Coupling a system to two different baths can lead to novel phenomena escaping the constraints of thermal equilibrium. In quantum materials inside optical cavities, this feature can be exploited as electrons and cavity photons are easily pulled away from their mutual equilibrium, even in the steady state. This offers new routes for a noninvasive control of material properties and functionalities. We show that the absence of thermal equilibrium between electrons and photons leads to reduced symmetries of the steady-state electronic distribution function. Moreover, by defining an effective temperature from the on-shell distribution function, we find a nonmonotonic behavior as a function of cavity frequency, consistent with recent experimental findings. Finally, we show that, the nonthermal behavior leads to qualitative modifications of the materials properties, as the standard Sommerfeld expansion for observables is modified by a leading-order correction linearly proportional to the temperature difference between the two baths and to the frequency derivative of the electron damping.

We develop a semiempirical model to describe the creation of an electron plasma in matter at extreme conditions, as excited by high-intensity, extreme ultraviolet (EUV) or x-ray radiation pulses currently available at free-electron laser (FEL) facilities. In typical FEL experiments, EUV and x-ray pulses are used to investigate highly excited states of matter produced by optical pump pulses. The pump pulses, promoting electrons from delocalized valence band states, excite the system often through multiphoton absorption processes and/or shock wave propagation. Instead, the use of FEL radiation as a pump allows triggering electron transitions from bound, core states to nearly free levels above the Fermi energy, promptly generating a highly excited state, toward the warm dense matter regime. We model such an excitation process starting from the optical properties of the analyzed materials and first-principle considerations, with the aim of motivating further studies in this scheme. We apply our model to predict the differences in the plasma creation process in insulators and metals, at varying exciting pulse wavelength and fluence, and we interpret the results in terms of the different nature of the investigated systems. Furthermore, we explore the effect of the finite core hole lifetime on the plasma formation and relaxation dynamics. We show the model is suited for the interpretation of real experimental data obtained from a prototypical insulating crystal (LiF) in a EUV FEL pump–optical transmission probe experiment.

We study the statistical physics of the classical Ising model in the so-called $\alpha$-Rényi ensemble, a finite-temperature thermal state approximation that minimizes a modified free energy based on the $\alpha$-Rényi entropy. We begin by characterizing its critical behavior in mean-field theory in different regimes of the Rényi index $\alpha$. Next, we re-introduce correlations and consider the model in one and two dimensions, presenting analytical arguments for the former and devising a Monte Carlo approach to the study of the latter. Remarkably, we find that while mean-field predicts a continuous phase transition below a threshold index value of $\alpha \sim 1.303$ and a first-order transition above it, the Monte Carlo results in two dimensions point to a continuous transition at all $\alpha$. We conclude by performing a variational minimization of the $\alpha$-Rényi free energy using a recurrent neural network (RNN) Ansatz where we find that the RNN performs well in two dimensions when compared to the Monte Carlo simulations. Our work highlights the potential opportunities and limitations associated with the use of the $\alpha$-Rényi ensemble formalism in probing the thermodynamic equilibrium properties of classical and quantum systems.

Time-periodic (Floquet) systems are one of the most interesting nonequilibrium systems. As the computation of energy eigenvalues and eigenstates of time-independent Hamiltonians is a central problem in both classical and quantum computation, quasienergy and Floquet eigenstates are the important targets. However, their computation has difficulty of time dependence; the problem can be mapped to a time-independent eigenvalue problem by the Sambe space formalism, but it instead requires additional infinite-dimensional space and seems to yield higher computational cost than the time-independent cases. It is still unclear whether they can be computed with guaranteed accuracy as efficiently as the time-independent cases. We address this issue by rigorously deriving the cutoff of the Sambe space to achieve the desired accuracy and organizing quantum algorithms for computing quasienergy and Floquet eigenstates based on the cutoff. The quantum algorithms return quasienergy and Floquet eigenstates with guaranteed accuracy like quantum phase estimation, which is the optimal algorithm for outputting energy eigenvalues and eigenstates of time-independent Hamiltonians. While the time periodicity provides the additional dimension for the Sambe space and defines two different kinds of eigenstates, the query complexity of the algorithms achieves the near-optimal scaling in allowable errors. In addition, as a by-product of these algorithms, we also organize a quantum algorithm for Floquet eigenstate preparation, in which a preferred gapped Floquet eigenstate can be deterministically implemented with nearly optimal query complexity in the gap. These results show that, despite the difficulty of time dependence, quasienergy and Floquet eigenstates can be computed almost as efficiently as time-independent cases, shedding light on the accurate and fast simulation of nonequilibrium systems on quantum computers.

We study switching current distributions in superconducting nanostrips using theoretical models and numerical simulations. Switching current distributions are commonly measured in experiments and may provide a window into the microscopic switching mechanisms. As the current through a superconducting strip is increased from zero it will at some point switch to the normal dissipative state. Due to thermal and/or quantum fluctuations the switching current will be random and follow a certain distribution depending on sweep rate, temperature, material properties, wire length, and width. By analyzing the resulting distribution it is possible to infer the transition rate for a switch, which can be related to the free-energy barrier separating the metastable superconducting state and the normal one. We study different switching scenarios and show using simulations how data taken for different sweep rates can be combined to obtain the switching rate over a wider interval of currents. In doing this, it is necessary to account for a time delay between the initiation of the switching event and its detection.

We present a systematic framework to construct model Hamiltonians that have unconventional superconducting pairing states as exact energy eigenstates, by incorporating multibody interactions (i.e., interactions among more than two particles). The multibody interactions are introduced in a form of the local density-density coupling in such a way that any pair configuration in real space has a constant interaction energy by canceling the two-body and multibody interactions. Our approach is applicable to both spinless and spinful models in any spatial dimensions and on any bipartite lattices, facilitating an exhaustive extension of Yang's $s\text{−}\mathrm{wave}\eta \text{−}\mathrm{pairing}$ state to various other unconventional pairing symmetries ($p\text{−}\mathrm{wave},$ $d\text{−}\mathrm{wave},$ $f\text{−}\mathrm{wave},$ etc.). Particularly, the constructed eigenstates have off-site pairs with finite center-of-mass momentum, which leads to superconducting states with either even-parity and spin-triplet or odd-parity and spin-singlet symmetry. We verify that the two-dimensional spinful Hubbard model on a square lattice with the multibody interactions has the spin-triplet $d\text{−}\mathrm{wave}$ pairing state as an energy eigenstate, which can be regarded as a quantum many-body scar state as evidenced from the numerical analysis of the pair correlation function, entanglement entropy, and level statistics. We also discuss other examples, including spin-triplet $f\text{−}\mathrm{wave}$ pairing states on a honeycomb lattice and spin-singlet $p\text{−}\mathrm{wave}$ pairing states in a one-dimensional chain. These findings open up the possibility of realizing nonequilibrium unconventional superconductivity in a long-lived manner protected against thermalization.

We present a method for generating homogeneous and tunable magnetic flux for bosonic particles in a lattice using Rydberg atoms. Our setup relies on Rydberg excitations hopping through the lattice by dipolar exchange interactions. The magnetic flux arises from complex hopping via ancilla atoms. Remarkably, the total flux within a magnetic unit cell directly depends on the ratio of the number of lattice sites to ancilla atoms, making it topologically protected to small changes in the positions of the atoms. This allows us to optimize the positions of the ancilla atoms to make the flux through the magnetic unit cell homogeneous. With this homogeneous flux, we get a topological band in the single-particle regime. In the many-body regime, we obtain indications of a bosonic fractional Chern insulator state at $\nu =1/2$ filling.

Understanding the time-dependent effects of charged vacancies on the electromechanical responses of materials is at the forefront of research for designing materials exhibiting metal-insulator transitions and memristive behavior. A Rayleighian approach is used to develop a model for studying the nonlinear kinetics of the reaction leading to generation of vacancies and electrons via the dissociation of vacancy-electron pairs. Also, diffusion and elastic effects of charged vacancies are considered to model polarization-electric potential and strain-electric potential hysteresis loops. The model captures multiphysics phenomena by introducing couplings among polarization, the electric potential, stress, strain, and concentrations of charged (multivalent) vacancies and electrons (treated as classical negatively charged particles), where the concentrations can vary due to association-dissociation reactions. A derivation of coupled time-dependent equations based on the Rayleighian approach is presented. Three limiting cases of the governing equations are considered, highlighting the effects of (1) nonlinear reaction kinetics on the generation of charged vacancies and electrons, (2) Vegard's law (i.e., the concentration-dependent local strain) on asymmetric strain-electric potential relations, and (3) coupling between a fast component and the slow component of the net polarization on the polarization-electric-field relations. The Rayleighian approach discussed in this work should pave the way for developing a multiscale modeling framework in a thermodynamically consistent manner while capturing multiphysics phenomena in ferroelectric materials.

Quasicrystal is state of matter with multiple crystalline orders, displaying a short-range density-wave order and a long-range disorder. As an exotic state lying in between crystals and Anderson insulators, its dynamical formation is little discussed. Here, we study the emergence of quasicrystalline order in fermionic superradiance, where Fermi statistics plays a crucial role, being distinct from its bosonic counterpart where interaction is the stabilizer. Owing to quasicrystalline orders, there exist multiple energy gaps opening at different quasicrystalline order strength, which can modify the density of states in excitation spectrum. As a result, the effective ground-state energy is strongly modified and the superradiant quasicrystal transition becomes first order and density dependent. This new mechanism leads to a linear V-shape kink of the fermionic superradiance transition line near a filling which uniquely relates to the quasicrystalline order. Our findings demonstrate that quasicrystal can be realized in fermionic superradiance as a result of Fermi statistics and be verified by characteristic density-dependent phenomena.

A comparative vacuum ultraviolet spectroscopy study conducted at ISOLDE-CERN of the radiative decay of the ${}^{229m}\mathrm{Th}$ nuclear clock isomer embedded in different host materials is reported. The ratio of the number of radiative decay photons and the number of ${}^{229m}\mathrm{Th}$ embedded are determined for single crystalline ${\mathrm{CaF}}_2, {\mathrm{MgF}}_2, {\mathrm{LiSrAlF}}_6$, AlN, and amorphous ${\mathrm{SiO}}_2$. For the latter two materials, no radiative decay signal was observed and an upper limit of the ratio is reported. The radiative decay wavelength was determined in ${\mathrm{LiSrAlF}}_6$ and ${\mathrm{CaF}}_2$, reducing its uncertainty by a factor of 2.5 relative to our previous measurement. This value is in agreement with the recently reported improved values from laser excitation.

As a typical representative of covalent superconductors, $\mathrm{S}{\mathrm{H}}_3$ has emerged as a significant milestone in superconductivity history and has greatly sparked interest in compressed hydrogen-rich superconductors. Authors of previous studies on theoretical design of ternary chalcogen-hydrogen compounds (known as chalcohydrides) have mainly focused on intercalating molecular motifs into the interstitial sites of the structural lattice or substituting atoms in the $\mathrm{S}{\mathrm{H}}_3$ backbone. Given the low electronegativity and small radius, lithium (Li) is empowered to serve as an excellent electron donor that can effectively turn the crystal structure and modulate the superconducting behavior. Here, we introduce Li into binary chalcohydrides and investigate ternary Li-$M$-H ($M$ = S, Se, and Te) compounds using the state-of-the-art structure prediction method in conjunction with first-principles calculations. As a result, five stable stoichiometries, including $\mathrm{LiS}{\mathrm{H}}_7, \mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{H}}_6$, LiSH, $\mathrm{L}{\mathrm{i}}_2\mathrm{SH}$, and $\mathrm{Li}{\mathrm{S}}_2\mathrm{H}$, are identified at 100–200 GPa. Notably, metallic $\mathrm{LiS}{\mathrm{H}}_7$ and $\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{H}}_6$ exhibit a layered structure where the covalent bond between sulfur and hydrogen breaks up and the hydrogen atoms are released to recombine into molecules confined into the interlayer. Furthermore, our electron-phonon simulations reveal that the estimated superconducting transition temperature ($T_c$) of $\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{H}}_6$ (46 K) is lower than that (77 K) of $\mathrm{L}{\mathrm{i}}_2\mathrm{Se}{\mathrm{H}}_6$ at 200 GPa, which contrasts with the general belief that the light-weight elemental compound (high Debye temperature) has higher superconductivity. Our results offer critical insights into designing high-temperature superconductors with layered structures among multinary hydrides.

We simulate quench dynamics of the Su-Schrieffer-Heeger (SSH) chain on IBM quantum computers, calculating the Rényi entanglement entropy, the twist order parameter, and the Berry phase. The latter two quantities can be deduced from a slow-twist operator defined in the Lieb-Schultz-Mattis theorem. The Rényi entropy is obtained using a recently developed randomized measurement scheme. The twist order parameter and the Berry phase are measured without the need for additional gates or ancilla qubits. We consider quench protocols in which a trivial initial state evolves dynamically in time under the topological SSH Hamiltonian in the fully dimerized limit (the flat-band limit). During these quenches, there are persistent and periodic oscillations in the time evolution of both entanglement entropy and twist order parameter. Through the implementation of error mitigation techniques using a global depolarizing ansatz and postselection, our simulations on the IBM devices yield results that closely match exact solutions.

We address the experimentally relevant situation, where a nonequilibrium state is created by injecting charge current into a chiral quantum Hall edge state. We show that the commonly accepted picture of the full equilibration of a nonequilibrium state at finite distances contradicts the charge-conservation requirement. We propose and solve the transmission line model that accounts for the local equilibration process and the charge- and energy-conserving dynamics of the collective mode. We find that the correction of the electron distribution function to its eventual equilibrium form scales down slowly as $1/\sqrt{L}$ at long distances $L$.

Nonreciprocal charge transport in solids, where resistance is different between rightward and leftward currents, is a key function of rectifying devices in modern electronics, as exemplified by $p\text{−}n$ semiconductor junctions. Recently, this was also demonstrated in noncentrosymmetric materials in magnetic fields since their band structure exhibits spin polarization coupled to the position of momentum space due to antisymmetric spin-orbit coupling. To enhance the magnitude of the nonreciprocal effect, it is essential to tune such spin-momentum coupling, which has been hampered in conventional materials owing to the difficulty in controlling the broken inversion symmetry built into the lattice and interfacial structures. Here, we report large nonreciprocal resistivity in the layered polar metal $\mathrm{BaMn}{X}_2$ ($X=\mathrm{Sb}$, Bi), where the spin-polarized Dirac dispersion depends on the in-plane polarization tunable by chemical substitution of the $X$ site. For $X=\mathrm{Sb}$ with a pair of single-type valleys, the nonreciprocal resistivity increases monotonically with decreasing temperature, while for $X=\mathrm{Bi}$ with multiple types of valleys it is reduced by about an order of magnitude and exhibits a peak at a low temperature. Theoretical calculations indicate that the nonreciprocal resistivity is sensitive not only to the spin-momentum (spin-valley) coupling but also to the Fermi energy and the Dirac dispersion. The observed significant variation of nonreciprocal transport in the same series of materials might be of great use in the design of junction-free rectifying devices and circuits.

Using time- and angle-resolved photoemission spectroscopy, we examine the unoccupied electronic structure and electron dynamics of the type-I Weyl semimetal ${\mathrm{PtBi}}_2$. Using the ability to change the probe photon energy over a wide range, we identify the predicted Weyl points in the unoccupied three-dimensional band structure and we discuss the effect of $k_{\perp }$ broadening in the normally unoccupied states. We characterize the electron dynamics close to the Weyl points and in other parts of three-dimensional Brillouin zone using $k$-means, an unsupervised machine-learning technique. This reveals distinct differences—in particular, that the electron dynamics close to the Weyl points are slower than in Brillouin zone regions close to the bulk Fermi surface.

Synthetic gauge fields have provided physicists with a unique tool to explore a wide range of fundamentally important phenomena. However, most experiments have focused on synthetic vector gauge fields. The very rich physics brought about by coupling tensor gauge fields to fracton phases of matter remains unexplored in laboratories. Here, we propose schemes to realize synthetic tensor gauge fields that address dipoles instead of single particles. A lattice tilted by a strong linear potential and a weak quadratic potential yields a rank-2 electric field for a dipole formed by a particle-hole pair. Such a rank-2 electric field leads to a new type of Bloch oscillations, which modulates the quadrupole moment and preserves the dipole moment of the system. In higher dimensions, the interplay between interactions and vector gauge potentials imprints a phase to the ring-exchange interaction and thus generates synthetic tensor gauge fields. Such tensor gauge fields make it possible to realize a dipolar Harper-Hofstadter model in laboratories. The resultant dipolar Chern insulators feature chiral edge currents of dipoles in the absence of net charge currents.

As part of the Vienna package for Erlangen LEED, low-energy electron diffraction (ViPErLEED) project, computer programs have been developed for facile and user-friendly data extraction from movies of LEED images. The programs make use of some concepts from astronomical image processing and analysis. As a first step, flat-field and dark-frame corrections reduce the effects of inhomogeneities of the camera and screen. In a second step, for identifying all diffraction maxima (“spots”), it is sufficient to manually mark and label a single spot or very few spots. Then the program can automatically identify all other spots and determine the distortions of the image. This forms the basis for automatic spot tracking (i.e., following the spots as they move across the LEED screen) and intensity measurement. Even for complex structures with hundreds to a few thousand diffraction beams, this step takes less than a minute. The package also includes a program for further processing of these $I\left(V\right)$ curves (averaging of equivalent beams, manual and/or automatic selection, smoothing) as well as several utilities. The software is implemented as a set of plugins for the public-domain image processing program ImageJ and provided as an open-source package.