Projected wave functions offer a means of incorporating local correlation effects in gapless electronic phases of matter, like metals. Although such wave functions can be readily specified formally, computing their associated physical observables is challenging. Tensor network approaches offer a modern numerical method for this task. In this paper, we develop and demonstrate a constructive tensor-network approach for obtaining physical quantities, like fermion two-point functions and density-density correlation functions, for one-dimensional projected Fermi sea states. We benchmark our method against exact analytical results for spin-$1/2$ electrons subjected to the Gutzwiller projection, and then present results on spinless fermions with nearest-neighbor repulsion. For a state with two pairs of Fermi points, we reveal a correlation-dependent tuning of the characteristic wave vector of the charge density modulation in the system.

In this work, the term “quantum chaos” refers to spectral correlations similar to those found in the random matrix theory. Quantum chaos can be diagnosed through the analysis of level statistics using, e.g., the spectral form factor, which detects both short- and long-range level correlations. The spectral form factor corresponds to the Fourier transform of the two-point spectral correlation function and exhibits a typical slope-dip-ramp-plateau structure (aka correlation hole) when the system is chaotic. We discuss how this structure could be detected through the quench dynamics of two physical quantities accessible to experimental many-body quantum systems: the survival probability and the spin autocorrelation function. The survival probability is equivalent to the spectral form factor with an additional filter. When the system is small, the dip of the correlation hole reaches sufficiently large values at times which are short enough to be detected with current experimental platforms. As the system is pushed away from chaos, the correlation hole disappears, signaling integrability or localization. We also provide a relatively shallow circuit with which the correlation hole could be detected with commercially available quantum computers.

Bound states and scattering resonances in the unoccupied continuum of a two-dimensional crystal, as predicted in [Phys. Rev. B 87, 041405(R) (2013)], are considered within an exactly solvable model. A close connection of the observed resonances with those arising in the Fano theory is revealed. The resonance occurs when the lateral scattering couples the layer-perpendicular incident electron wave to a strictly bound state. The coupling strength determines the location of the pole in the scattering amplitude in the complex energy plane, which is analytically shown to lead to a characteristic Fano lineshape of the energy dependence of the electron transmissivity through the crystal. The implications for the temporal characteristics of the resonance scattering are discussed. The analytical results are illustrated by ab initio calculations for a graphene monolayer.

Small particles are transported over long distances in the atmosphere, with significant environmental impact. The transport of symmetric particles is well understood, but atmospheric particles, such as curved microplastic fibers or ash particles, are generally asymmetric. This makes the description of their transport properties uncertain. Here, we derive a model of how planar curved fibers settle in quiescent air. The model explains that fluid-inertia torques may align such fibers at oblique angles with gravity as seen in recent laboratory experiments, and shows that inertial alignment is a general and thus important factor for the transport of atmospheric particles.

Research on the emergence of thermodynamics in closed quantum systems under unitary time evolution arrived at the consensus that generic systems equilibrate under rather general assumptions. A new focus of the field is thus on exceptions. Persistent oscillations are one possible hallmark of nonergodic time evolution. While time-crystalline behavior results from, e.g., many-body localization, here we show that ever-revolving solitary waves emerge in flatband Heisenberg quantum spin systems. This phenomenon is rather general for a variety of frustrated spin systems in one, two, and three dimensions as well as for Hubbard systems.

Quantum neural networks hold promise for achieving lower generalization error bounds and enhanced computational efficiency in processing certain datasets. However, the integration of quantum superposition in data parallelism—a key aspect in the applications of classical neural network—has been largely unexplored in the context of quantum neural networks. Here, we demonstrate the effective application of quantum parallelism, via quantum superposition and entanglement, to achieve data parallelism in generic quantum neural network models. We address two classes of encoding schemes for the training data: as orthogonal and nonorthogonal quantum states, respectively. With orthogonal encoding, we rigorously prove that parallel and individual training are equivalent in terms of the loss function, a result directly derived from quantum entanglement; with nonorthogonal encoding, we show that parallel computing remains viable, and quantum interference among data becomes notable. We perform a comparative numerical analysis in these different scenarios, exploring various quantum neural networks with or without superposed data input, highlighting the validity of quantum data parallelism.

Hollow atoms (HAs) are an exotic type of matter formed when a highly charged ion impacts on a surface, capturing multiple electrons into highly excited states, leaving intermediate electronic states empty. Although experimental fingerprints of HAs were found in high-resolution x-ray spectra, it has been widely believed that HAs decay too rapidly to be studied directly. Using a simulation code for the full deexcitation cascade of the HAs, based on rates from atomic structure codes for highly excited states, we show that the lifetime of an HA, scattered under very grazing angles from a surface, can reach several tens of ps in free decay. In additional experiments, we use ${\mathrm{Ar}}^{14+}$ ions on a Ni(110) surface under incidence angles below $0.5^{\circ }$. Comparing ion charge state distributions after scattering between experiment and simulation, we discuss the role of surface-stimulated decay in most interactions. We show that by minimizing the incidence angle, the influence of surface-near processes can be reduced and that HAs can exist for several picoseconds in free space.

The cytoskeletal component actomyosin is a canonical example of active matter since the power-stroke cycle locally converts chemical energy in the form of adenoside triphosphate (ATP) into mechanical work for remodeling. Observing myosin II minifilaments as they remodel actin in vitro, we now report that at high concentrations of ATP, myosin minifilaments form metastable swirling patterns that are characterized by recurrent vortex and spiral-like motifs, whereas at low concentrations of ATP, such structures give way to asterlike patterns. To explain this, we construct the (quasi)steady states of a polar active hydrodynamic theory of actomyosin whose ATP-scaling is obtained from a microscopic, stochastic description for the ATP-dependent binding of the heads of single myosin II minifilaments. The latter codifies the heuristic that, since the power-stroke cycle involves the unbinding of myosin II heads from actin, increases in the concentration of ATP reduce the likelihood that a given myosin II minifilament has more than one head bound simultaneously, reducing its ability to generate contractile forces and increasing the relative likelihood of processive motion. This reproduces several qualitative and some quantitative aspects of experiments, providing evidence for the central phenomenon of the theory: an ATP-dependent active contractile instability. ATP therefore controls not only the rate at which work is done—i.e., the power—but also the mode by which this occurs.

We have investigated the surface and bulk electronic structures of the superconducting type-II Dirac semimetal 1T-PdSeTe. The superconducting transition temperature $T_{\text{c}}=3.2$ K was almost twice as high as $T_{\text{c}}=1.6$ K in 1T-${\mathrm{PdTe}}_2$. Scanning transmission electron microscopy measurements showed homogeneously mixed Se and Te atoms in the chalcogen layers, consistent with the ${\mathrm{CdI}}_2$-type crystal structure. Angle-resolved photoemission spectroscopy measurements and density functional theory calculations indicated the existence of the topological surface states, and the overall band structures were similar to those of 1T-${\mathrm{PdTe}}_2$. These results suggest that the ${\mathrm{CdI}}_2$-type lattice symmetry dictates the band dispersion, regardless of atomic disorder in the chalcogen layers. As the electronic band dispersion and the local structures were persistent upon substitution, the enhancement of $T_{\text{c}}$ is likely associated with the chemical pressure. Our results provide insight into the effects of the solid solution on the surface and bulk electronic states as well as the superconducting transition temperature.

Concepts from non-Hermitian quantum mechanics have proven useful in understanding and manipulating a variety of classical systems, such as those encountered in optics, classical mechanics, and metamaterial design. Recently, the non-Hermitian analog of the Berry phase for adiabatic processes was experimentally measured. In non-Hermitian systems, the Berry phase can have an imaginary part, which contributes to the amplification or decay of the total wave intensity. When the imaginary part of the Berry curvature is zero, this geometric amplification factor is determined solely by the initial and final points of the adiabatic path in parameter space, and it does not depend on how these points are connected by the path. We list classes of non-Hermitian Hamiltonians where this path independence is guaranteed by suitable symmetries, and we find that, for some of these classes, the amplification factor can be written only in terms of the Petermann factors of the initial and final points. Our result can, in turn, be used to experimentally obtain the Petermann factor by observing how the norm of the wave function changes under adiabatic processes. We validate our theory using a couple of concrete examples of physical relevance.

We investigate the relationship between the K-edge fine structure of isolated single-wall carbon nanotubes (SWCNTs) and the Van Hove singularities (VHSs) in the conduction-band density of states. To this end, we model x-ray absorption spectra of SWCNTs using the final-state approximation and the Bethe-Salpeter equation (BSE) method. Both methods can reproduce the experimental fine structure, where the BSE results improve on peak positions and amplitude rations compared to the final-state approximation. When the fine structure in the modeled spectra is related to the VHSs, significant differences are found. We suggest that these differences arise due to modifications of the core exciton wave functions induced by the confinement along the circumference. Additionally, we analyze the character of core excitons in SWCNTs, and we find that the first bright excitons are Frenkel excitons, while higher-lying excitons are charge resonance states. Finally, we suggest that the qualitative picture based on VHSs in the density of states holds when there is a large energy gap between successive VHSs.

We report on an experiment achieving the dynamical generation of non-Gaussian states of motion of a levitated optomechanical system. We access intrinsic Duffing-like nonlinearities by thermal squeezing of an oscillator's state of motion by rapidly switching the frequency of its trap. We characterize the experimental non-Gaussian state versus expectations from simulations and give prospects for the emergence of genuine nonclassical features.

The appearance of quantized vortices in a superfluid and a Bose-Einstein condensate (BEC) stems from their nontrivial response to broken time-reversal symmetry (TRS). Here, we show that breaking of the TRS by, for example, rotation or an external synthetic magnetic field in a transversely driven BEC coupled to a single mode of an optical cavity modifies drastically Dicke superradiance and self-ordering phenomena in this system. In particular, photon scattering from the pump laser into the cavity is amplified by rotational motion of the BEC, leading to so-called “rotational superradiance”—in a loose analogy to black-hole physics—with distinct critical scaling properties. In turn, cavity photons mediate long-range, periodic attractive interactions among the vortices, which compete with pairwise logarithmic repulsive vortex interactions and deform the Abrikosov triangular vortex lattice favoring a stripe-like pattern. Remarkably, the rotation of the BEC and the topological properties of the vortex lattice can be monitored nondestructively through the cavity output field.

In the low-energy regime, the Su-Schrieffer-Heeger (SSH) model's key characteristics are encapsulated by a Dirac-type Hamiltonian in continuum space, i.e., localized states emerge at the boundaries. Building on this, we have developed an effective Hamiltonian to model ultracold interacting bosons on an SSH-like lattice through variational minimization under the mean-field approximation. To pinpoint the boundary states, we have developed an algorithm by generalizing the imaginary time propagator, where an initial state evolves under the squared Hamiltonian to converge to the targeted state. This algorithm has broader applicability, enabling the identification of specific eigenstates in various contexts. Furthermore, we draw a parallel to an experimentally physical setup involving a gas of ultracold bosons confined to an array of potential wells with alternating depths. By establishing the system's analogy with the SSH system, we apply our algorithm to investigate boundary states in the presence of interactions, demonstrating how these findings align with those of the continuous system.

An intense terahertz pulse has the potential to control the polarization of ferroelectrics at the subpicosecond time scale. In some ferroelectrics, it has been reported that irradiation with a terahertz pulse on the order of 100 kV/cm modulates the polarization in a manner proportional to the electric field. If a stronger electric field is applied, it may be possible to alter the polarization nonlinearly or even cause it to disappear. However, such attempts have not been reported to date. In this paper, we aim to nonlinearly control ferroelectric polarization by irradiating an organic molecular compound tetrathiafulvalene-$p$-chloranil (TTF-CA), a typical electronic-type ferroelectric, with a terahertz pulse having an electric-field amplitude of up to 2.2 MV/cm. When the temperature is reduced, TTF-CA undergoes a neutral (N) to ionic (I) phase transition at $T_{\mathrm{c}}=81\mathrm{K}$, and the degree of charge transfer $\rho$ from CA to TTF changes from 0.3 to 0.6. In the I phase, it shows the ferroelectricity owing to the collective intermolecular charge transfer at $T_{\mathrm{c}}$. Therefore, if the I-to-N conversion is induced by a strong electric field, the ferroelectric polarization $P$ may change significantly. To investigate the changes in $\rho$ and $P$ along the electric-field waveform of the terahertz pulse in the ferroelectric I phase of TTF-CA, in this paper, a terahertz pump-optical reflectivity probe and second harmonic generation (SHG) probe measurements are performed. For the terahertz electric-field amplitudes $E_{\mathrm{THz}}<0.5\mathrm{MV}/\mathrm{cm}$, the changes in $\rho$ and $P$ are proportional to $E_{\mathrm{THz}}$. When $E_{\mathrm{THz}}>0.5\mathrm{MV}/\mathrm{cm}$ is applied parallel to $P$, the increases in $\rho$ and $P$ tend to become saturated. The increase in P is only ∼10% at a maximum electric-field amplitude of 2.2 MV/cm. In contrast, when $E_{\mathrm{THz}}$ is antiparallel to $P$, the decreases in $\rho$ and $P$ are proportional to $E_{\mathrm{THz}}$ up to 1.5 MV/cm. When $E_{\mathrm{THz}}$ is further increased to values >1.5 MV/cm, a partial I-to-N conversion occurs via quantum electron tunneling processes, and $P$ decreases in a nonlinear manner. The decrease in P reaches ∼30% at 2.2 MV/cm. These nonreciprocal changes in $\rho$ and $P$ are discussed while considering the valence instability characteristic of TTF-CA. Furthermore, several coherent oscillations are observed in the changes of reflectivity and SHG, which reflect the modulations of $\rho$ and $P$ due to molecular vibrations (lattice modes), respectively. A detailed analysis of the amplitudes and phases of these coherent oscillations reveals that a molecular vibration with a frequency of ∼90 $\mathrm{c}{\mathrm{m}}^{-1}$ is driven by changes in $\rho$ induced by a terahertz electric field and modulates $P$ significantly through the changes in the dimeric molecular displacements along the molecular stacking axis, that is, the polarization direction.

Itinerant ferromagnetism is one of the most-studied quantum phase transitions, the transition point and the nature of this phase transition being widely discussed. In dilute Fermi liquids, this analysis has been carried out up to second order in the gas parameter, where the results for any spin degeneracy are universal in terms of only the $s$-wave scattering length $a_0$. We extend this analysis to third order, where energies depend not only on $a_0$, but also on the $s$-wave effective range $r_0$ and the $p$-wave scattering length $a_1$. The introduction in the theory of these new parameters changes the transition point, with respect to the second-order estimation, and also can modify the nature of the phase transition itself. We analyze these interaction effects on the phase transition for different spin values. The emerging phase diagram shows that the type of ferromagnetic transition changes dramatically as a function of $r_0$ and $a_1$ and, importantly, that this classification is not solely determined by the spin value, as happens at second order.

We study a two-terminal Josephson junction with conventional superconductors and a normal region with a Rashba spin-orbit interaction, characterized by two Aharonov-Casher (AC) fluxes. When the superconducting phase difference equals $\pi$, the Andreev subgap spectrum may host zero-energy Weyl singularities associated with a vanishing normal-state reflection eigenvalue. With one of the AC fluxes playing the role of a quasimomentum, the junction can be viewed as an artificial one-dimensional chiral topological insulator. Its topological phase can be tuned by crossing a Weyl singularity by means of varying the remaining AC flux. By associating an additional component of the quasimomentum with the superconducting phase difference, an artificial Chern insulator is realized.

We employ real-space variational quantum Monte Carlo methods with resonating valence bond many-body wave functions to investigate electron correlation effects in the kagome system ${\text{Zn}}_x{\text{Cu}}_{4-x}{\text{O}}_6$. Using three trial wave functions of the Slater-Jastrow type, where (i) only the Jastrow correlation factor is optimized and the orbitals obtained by density functional theory, (ii) both the Jastrow factor and the Slater determinant are optimized, and (iii) additionally the Slater determinant is substituted by an antisymmetrized-geminal power-wave function, we analyze static and dynamic correlation energies across concentrations $x=0,\frac{1}{3},\frac{2}{3},1$. Optimizing the Slater determinant significantly enhances the correlation energy by approximately $\sim -73\left(4\right)\text{mHa}$ per electron. Eventually, the emergence of a quantum spin liquid state driven by a long-range correlation energy is discussed.