Understanding the thermodynamics of driven quantum systems strongly coupled to thermal baths is a central focus of quantum thermodynamics and mesoscopic physics. A variety of different methodological approaches exist in the literature, all with their own advantages and disadvantages. The mesoscopic leads approach was recently generalized to steady-state thermal machines and has the ability to replicate Landauer-Büttiker theory in the noninteracting limit. In this approach a set of discretized lead modes, each locally damped, provide a Markovian embedding for the baths. In this work we further generalize this approach to incorporate an arbitrary time dependence in the system Hamiltonian. Following a careful discussion of the calculation of thermodynamic quantities we illustrate the power of our approach by studying several driven mesoscopic examples coupled to finite-temperature fermionic baths, replicating known results in various limits. In the case of a driven noninteracting quantum dot we show how fast driving can be used to induce heat rectification.

A quasistatic theory for slowly rotating electromagnetic systems observed in their rest frame of reference is developed. Rotation-induced electrodynamic effects are explored, and their electric circuitry implications are discussed. It is shown that rotation may induce fictitious charges that affect lumped device dynamics and offer various device functionalities such as voltage-excited magnetic fields leading to rotation-induced memristors of positive or even negative memristance and their dualities. Rotation-induced electromagnetic gain and instabilities may exist, manifested either as parasitic processes that hamper electric circuitry functionality or as a mean for possible energy harvesting methodology in which the large-scale rotating platform serves as an essentially unlimited energy reservoir. Furthermore, as many artificially engineered electromagnetic materials consist of meta-atoms whose internal dynamics is essentially quasistatic, the study also potentially paves the way for new types of metamaterials. These effects depend on the rotation rate $\mathit{\Omega }$ but are essentially independent of the axis location. This fundamental property renders them extremely robust and has far-reaching ramifications for a plethora of applications. A preliminary quantitative analysis for $\mathit{\Omega }$ typical of large-scale platforms ranging from planets to artificial gravity structures is presented.

Recent advances in ultrafast pump-probe spectroscopy provide access to hidden phases of correlated matter, including light-induced superconducting states. The theoretical understanding of these nonequilibrium phases remains limited, particularly for correlated materials on frustrated lattices. Here we demonstrate that photodoping can induce a new type of chiral superconducting phase in frustrated Mott insulators by forming a condensate of doublons and holons. This metastable phase features a spatially varying order parameter with a ${120}^{\circ }$ phase twist which breaks both time-reversal and inversion symmetry. Under an external electric pulse, the ${120}^{\circ }$ chiral superconducting state can exhibit a second-order supercurrent perpendicular to the field in addition to a first-order parallel response, similar to a nonlinear anomalous Hall effect. Light-induced artificial gauge fields may be used to further stabilize this hidden phase. The presented results demonstrate that the light-induced superconducting state on a triangular lattice is of chiral nature and exhibits distinguishing properties which can be revealed in pump-probe experiments. The general mechanism applies to Mott insulators on various frustrated lattices and is tunable by optical means.

Non-Hermiticity, usually represented in the context of gain and loss, gives rise to many exotic topological phenomena and offers more opportunities to steering topological functions. In modern acoustics, it has been widely perceived that the topological mode will be altered if topological phase changes. Our work shows otherwise in non-Hermitian acoustic crystals. We experimentally demonstrate an acoustic quadrupole topological insulator, whose topological corner, edge, and bulk modes could be arbitrarily engineered at any desired positions with its topological phase maintained. These non-Hermiticity-controlled topological modes bestow a bulk structure with unique features beyond the classical bulky state, offering a reconfigurable and versatile approach to manipulating topological phenomena. This non-Hermitian scheme can be readily generalized to other topological systems in various dimensions, such as the three-dimensional photonic/phononic lattices, which offer advanced and externally controllable recipes for manipulating topological phenomena.

We use a recently developed high-resolution coherent probe spectroscopy method to investigate the dispersion of collective excitations of a polaritonic quantum fluid. We measure the dispersion relation with high energy and wave-number resolution, which allows to determine the speed of sound in the fluid and to evidence the contribution of an excitonic reservoir. We report on the generation of collective excitations at negative energies, on the ghost branch of the dispersion curve. Precursors of dynamical instabilities are also identified. Our methods open the way to the precise study of quantum hydrodynamics of quantum fluids of light.

The interplay of superconductivity and disorder generates a wealth of complex phenomena. In particular, the peculiar structure of diffusive electronic wave functions is predicted to increase the superconducting critical temperature in some range of disorder. In this work, we use an epitaxial monolayer of lead showing a simple band structure and homogeneous structural disorder as a model system of a two-dimensional superconductor in the weak-antilocalization regime. Then, we perform an extensive study of the emergent fluctuations of the local density of states (LDOS) and spectral energy gap in this material and compare them with both analytical results and the numerical solution of the attractive Hubbard model. We show that mesoscopic LDOS fluctuations allow us to probe locally both the elastic and inelastic scattering rates, which are notoriously difficult to measure in transport measurements.

The highly frustrated rare-earth-based magnet ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$ is a promising candidate in the search for proximate spin-liquid physics. We present a thorough investigation on single crystals of this material using bulk and microscopic techniques. Magnetization data reveal a fractional magnetization plateau for three different investigated field directions. The magnetic phase diagram is mapped out from calorimetric data and exhibits several domes of magnetic order below 0.3 K. Propagation vectors for all ordered phases are presented. The results suggest complex ordering in this material, and unveil the existence of a commensuration transition of the propagation vector at zero magnetic field. A scenario where interplane exchange interactions are essential to a magnetic model of ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$ is discussed.

The electric field of an intense laser pulse can directly modify the electronic properties of materials via electromodulation up to the petahertz regime. In this regime, the energy of the quiver motion of the electron-hole pair is comparable with the photon energy, which results in complex nonadiabatic dynamics. This regime opens opportunities to probe the electronic structure of materials on the attosecond timescale. Here, we show how the quasistatic electromodulation spectroscopy based on the Franz-Keldysh effect (FKE) connects with its nonadiabatic limit, which we find to be determined by resonant transitions between Floquet-Bloch states. This insight can be applied to measure the effective mass, ponderomotive and binding energies of the electron-hole pair on a few-femtosecond timescale. We demonstrate this by experimentally investigating laser-field-driven fused silica, a prototypical material for light-wave electronics, with extreme-ultraviolet attosecond pulse trains. We reproduce the experimental transient absorption spectra with an effective band structure and a dynamical Franz-Keldysh model, offering a simple parametrization for a theoretically challenging but technologically abundant material. Ab initio calculations in α-quartz highlight the contributions of specific bands, symmetry, and crystal orientation that are hidden in the experimental data due to randomized crystallographic orientation and finite temporal and spatial coherence. We show that the dynamical FKE can be explained as a third-order nonlinear process in the weak-field regime. The delay-dependent position of the absorption maxima and minima has a minimum tilt angle, determined by transitions between the underlying Floquet-Bloch states. In our analysis, we discuss the main experimental observables and show their connection to the parameters of the solid, providing the basis for nonadiabatic electromodulation spectroscopy.

The high level of control and precision achievable in current synthetic quantum matter setups has enabled first attempts at quantum-simulating various intriguing phenomena in condensed matter physics, including those probing thermalization or its absence in closed quantum systems. In the companion Letter to this article [J.-Y. Desaules et al., Phys. Rev. B 107, L201105 (2023)], we have shown that quantum many-body scars, special low-entropy eigenstates that weakly break ergodicity in nonintegrable systems, arise in spin-$S$ quantum link models that converge to $(1+1)$-dimensional lattice quantum electrodynamics (Schwinger model) in the Kogut-Susskind limit $S\to \infty$. In this work, we further demonstrate that quantum many-body scars exist in a truncated version of the Schwinger model, and are qualitatively more prominent than their counterparts in spin-$S$ quantum link models. We illustrate this by, among other things, performing a finite-$S$ scaling analysis that strongly suggests that scarring persists in the truncated Schwinger model in the limit $S\to \infty$. Although it does not asymptotically converge to the Schwinger model, the truncated formulation is relevant to synthetic quantum matter experiments, and also provides fundamental insight into the nature of quantum many-body scars, their connection to lattice gauge theories, and the thermalization dynamics of the latter. Our conclusions can be readily tested in current cold-atom setups.

Motivated by recent advances in the fabrication of twisted bilayers of two-dimensional materials, we consider the low-energy properties of a twisted pair of two-dimensional nodal superconductors. We study both the cases of singlet and triplet superconductors. It is demonstrated that the Bogoliubov–de Gennes (BdG) quasiparticle dispersion undergoes dramatic reconstruction due to the twist. In particular, the velocity of the neutral massless Dirac excitations near the gap nodes is strongly renormalized by the interlayer hopping and vanishes at a “magic angle” where in the limit of a circular Fermi surface a quadratic band touching is formed. In addition, it is shown that the BdG dispersion can be tuned with an interlayer displacement field, magnetic field, and current, which can suppress the velocity renormalization, create finite BdG Fermi surfaces, or open a gap, respectively. Finally, interactions between quasiparticles are shown to lead to the emergence of a correlated superconducting state breaking time-reversal symmetry in the vicinity of the magic angle. Estimates of the magic angle in a variety of nodal superconductors are presented, ranging from the cuprates to the organic and heavy-fermion superconductors, all of which are shown to be promising for the experimental realization of our proposal.

The competition between magnetic order and Kondo effect is essential for the rich physics of heavy fermion systems. Nevertheless, how such competition is manifested in the quasiparticle bands in a real periodic lattice remains elusive in spectroscopic experiments. Here we report a high-resolution photoemission study of the antiferromagnetic Kondo lattice system ${\mathrm{CeCoGe}}_3$ with a high $T_{N1}$ of 21 K. Our measurements reveal a weakly dispersive $4f$ band at the Fermi level near the $Z$ point, arising from a moderate Kondo effect. The intensity of this heavy $4f$ band exhibits a logarithmic increase with lowering temperature and begins to deviate from this Kondo-like behavior below $\sim 25$ K, just above $T_{N1}$, and eventually ceases to grow below $\sim 12$ K. Our work provides direct spectroscopic evidence for the competition between magnetic order and the Kondo effect in a Kondo lattice system with local-moment antiferromagnetism, indicating a distinct scenario for the microscopic coexistence and competition of these phenomena, which might be related to the real-space modulation.

The bulk states of Hermitian systems are believed insensitive to local Hermitian impurities or perturbations except for a few impurity-induced bound states. Thus, it is important to ask whether local non-Hermiticity can cause drastic changes to the original Hermitian systems. Here we address this issue affirmatively and present exact solutions for the double chain model with local non-Hermitian terms possessing parity-time $\left(\mathcal{PT}\right)$ symmetry. Induced by the non-Hermiticity, the system undergoes a sequence of $\mathcal{PT}$-symmetry breakings, after which the eigenenergies appear in complex conjugate pairs. The associated extended bulk states then become scale-free localized and unidirectionally accumulated around the impurity. There exist mobility edges separating the residual extended states until a full scale-free localization of all eigenstates. Further increasing the non-Hermitity counterintuitively brings the system to a $\mathcal{PT}$-restoration regime with fully real spectra except for a pair of complex bound states. We demonstrate that the local non-Hermiticity generated scale-free localization is a general phenomenon and can even survive the quasiperiodic disorder. Our results indicate that the bulk properties of the original Hermitian system can be globally reshaped by local non-Hermiticity.

Heavy-metal (HM)/magnet bilayers host many magnetoresistance (MR) and spin caloritronic effects. Here, we show that the spin Peltier effect and electron-phonon scattering produce much larger unidirectional MR of an HM on a magnetic insulator than existing theories that neglect the interplay between MR and spin caloritronic effects. By accounting for local nonequilibrium in both the magnon chemical potential and temperature, our analytical model attributes the observed frequency dependence of the spin Peltier MR and the spin Seebeck effect to the reduction of the thermal penetration depth, which approaches the $1\text{−}\mu \mathrm{m}$-scale magnon spin-diffusion length at high frequencies.

Recent developments in quantum hardware and quantum algorithms have made it possible to utilize the capabilities of current noisy intermediate-scale quantum devices for addressing problems in quantum chemistry and condensed-matter physics. Here we report a demonstration of solving the dynamical mean-field theory (DMFT) impurity problem for the Hubbard-Holstein model on the IBM Quantum Processor Kawasaki, including self-consistency of the DMFT equations. This opens up the possibility to investigate strongly correlated electron systems coupled to bosonic degrees of freedom and impurity problems with frequency-dependent interactions. The problem involves both fermionic and bosonic degrees of freedom to be encoded on the quantum device, which we solve using a recently proposed Krylov variational quantum algorithm to obtain the impurity Green's function. We find the resulting spectral function to be in good agreement with the exact result, exhibiting both correlation and plasmonic satellites and significantly surpassing the accuracy of standard Trotter-expansion approaches. Our results provide an essential building block to study electronic correlations and plasmonic excitations on future quantum computers with modern ab initio techniques.

Moiré twist angle underpins the interlayer interaction of excitons in twisted van der Waals hetero- and homostructures. The influence of twist angle on the excitonic absorption of twisted bilayer tungsten diselenide (${\mathrm{WSe}}_2$) has been investigated using electron energy-loss spectroscopy. Atomic-resolution imaging by scanning transmission electron microscopy was used to determine key structural parameters, including the nanoscale measurement of the relative twist angle and stacking order. Detailed spectral analysis revealed a pronounced blueshift in the high-energy excitonic peak C with increasing twist angle, up to 200 meV when compared to the ${\mathrm{AA}}^{\prime }$ stacking. The experimental findings have been discussed relative to first-principle calculations of the dielectric response of the ${\mathrm{AA}}^{\prime }$-stacked bilayer ${\mathrm{WSe}}_2$ as compared to monolayer ${\mathrm{WSe}}_2$ by employing the $\mathit{GW}$ plus Bethe-Salpeter equation approaches, resolving the origin of higher energy spectral features from ensembles of excitonic transitions, and thus any discrepancies between previous calculations. Furthermore, the electronic structure of moiré supercells spanning twist angles of $\sim 9.5–46.5^{\circ }$ calculated by density functional theory were unfolded, showing an uplifting of the conduction band minimum near the $Q$ point and minimal change in the upper valence band concurrently. The combined experiment/theory investigation provides valuable insight into the physical origins of high-energy absorption resonances in twisted bilayers, which enables one to track the evolution of interlayer coupling from tuning of the exciton C transitions by absorption spectroscopy.

Intermetallic narrow-gap semiconductors have been intensively explored due to their large thermoelectric power at low temperatures and a possible role of strong electronic correlations in their unusual thermodynamic and transport properties. Here we study the optical spectra and vibrational properties of ${\mathrm{FeGa}}_3$ single crystal. The optical conductivity indicates that ${\mathrm{FeGa}}_3$ has a direct band gap of $\approx 0.7$ eV, consistent with density functional theory (DFT) calculations. Most importantly, we find a substantial spectral weight also below 0.4 eV, which is the energy of the indirect (charge) gap found in resistivity measurements and ab initio calculations. We find that the spectral weight below the gap decreases with increasing temperature, which indicates that it originates from the impurity states and not from the electronic correlations. Interestingly, we did not find any signatures of the impurity states in vibrational spectra. The infrared and Raman vibrational lines are narrow and weakly temperature dependent. The vibrational frequencies are in excellent agreement with our DFT calculations, implying a modest role of electronic correlations. Narrow Mössbauer spectral lines also indicate high crystallinity of the sample.

The structural properties of the hexagonal multiferroic $h\text{−}{\mathrm{HoMnO}}_3$ under high pressure have been explored using synchrotron x-ray diffraction and x-ray absorption spectroscopy in diamond anvil cells. The structure was found to undergo a pressure-induced phase transition at $\sim 24$ GPa to a rhombohedrally distorted superstructure, which is isostructural to the oxygen-loaded h-R${\mathrm{MnO}}_{3+\delta }$ ($R=\mathrm{Y},\mathrm{Dy},\mathrm{Ho},\mathrm{Er}$; $\delta \approx 0.28)$ phases found in the same systems. The driving force behind the phase transition is the highly compressible $\mathit{ab}$ plane which facilitates a gradual charge disproportionation of Mn(III) with pressure. We speculate this stabilizes the spin-liquid phase due to ferromagnetic coupling between neighboring Mn(II)/Mn(IV) and Mn(III). In addition, we demonstrate that the structural behavior is highly susceptible to nonhydrostatic conditions and the choice of pressure medium should be carefully made.

Spectral probes, such as neutron scattering, are crucial for characterizing excitations in quantum many-body systems and the properties of quantum materials. Among the most elusive phases of matter are quantum spin liquids, which have no long-range order even at zero temperature and host exotic fractionalized excitations with nontrivial statistics. These phases can occur in frustrated quantum magnets, such as the paradigmatic Heisenberg model with nearest- and next-nearest-neighbor exchange interactions on the triangular lattice, the so-called $J_1\text{−}{J}_2$ model. In this work, we compute the spectral function using large-scale matrix product state simulations across the three different phases of this model's phase diagram, including a quantum spin-liquid phase at intermediate $J_2/J_1$. Despite a plethora of theoretical and experimental studies, the exact nature of this phase is still contested, with the dominant candidates being a gapped ${\mathbb{Z}}_2$, a gapless $U\left(1\right)$ Dirac, and a spinon Fermi surface quantum spin-liquid state. We find a V-shaped spectrum near the center of the Brillouin zone ($\mathrm{\Gamma }$ point), a key signature of a spinon Fermi surface, observed in prior neutron scattering experiments. However, we find a small gap near the $\mathrm{\Gamma }$ point, ruling out such a phase. Furthermore, we find localized gapless excitations at the corner of the Brillouin zone boundary (K point) and the middle of the edge of the Brillouin zone boundary (M point), ruling out the gapped ${\mathbb{Z}}_2$ spin-liquid phase. Our results imply that the intermediate spin-liquid phase is a gapless $U\left(1\right)$ Dirac spin liquid, and provide clear signatures to detect this phase in future neutron scattering experiments.

We report the detailed high-field and high-frequency electron spin resonance spectroscopic study of the single-crystalline van der Waals compounds ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ and ${\mathrm{MnNiP}}_2{\mathrm{S}}_6$. Analysis of magnetic excitations shows that in comparison to ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ increasing the Ni content yields a larger magnon gap in the ordered state and a larger $g$-factor value and its anisotropy in the paramagnetic state. The studied compounds are found to be strongly anisotropic having each the unique ground state and type of magnetic order. Stronger deviation of the $g$ factor from the free electron value in the samples containing Ni suggests that the anisotropy of the exchange is an important contributor to the stabilization of a certain type of magnetic order with particular anisotropy. At temperatures above the magnetic order, we have analyzed the spin-spin correlations resulting in a development of slowly fluctuating short-range order. They are much stronger pronounced in ${\mathrm{MnNiP}}_2{\mathrm{S}}_6$ compared to ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. The enhanced spin fluctuations in ${\mathrm{MnNiP}}_2{\mathrm{S}}_6$ are attributed to the competition of different types of magnetic order. Finally, the analysis of the temperature-dependent critical behavior of the magnon gaps below the ordering temperature in ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ suggests that the character of the spin wave excitations in this compound undergoes a field-induced crossover from a 3D-like toward a 2D XY regime.

${\mathrm{KAgF}}_3$ is a quasi-one-dimensional quantum antiferromagnet hosting a series of intriguing structural and magnetic transitions. Here we use powder neutron diffraction, $\mu \mathrm{SR}$ spectroscopy, and density functional theory calculations to elucidate the low-temperature magnetic phases. Below $T_{\mathrm{N}1}=29\mathrm{K}$ we find that the material orders as an A-type antiferromagnet with an ordered moment of $0.52{\mu }_{\mathrm{B}}$. Both neutrons and muons provide evidence for an intermediate phase at temperatures $T_{\mathrm{N}1}<T<T_{\mathrm{N}2}$ with $T_{\mathrm{N}2}\approx 66$ K from a previous magnetometry study. However, the evidence is at the limit of detection and its nature remains an open problem.