Few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with optical 500–1000-nm supercontinuum and broadband XUV pulses (30–50 eV), simultaneously probes dynamics of photoexcited carriers in $\mathrm{W}{\mathrm{S}}_2$ at the W ${\mathrm{O}}_3$ edge (37–45 eV) and carrier-induced modifications of core-exciton absorption at the $\mathrm{W}{\mathrm{N}}_{6,7}$ edge (32–37 eV). Access to continuous core-to-conduction-band absorption features and discrete core-exciton transitions in the same XUV spectral region in a semiconductor provides a means to investigate the effect of carrier excitation on core-exciton dynamics. The core-level transient absorption spectra, measured with either pulse arriving first to explore both core-level and valence carrier dynamics, reveal that core-exciton transitions are strongly influenced by the photoexcited carriers. A $1.2\pm 0.3$ ps hole-phonon relaxation time and a $3.1\pm 0.4$ ps carrier recombination time are extracted from the XUV transient absorption spectra from the core-to-conduction-band transitions at the W ${\mathrm{O}}_3$ edge. Global fitting of the transient absorption signal at the W ${\mathrm{N}}_{6,7}$ edge yields $\sim 10$ fs coherence lifetimes of core-exciton states and reveals that the photoexcited carriers, which alter the electronic screening and band filling, are the dominant contributor to the spectral modifications of core excitons and direct field-induced changes play a minor role. This work provides a first look at the modulations of core-exciton states by photoexcited carriers and advances our understanding of carrier dynamics in metal dichalcogenides.

We develop a semiclassical theory of sound absorption in Weyl semimetals in magnetic fields. We focus on the contribution to the absorption that stems from the existence of Berry monopoles in the band structure of such materials or, equivalently, due to the chiral anomaly and chiral magnetic effect. Sound absorption is shown to come primarily from the motion of Weyl nodes in energy space, associated with the propagation of a sound wave. We argue that acoustic magnetochiral dichroism, which occurs when absorption of sound is different for opposite propagation directions and for opposite directions of the magnetic field, can be a definitive probe of band topology. The part of the monopole-related sound magnetoabsorption that is even in the magnetic field is negative in time-reversal Weyl materials. The difference in the sign of the effect as compared to the positive anomaly-related transport magnetoconductance stems from the existence of valley electrochemical imbalances without magnetic field in the sound propagation problem. In centrosymmetric Weyl semimetals with few Weyl nodes, the magnetoabsorption is negative at low frequencies but can change signs with increasing frequency.

We evaluate the sound attenuation in a Weyl semimetal subject to a magnetic field or a pseudomagnetic field associated with a strain. Due to the interplay of intra- and internode scattering processes as well as screening, the fields generically reduce the sound absorption. A nontrivial dependence on the relative direction of the magnetic field and the sound wave vector, i.e., the magnetic sound dichroism, can occur in materials with nonsymmetric Weyl nodes (e.g., different Fermi velocities and/or relaxation times). It is found that the sound dichroism in Weyl materials can also be activated by an external strain-induced pseudomagnetic field. In view of the dependence on the field direction, the dichroism may lead to a weak enhancement of the sound attenuation compared with its value at vanishing fields.

Raman heterodyne spectroscopy is a powerful tool for characterizing the energy and dynamics of spins. The technique uses an optical pump to transfer coherence from a spin transition to an optical transition where the coherent emission is more easily detected. Here Raman heterodyne spectroscopy is used to probe an isotopically purified ensemble of erbium dopants in a yttrium orthosilicate $\left({\mathrm{Y}}_2{\mathrm{SiO}}_5\right)$ crystal coupled to a microwave cavity. Because the erbium electron spin transition is strongly coupled to the microwave cavity, we observed Raman heterodyne signals at the resonant frequencies of the hybrid spin-cavity modes (polaritons) rather than the bare erbium spin-transition frequency. Using the coupled system, we made saturation recovery measurements of the ground-state spin relaxation time $T_1=10\pm 3$ s and also observed Raman heterodyne signals using an excited state spin transition. We discuss the implications of these results for efforts toward converting microwave quantum states to optical quantum states.

Random measurements have been shown to induce a phase transition in an extended quantum system evolving under chaotic unitary dynamics, when the strength of measurements exceeds a threshold value. Below this threshold, a steady state with a subthermal volume law entanglement emerges, which is resistant to the disentangling action of measurements, suggesting a connection to quantum error-correcting codes. We quantify these notions by identifying a power-law decay of the mutual information $I(\left\{x\right\}:\overline{A})\propto x^{-3/2}$ in the volume-law-entangled phase, between a qudit located a distance $x$ from the boundary of a region $A$, and the complement $\overline{A}$, which implies that a measurement of this qudit will retrieve very little information about $\overline{A}$. We also find a universal logarithmic contribution to the volume law entanglement entropy $S^{\left(2\right)}\left(A\right)=\kappa L_A+\frac{3}{2}log{L}_A$ which is intimately related to the first observation. We obtain these results by relating the entanglement dynamics to the imaginary time evolution of an Ising model, to which we apply field-theoretic and matrix-product-state techniques. Finally, exploiting the error-correction viewpoint, we assume that the volume-law state is an encoding of a Page state in a quantum error-correcting code to obtain a bound on the critical measurement strength $p_c$ as a function of the qudit dimension $d$: $p_{c}log\left[(d^2-1)\left(p_c^{-1}-1\right)\right]\le log\left[(1-p_c)d\right]$. The bound is saturated at $p_c(d\to \infty )=1/2$ and provides a reasonable estimate for the qubit transition: $p_c(d=2)\le 0.1893$.

We explore the nonunitary dynamics of $(2+1)$-dimensional free fermions and show that the obtained steady state is critical regardless the strength of the nonunitary evolution. Numerical results indicate that the entanglement entropy has a logarithmic violation of the area law and the mutual information between two distant regions decays as a power-law function. In particular, we provide an interpretation of these scaling behaviors in terms of a simple quasiparticle pair picture. In addition, we study the dynamics of the correlation function and demonstrate that this system has dynamical exponent $z=1$. We further demonstrate the dynamics of the correlation function can be well captured by a classical nonlinear master equation. Our method opens a door to a vast number of nonunitary random dynamics in free fermions and can be generalized to any dimensions.

We study stabilizer quantum error correcting codes (QECC) generated under hybrid dynamics of local Clifford unitaries and local Pauli measurements in one dimension. Building upon (1) a general formula relating the error susceptibility of a subregion to its entanglement properties, and (2) a previously established mapping between entanglement entropies and domain wall free energies of an underlying spin model, we propose a statistical mechanical description of the QECC in terms of “entanglement domain walls.” Free energies of such domain walls generically feature a leading volume law term coming from its “surface energy,” and a subvolume law correction coming from thermodynamic entropies of its transverse fluctuations. These are most easily accounted for by capillary-wave theory of liquid-gas interfaces, which we use as an illustrative tool. We show that the information-theoretic decoupling criterion corresponds to a geometric decoupling of domain walls, which further leads to the identification of the “contiguous code distance” of the QECC as the crossover length scale at which the energy and entropy of the domain wall are comparable. The contiguous code distance thus diverges with the system size as the subleading entropic term of the free energy, protecting a finite code rate against local undetectable errors. We support these correspondences with numerical evidence, where we find capillary-wave theory describes many qualitative features of the QECC; we also discuss when and why it fails to do so.

Most material properties of great physical interest are directly related to nuclear dynamics, e.g., the ionic thermal conductivity, Raman/IR vibrational spectra, inelastic x-ray, and neutron scattering. A theory able to compute from first principles these properties, accounting for the anharmonicity and quantum fluctuations in the nuclear energy landscape that can be implemented in systems with hundreds of atoms is missing. Here, we derive an approximate theory for the quantum time evolution of lattice vibrations at finite temperature. This theory introduces the time dynamics in the self-consistent harmonic approximation (SCHA) and shares with the static case the same computational cost. It is nonempirical, as pure states evolve according to the Dirac least action principle and the dynamics of the thermal ensemble conserves both energy and entropy. The static SCHA is recovered as a stationary solution of the dynamical equations. We apply perturbation theory around the static SCHA solution and derive an algorithm to compute efficiently quantum dynamical response functions. Thanks to this new algorithm, we have access to the response function of any general external time-dependent perturbation, enabling the simulation of phonon spectra without following any perturbative expansion of the nuclear potential or empirical methods. We benchmark the algorithm on the IR and Raman spectroscopy of high-pressure hydrogen phase III, with a simulation cell of 96 atoms. Our work also explores the nonlinear regime of the dynamical nuclear motion, providing a paradigm to simulate the interaction with intense or multiple probes, as in pump-probe spectroscopy, or chemical reactions involving light atoms, as the proton transfer in biomolecules.

We study the late time relaxation dynamics of a pure $U\left(1\right)$ lattice gauge theory in the form of a dimer model on a bilayer geometry. To this end, we first develop a proper notion of hydrodynamic transport in such a system by constructing a global conservation law that can be attributed to the presence of topological solitons. The correlation functions of local objects charged under this conservation law can then be used to study the universal properties of the dynamics at late times, applicable to both quantum and classical systems. Performing the time evolution via classically simulable automata circuits unveils a rich phenomenology of the system's nonequilibrium properties: For a large class of relevant initial states, local charges are effectively restricted to move along one-dimensional “tubes” within the quasi-two-dimensional system, displaying fracton-like mobility constraints. The timescale on which these tubes are stable diverges with increasing systems size, yielding a novel mechanism for nonergodic behavior in the thermodynamic limit. We further explore the role of geometry by studying the system in a quasi-one-dimensional limit, where the Hilbert space is strongly fragmented due to the emergence of an extensive number of conserved quantities. This provides an instance of a recently introduced concept of “statistically localized integrals of motion,” whose universal anomalous hydrodynamics we determine by a mapping to a problem of classical tracer diffusion. We conclude by discussing how our approach might generalize to study transport in other lattice gauge theories.

We develop a general kinetic theory framework to describe the hydrodynamics of strongly interacting, nonequilibrium quantum systems in which integrability is weakly broken, leaving a few residual conserved quantities. This framework is based on a generalized relaxation-time approximation; it gives a simple, but surprisingly accurate, prescription for computing nonequilibrium transport even in strongly interacting systems. We validate the predictions of this approximation against matrix product operator calculations on chaotic quantum spin chains, finding surprisingly good agreement. We show that despite its simplicity, our framework can capture phenomena distinctive to strongly interacting systems, such as widely separated charge and energy diffusion constants.

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel $M_{2,3}$ edge. It is observed that the core-level absorption line shape of photoexcited nickel can be described by a Gaussian broadening ($\sigma$) and a red shift (${\omega }_s$) of the ground-state absorption spectrum. Theory predicts and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ($\mathrm{\Delta }T$) is linearly proportional to the Gaussian broadening factor $\sigma$, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of $640\pm 80$ fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of ${\omega }_s\propto \mathrm{\Delta }{T}^{1.5}$. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4-fs photoexcitation pulse until the carriers reach a quasithermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.

Motivated by the recent experimental realization of twisted transition metal dichalcogenide bilayers, we study a simplified model driven by different forms of monochromatic light. As a concrete and representative example we use parameters that correspond to a twisted ${\mathrm{MoTe}}_2$ homobilayer. First, we consider irradiation with circularly polarized light in free space and demonstrate that the corresponding Floquet Hamiltonian takes the same form as the static Hamiltonian, only with a constant overall shift in quasienergy. This is in stark contrast to twisted bilayer graphene, where new terms are typically generated under an analogous drive. Longitudinal light, on the other hand, which can be generated from the transverse magnetic mode in a waveguide, has a much more dramatic effect—it renormalizes the tunneling strength between the layers, which effectively permits the tuning of the twist angle in situ. We find that, by varying the frequency and amplitude of the drive, one can induce a topological transition that cannot be obtained with the traditional form of the Floquet drive in free space. Furthermore, we find that strong drives can have a profound effect on the layer pseudospin texture of the twisted system, which coincides with multiple simultaneous band gap closings in the infinite-frequency limit. Surprisingly, these band-gap closings are not associated with topological transitions. For high but finite drive frequencies near 0.7 eV, the infinite-frequency band crossings become band gap minima of the order of ${10}^{-6}$ eV or smaller.

The relaxation of a classical spin, exchange coupled to the local magnetic moment at an edge site of the one-dimensional spinful Su-Schrieffer-Heeger model, is studied numerically by solving the full set of equations of motion. A Lindblad coupling of a few sites at the opposite edge to an absorbing bath ensures that convergence with respect to the system size is achieved with only a moderate number of core sites. This allows us to numerically exactly study the long-time limit and to determine the parameter regimes where spin relaxation takes place. Corresponding dynamical phase diagrams for the topologically trivial and the nontrivial cases are constructed. The dynamical phase boundaries, the role of the topological edge state, and its internal Zeeman splitting for the spin-relaxation process, as well as incomplete spin relaxation on long time scales can be explained within the framework of a renormalized linear-response approach when explicitly taking retardation effects and nonequilibrium spin-exchange processes into account.

Understanding the spreading of quantum correlations in out-of-equilibrium many-body systems is one of the major challenges in physics. For isolated systems, a hydrodynamic theory explains the origin and spreading of entanglement via the propagation of quasiparticle pairs. However, when systems interact with their surrounding much less has been established. Here we show that the quasiparticle picture remains valid for open quantum systems: While information is still spread by quasiparticles, the environment modifies their correlation and introduces incoherent and mixing effects. For free fermions with gain/loss dissipation we provide formulas fully describing incoherent and quasiparticle contributions in the spreading of entropy and mutual information. Importantly, the latter is not affected by entanglement of the system with the external environment. The mutual information is exponentially damped at short times and eventually vanishes signaling the onset of a classical limit. The behavior of the logarithmic negativity is similar and this scenario is common to other dissipations. For weak dissipation, the presence of quasiparticles underlies remarkable scaling behaviors.

Crystals form regular and robust structures that under extreme conditions can melt and recrystallize into different arrangements in a process that is called crystal metamorphism. While crystals exist due to the breaking of a continuous translation symmetry in space, it was recently proposed that discrete crystalline order can also emerge in time and give rise to a novel phase of matter named the discrete time crystal (DTC). In this paper, we join these two ideas and propose a model for quantum metamorphism between two DTCs with different periodicities, $2T$- and $4T$-DTCs. In our model the conditions for metamorphism come from the modulation of perturbative terms in the $4T$-DTC Hamiltonian that gradually melt its structure and transform it into a $2T$-DTC. This process is studied in detail from the viewpoint of many-body physics of periodically driven systems. We also propose a protocol to experimentally observe quantum metamorphism using current quantum technology.

The concept of geometrical frustration has led to rich insights into condensed matter physics, especially as a mechanism to produce exotic low-energy states of matter. Here we show that frustration provides a natural vehicle to generate models exhibiting anomalous thermalization of various types within high-energy states. We consider three classes of nonintegrable frustrated spin models: (I) systems with local conserved quantities where the number of symmetry sectors grows exponentially with the system size but more slowly than the Hilbert space dimension, (II) systems with exact eigenstates that are singlet coverings, and (III) flatband systems hosting magnon crystals. We argue that several one- and two-dimensional models from class I exhibit disorder-free localization in high-energy states so that information propagation is dynamically inhibited on length scales greater than a few lattice spacings. We further show that models of classes II and III exhibit quantum many-body scars: eigenstates of nonintegrable Hamiltonians with finite-energy density and anomalously low-entanglement entropy. Our results demonstrate that magnetic frustration supplies a means to systematically construct classes of nonintegrable models exhibiting anomalous thermalization in mid-spectrum states.

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the electronic structure of solids under optical excitation, and is a powerful technique for studying the coupling between electrons and collective modes. One approach to infer electron-boson coupling is through the relaxation dynamics of optically excited electrons, and the characteristic timescales of energy redistribution. A common description of electron relaxation dynamics is through the effective electronic temperature. Such a description requires that thermodynamic quantities are well-defined, an assumption that is generally violated at early delays. Additionally, precise estimation of the nonthermal window—within which effective temperature models may not be applied—is challenging. We perform TR-ARPES on graphite and show that Boltzmann rate equations can be used to calculate the time-dependent electronic occupation function $f(\epsilon ,t)$, and reproduce experimental features given by nonthermal electron occupation. Using this model, we define a quantitative measure of nonthermal electron occupation and use it to define distinct phases of electron relaxation in the fluence-delay phase space. More generally, this approach can be used to inform the nonthermal-to-thermal crossover in pump-probe experiments.

This paper investigates the temperature dependence of quantum information scrambling in local systems with an energy gap, $m$, above the ground state. We study the speed and shape of growing Heisenberg operators as quantified by out-of-time-order correlators, with particular attention paid to so-called contour dependence, i.e., dependence on the way operators are distributed around the thermal circle. We report large-scale tensor network numerics on a gapped chaotic spin chain down to temperatures comparable to the gap which show that the speed of operator growth is strongly contour-dependent. The numerics also show a characteristic broadening of the operator wavefront at finite temperature $T$. To study the behavior at temperatures much below the gap, we perform a perturbative calculation in the paramagnetic phase of a $2+1\mathrm{D} \mathrm{O}(N$) nonlinear sigma model, which is analytically tractable at large $N$. Using the ladder diagram technique, we find that operators spread at a speed $\sqrt{T/m}$ at low temperatures, $T\ll m$. In contrast to the numerical findings of spin chain, the large $N$ computation is insensitive to the contour dependence and does not show broadening of operator front. We discuss these results in the context of a recently proposed state-dependent bound on scrambling.

Nonlocal thermal transport is generally described by the Peierls-Boltzmann transport equation (PBE). However, solving the PBE for a general space-time dependent problem remains a challenging task due to the high dimensionality of the integro-differential equation. In this work, we present a direct solution to the space-time dependent PBE with a linearized collision matrix using an eigendecomposition method. We show that there exists a generalized Fourier-type relation that links heat flux to the local temperature, and this constitutive relation defines a thermal conductivity that depends on both time and space. Combining this approach with ab initio calculations of phonon properties, we demonstrate that the space-time dependent thermal conductivity gives rise to an oscillatory response in temperature in a transient grating geometry in high thermal conductivity materials. The present solution method allows us to extend the reach of our computational capability for heat conduction to space-time dependent nondiffusive transport regimes. This capability will not only enable a more accurate interpretation of thermal measurements that observe nonlocal thermal transport, but also enhance our physical understanding of nonlocal thermal transport in high thermal conductivity materials that are promising candidates for nanoscale thermal management applications.

Scalar waves such as airborne sound lack an intrinsic spin degree of freedom, making the realization of sonic ${\mathbb{Z}}_2$ topological phases based on spin degeneracy challenging. Here, we demonstrate the relevance of synthetic dimensions and higher-dimensional topological physics for exploring topological phases based on acoustic pseudo-spin with exact Kramers degeneracy. Interestingly, we find that a carefully designed two-band one-dimensional Hamiltonian with two additional phason degrees of freedom can enter a ${\mathbb{Z}}_2$ semimetallic phase with nonzero topological invariants carried by pairs of Weyl points in a three-dimensional synthetic momentum space. Taking advantage of the high localization of sonic quasibound states, embedded in the modal continuum of a one-dimensional acoustic waveguide, we implement a ${\mathbb{Z}}_2$ topological Weyl system and experimentally observe its signature in far-field sound scattering experiments. Our findings establish sonic quasibound states in continuum as a fertile ground for exploring higher dimensional Weyl physics in scattering media, and provide a viable experimental path to study spin-related topological effects in acoustics.