quantum error correction code.

We present a stochastic algorithm for constructing a topologically disordered (i.e., nonregular) spatial lattice with nodes of constant coordination number, the CC lattice. The construction procedure dramatically improves on an earlier proposal [Schrauth, Richter, and Portela, Phys. Rev. E 97, 022144 (2018)] with respect to both computational complexity and finite-size scaling properties—making the CC lattice an alternative to proximity graphs which, especially in higher dimensions, is significantly faster to build. Among other applications, physical systems such as certain amorphous materials with low concentration of coordination defects are an important example of disordered, constant-coordination lattices in nature. As a concrete application, we characterize the criticality of the 3D Ising model on the CC lattice. We find that its phase transition belongs to the clean Ising universality class, establishing that the disorder present in the CC lattice is a nonrelevant perturbation for this model in the sense of renormalization group theory.

The nonequilibrium random-field Ising model is well studied, yet there are outstanding questions. In two dimensions, power-law scaling approaches fail and the critical disorder is difficult to pin down. Additionally, the presence of faceting on the square lattice creates avalanches that are lattice dependent at small scales. We propose two methods which we find solve these issues. First, we perform large-scale simulations on a Voronoi lattice to mitigate the effects of faceting. Second, the invariant arguments of the universal scaling functions necessary to perform scaling collapses can be directly determined using our recent normal form theory of the renormalization group. This method has proven useful in cleanly capturing the complex behavior which occurs in both the lower and upper critical dimensions of systems and here captures the two-dimensional nonequilibrium random-field Ising model behavior well. The obtained scaling collapses span a range of a factor of 10 in the disorder and a factor of ${10}^4$ in avalanche cutoff. They are consistent with a critical disorder at zero and with a lower critical dimension for the model equal to 2.

The dynamic structure factor of materials is proportional to their linear electronic response and it displays their excitation spectra. Usually the response is measured on the same length scale as the perturbation. Here, we illustrate that much can be gained by studying also the mixed dynamic structure factor, which connects different spatial components of perturbation and response. We extend state-of-the-art ab initio calculations to access the mixed dynamic structure factor, including excitonic effects. Using bulk silicon and lithium fluoride as prototype examples, we show that these effects play a crucial role above and below the quasiparticle gap, and are needed in order to explain coherent inelastic x-ray scattering experiments. Our approach also yields important information concerning the microscopic structure of time-dependent density functional theory.

By measuring the Josephson emission of a diffusive superconductor–normal metal–superconductor (SNS) junction we access the harmonic content of the current-phase relation (CPR). We experimentally identify a nonadiabatic regime in which the CPR is modified by high frequency microwave irradiation. This observation is explained by the excitation of quasiparticles in the normal wire induced by the electromagnetic field. The distortion of the CPR originates from the phase-dependent out-of-equilibrium distribution function which is strongly affected by the ac response of the spectral supercurrent. For a phase difference approaching $\pi$, transitions across the minigap are dynamically favored, leading to a supercurrent reduction. This finding is supported by a comparison with the quasiclassical Green's function theory of superconductivity in diffusive SNS junctions under microwave irradiation.

Achieving atomically flat and stoichiometric films of complex multicomponent oxides is crucial to integrating these materials in both established and emerging technologies. While pulsed laser deposition (PLD) can in principle produce these high-quality films, growth experiments often result in unsatisfactory morphologies with rough surfaces and nonstoichiometric compositions. To understand the cause, the growth needs to be followed at an atomic level from its early stages as a function of the growth conditions. By combining PLD with atomically resolved scanning tunneling microscopy, as well as surface spectroscopic and diffraction techniques, we address the origin of surface roughening in ${\text{SrTiO}}_3$(110) homoepitaxial films and pinpoint optimal growth conditions. We highlight the importance of surface reconstructions at all stages of growth: The different sticking on coexisting surface structures is responsible for the roughening of ${\text{SrTiO}}_3$(110) films and affects their stoichiometry.

We reexamine the azimuthal angle dependence of the local spin polarization of hyperons in heavy ion collisions at 200 GeV in the framework of the $\left(3+1\right)$-dimensional viscous hydrodynamic model CLVisc. Two different initial conditions are considered in our simulation: the optical Glauber initial condition without an initial orbital angular momentum and the AMPT initial condition with an initial orbital angular momentum. We find that the azimuthal angle dependence of the hyperon polarization strongly depends on the choice of the so-called spin chemical potential ${\mathrm{\Omega }}_{\mu \nu }$. With ${\mathrm{\Omega }}_{\mu \nu }$ chosen to be proportional to the temperature vorticity, our simulation shows qualitatively coincidental results with the recent measurements at the BNL Relativistic Heavy Ion Collider for both the longitudinal and the transverse polarization. We argue that such a coincidence may be related to the fact that the temperature vorticity is approximately conserved in the hot quark-gluon matter.

A many-particle system must possess long-range interactions in order to be hyperuniform at thermal equilibrium. Hydrodynamic arguments and numerical simulations show, nevertheless, that a three-dimensional elastic-line array with short-ranged repulsive interactions, such as vortex matter in a type-II superconductor, forms at equilibrium a class-II hyperuniform two-dimensional point pattern for any constant-$z$ cross section. In this case, density fluctuations vanish isotropically as $\sim q^{\alpha }$ at small wave vectors $q$, with $\alpha =1$. This prediction includes the solid and liquid vortex phases in the ideal clean case and the liquid in presence of weak uncorrelated disorder. We also show that the three-dimensional Bragg glass phase is marginally hyperuniform, while the Bose glass and the liquid phase with correlated disorder are expected to be nonhyperuniform at equilibrium. Furthermore, we compare these predictions with experimental results on the large-wavelength vortex density fluctuations of magnetically decorated vortex structures nucleated in pristine, electron-irradiated, and heavy-ion-irradiated superconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ samples in the mixed state. For most cases, we find hyperuniform two-dimensional point patterns at the superconductor surface with an effective exponent ${\alpha }_{\text{eff}}\approx 1$. We interpret these results in terms of a large-scale memory of the high-temperature line-liquid phase retained in the glassy dynamics when field cooling the vortex structures into the solid phase. We also discuss the crossovers expected from the dispersivity of the elastic constants at intermediate length-scales, and the lack of hyperuniformity in the $x\text{−}y$ plane for lengths $q^{-1}$ larger than the sample thickness due to finite-size effects in the $z$ direction. We argue these predictions may be observable and propose further imaging experiments to test them independently.

A common difficulty in applications of machine learning is the lack of any general principle for guiding the choices of key parameters of the underlying neural network. Focusing on a class of recurrent neural networks—reservoir computing systems, which have recently been exploited for model-free prediction of nonlinear dynamical systems—we uncover a surprising phenomenon: the emergence of an interval in the spectral radius of the neural network in which the prediction error is minimized. In a three-dimensional representation of the error versus the time and spectral radius, the interval corresponds to the bottom region of a “valley.” Such a valley arises for a variety of spatiotemporal dynamical systems described by nonlinear partial differential equations, regardless of the structure and the edge-weight distribution of the underlying reservoir network. We also find that, while the particular location and size of the valley depend on the details of the target system to be predicted, the interval tends to be larger for undirected than for directed networks. The valley phenomenon can be beneficial to the design of optimal reservoir computing, representing a small step forward in understanding these machine-learning systems.

We find that the Hanford and Livingston detectors of Advanced LIGO derive a distinct posterior probability distribution of binary tidal deformability $\tilde{\mathrm{\Lambda }}$ of the first binary-neutron-star merger GW170817. By analyzing public data of GW170817 with a nested-sampling engine and the default TaylorF2 waveform provided by the LALInference package, the probability distribution of the binary tidal deformability derived by the LIGO-Virgo detector network turns out to be determined dominantly by the Hanford detector. Specifically, by imposing the flat prior on tidal deformability of individual stars, symmetric 90% credible intervals of $\tilde{\mathrm{\Lambda }}$ are estimated to be ${527}_{-345}^{+619}$ with the Hanford detector, ${927}_{-619}^{+522}$ with the Livingston detector, and ${455}_{-281}^{+668}$ with the LIGO-Virgo detector network. Furthermore, the distribution derived by the Livingston detector changes irregularly when we vary the maximum frequency of the data used in the analysis. This feature is not observed for the Hanford detector. While they are all consistent, the discrepancy and irregular behavior suggest that an in-depth study of noise properties might improve our understanding of GW170817 and future events.

Recently, it was realized that anomalies can be completely classified by topological orders, symmetry protected topological orders, and symmetry enriched topological orders in one higher dimension. The anomalies that people used to study are invertible anomalies that correspond to invertible topological orders and/or symmetry protected topological orders in one higher dimension. In this paper, we introduce a notion of noninvertible anomaly, which describes the boundary of generic topological order. It is characterized by two features. First, a theory with noninvertible anomaly has a multicomponent partition function. Second, under the mapping class group transformation of space-time, the vector of partition functions transform covariantly. In fact, the anomalous partition functions transform in the same way as the degenerate ground states of the corresponding topological order in one higher dimension. This general theory of noninvertible anomaly may have wide applications. As an example, we show that the irreducible gapless boundary of 2+1D double-semion topological order must have central charge $c=\overline{c}>\frac{25}{28}$.

Vortices are localized planar structures that attain topological stability and can be used to describe collective behavior in a diversity of situations of current interest in nonlinear science. In high-energy physics, vortices engender an integer winding number and appear under the presence of a local Abelian symmetry. In this work, we study vortices in a Maxwell-Higgs model, in which the gauge symmetry is enhanced to accommodate additional symmetries, responsible to generate localized structures to be used to constrain the vortex structure in a given region in the plane. The main aim is to examine how the vortex profile changes when it inhabits a limited region, an issue of current interest to the study of vortices at the nanometric scale.

Antiferromagnets (AFMs) are widely believed to be superior to ferromagnets in spintronics because of their high stability due to the vanishingly small stray field. It is thus expected that the order parameter of AFMs should always align along the easy axis of the crystalline anisotropy. In contrast to this conventional wisdom, we find that the AFM order parameter switches away from the easy axis below a critical anisotropy strength when an AFM is properly tailored into a nanostructure. The switching time first decreases and then increases with the damping. Above the critical anisotropy, the AFM order parameter is stable and precesses under a microwave excitation. However, the absorption peak is not at resonance frequency even for magnetic damping as low as 0.01. To resolve these anomalies, we first ascertain the hidden role of dipolar interaction that reconstructs the energy landscape of the nanosystem and propose a model of a damped nonlinear pendulum to explain the switching behavior. In this framework, the second anomaly appears when an AFM is close to the boundary between underdamped and overdamped phases, where the observed absorption line shape has a small quality factor and thus is not reliable any longer. Our results should be significant to extract the magnetic parameters through resonance techniques.

We show that in a generic, ergodic quantum many-body system the interactions induce a nontrivial topology for an arbitrarily small non-Hermitian component of the Hamiltonian. This is due to an exponential-in-system-size proliferation of exceptional points which have the Hermitian limit as an accumulation (hyper)surface. The nearest-neighbor level repulsion characterizing Hermitian ergodic many-body systems is thus shown to be a projection of a richer phenomenology, where actually all the exponentially many eigenvalues are pairwise connected in a topologically robust fashion via exceptional points.

The photofragmentation behavior of the lead-doped bismuth cluster ${}^{209}\mathrm{Bi}_7{}^{208}\mathrm{Pb}^{+}$ is investigated by use of multi-reflection time-of-flight mass spectrometry (MR-ToF MS), which serves both as precursor selector on a scale of single atomic mass units as well as high-resolution fragment analyzer. This is an implementation of a tandem MR-ToF study, where two instances of high-resolution mass spectrometry are used, namely for pre-selection and post-excitation analysis, in quick succession within a single device to determine complex dissociation patterns. The full information of the fragmentation pathways is obtained, including branching ratios describing the retention of the charge on the pure or compound cluster. The results are compared to the behavior of the analogous pure bismuth cluster ${}^{209}\mathrm{Bi}_8^{+}$ and interpreted within the framework of well-established cluster stability rules.

The interplay between spin and orbital angular momentum in the up-conversion process allows us to control the macroscopic wave front of high harmonics by manipulating the microscopic polarizations of the driving field. We demonstrate the control of orbital angular momentum in high-harmonic generation from both solid- and gas-phase targets using the selection rules of spin angular momentum. The gas-phase harmonics extend the control of angular momentum to the extreme-ultraviolet wavelength. We also propose a bicolor scheme to produce spectrally separated extreme-ultraviolet radiation carrying orbital angular momentum.

It is known that, under the presence of a static external magnetic field $B_e$, an electromagnetic field rotates its polarization axes (Faraday rotation) when transmitted across a graphene single layer. Graphene provides record values of Faraday angle per layer thickness, but at frequencies of the order of the cyclotron frequency. This impedes applications, as fields of the order of 10 T would be required even for terahertz operation. Here we show that this condition is relaxed in strained graphene, where the potentially large induced pseudomagnetic field $B_{\text{ps}}$, when combined with a small $B_e$ (needed to break time-reversal symmetry), provides large Faraday rotation at arbitrary frequencies. It is found that the Faraday rotation in this system presents a very rich dependence on all different parameters, being greatly enhanced when the number of occupied Landau levels (governed by $B_e\pm B_{\text{ps}}$) is different in the two graphene valleys.