High harmonic generation (HHG) in solids reflects the underlying nonperturbative nonlinear dynamics of electrons in a strong light field and is a powerful tool for ultrafast spectroscopy of electronic structures. Photocarrier doping allows us to understand the carrier dynamics and the correlations between the carriers in the HHG process. Here, we study the effect of incoherent electron-hole pairs on HHG in an atomically thin semiconductor. The experimentally observed response to photocarrier doping is successfully reproduced in numerical simulations incorporating the photoexcited carrier distribution, excitonic Coulomb interaction, and electron-electron scattering effects. The simulation results reveal that the presence of photocarriers enhances the intraband current that contributes to high harmonics below the absorption edge. We also clarify that the excitation-induced dephasing process rather than the phase-space filling effect is the dominant mechanism reducing the higher-order harmonics above the absorption edge. Our work provides a deeper understanding of high harmonic spectroscopy and the optimum conditions for generating extreme ultraviolet light from solids.

Compact inductor-capacitor (LC) resonators, in contrast to coplanar waveguide (CPW) resonators, have a simple lumped-element circuit representation but usually call for sophisticated finite-element method (FEM) simulations for an accurate modeling. Here we present a simple analytical model for a family of coplanar LC resonators where the electrical properties are directly obtained from the circuit geometry with a satisfying accuracy. Our experimental results on ten high-internal-quality-factor resonators $(Q_{\mathrm{i}}\gtrsim 2\times {10}^5)$, with frequencies ranging from $300\mathrm{MHz}$ to $1\mathrm{GHz}$, show an excellent consistency with both the derived analytical model and detailed FEM simulations. These results showcase the ability to design sub-gigahertz resonators with less than $2\%$ deviation in the resonance frequency, which has immediate applications, for example, in the implementation of ultrasensitive cryogenic detectors. The achieved compact resonator size of the order of a square millimeter indicates a feasible way to integrate hundreds of microwave resonators on a single chip for realizing photonic lattices.

We investigate squeezing of magnons in a conical spin spiral configuration. We find that while the energy of magnons propagating along the $\mathit{k}$ and the $-\mathit{k}$ directions can be different due to the nonreciprocal dispersion, these two modes are connected by the squeezing, and hence can be described by the same squeezing parameter. The squeezing parameter diverges at the center of the Brillouin zone due to the translational Goldstone mode of the system, but the squeezing also vanishes for certain wave vectors. We discuss possible ways of detecting the squeezing.

The critical field behavior of a layered Ising superconductor with finite number of layers is studied. Under in-plane magnetic fields, the finite-momentum superconductivity dubbed as the orbital Fulde-Ferrell-Larkin-Ovchinnikov state is found in the regime of low field and high temperature. Our theory agrees well with the experimental results in Nature 619, 46 (2023).

Control of quantum matter through resonant electromagnetic cavities is a promising route towards establishing control over material phases and functionalities. Quantum paraelectric insulators—materials that are nearly ferroelectric—are particularly promising candidate systems for this purpose since they have strongly fluctuating collective modes that directly couple to the electric field. In this work, we explore this possibility in a system comprised of a quantum paraelectric sandwiched between two high-quality metal mirrors, realizing a Fabry-Perot-type cavity. By developing a full multimode, continuum description we are able to study the effect of the cavity in a spatially resolved way for a variety of system sizes and temperatures. Surprisingly, we find that once a continuum of transverse modes is included the cavity ends up suppressing ferroelectric correlations. This effect arises from the screening out of transverse photons at the cavity boundaries and, as a result, is confined to the surface of the paraelectric sample. We also explore the temperature dependence of this effect and find it vanishes at high temperatures, indicating it is a purely quantum mechanical effect. We connect our result to calculations of Casimir and Van der Waals forces, which we argue are closely related to the dipolar fluctuations in the quantum paraelectric. Our results are based on a general formalism and are expected to be widely applicable, paving the way towards studies of the quantum electrodynamics of heterostructures featuring multiple materials and phases.

B-rich metal compounds have garnered significant attention due to their diverse polymeric boron structures and versatile properties including high melting temperature, exceptional hardness, and superconductivity. In this paper, we systematically investigate the ternary phase diagram of the Li-Mn-B system at 0–100 GPa, employing a combination of crystal structure prediction methods and first-principles calculations. We identify five pressure-stabilized compounds with stoichiometries of ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{B}}_2, \mathrm{LiMn}{\mathrm{B}}_3, \mathrm{LiMn}{\mathrm{B}}_4, {\mathrm{Li}}_2\mathrm{Mn}{\mathrm{B}}_8$, and $\mathrm{LiMn}{\mathrm{B}}_{10}$. These pressure-induced compounds can be recovered to ambient pressure conditions. Notably, the boron atoms in these compounds exhibit diverse polymerization patterns, forming structures such as dimers, zigzag chains, ribbons, kagome lattices, and three-dimensional channel frameworks. The five Li-Mn-B compounds are potential hard materials, with $\mathrm{LiMn}{\mathrm{B}}_4$ standing out with a remarkable hardness value of 39.47 GPa. $\mathrm{LiMn}{\mathrm{B}}_4$ and $\mathrm{LiMn}{\mathrm{B}}_{10}$ are indirect semiconductors with band gaps of 0.92 and 0.64 eV, respectively. Notably, $\mathrm{LiMn}{\mathrm{B}}_{10}$ exhibits a lower effective mass than silicon, superior solar cell efficiency compared with silicon, and a work function comparable with that of silicon. These properties highlight the considerable potential of $\mathrm{LiMn}{\mathrm{B}}_{10}$ as a viable material for photovoltaic applications. ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{B}}_8$ is predicted to be a superconductor with a critical temperature of 15 K at 50 GPa. When the pressure is reduced to 0 GPa, ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{B}}_8$ displays antiferromagnetic properties. $\mathrm{LiMn}{\mathrm{B}}_3$ is a suitable precursor to obtain a ${\mathrm{B}}_{20}$-caged structure with remarkable superconductivity at ambient pressure, reaching a $T_{\mathrm{c}}$ of 31 K. In this paper, we provide comprehensive insights into the ternary phase diagram of the Li-Mn-B system at high pressure, elucidating the formation of various stable compounds with intriguing properties.

Universal topological data of topologically ordered phases can be captured by topological quantum field theory in continuous space time by taking the limit of low energies and long wavelengths. While previous continuum field-theoretical studies of topological orders in three-dimensional (3D) real space focused on either self-statistics, braiding statistics, shrinking rules, fusion rules, or quantum dimensions, it has yet to systematically put all topological data together in a unified continuum field-theoretical framework. Here, we construct the topological $BF$ field theory with twisted terms (e.g., $AAdA$ and $AAB$) as well as a $K$-matrix $BB$ term to simultaneously explore all such topological data and reach anomaly-free topological orders. Following the spirit of the famous $K$-matrix Chern-Simons theory of two-dimensional topological orders, we present general formulas and systematically show how the $K$-matrix $BB$ term confines topological excitations, and how self-statistics of particles is transmuted between bosonic one and fermionic one. To reach anomaly-free topological orders, we explore, within the present continuum field-theoretical framework, how the principle of gauge invariance fundamentally influences possible realizations of topological data. More concretely, we present the topological actions of (i) particle-loop braidings with emergent fermions, (ii) multiloop braidings with emergent fermions, and (iii) Borromean rings braidings with emergent fermions, and calculate their universal topological data. Together with previous efforts, our paper paves the way toward a more systematic and complete continuum field-theoretical analysis of exotic topological properties of 3D topological orders. Several interesting future directions are also discussed.

The quest for a half-quantized thermal Hall effect of a Kitaev system represents an important tool to probe topological edge currents of emergent Majorana fermions. Pertinent experimental findings for $\alpha \text{−}{\mathrm{RuCl}}_3$ are, however, strongly debated, and it has been argued that the thermal Hall signal stems from phonons or magnons rather than from Majorana fermions. Here, we investigate the thermal Hall effect of the Kitaev candidate material ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, and we show that the measured signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the longitudinal and transversal heat conductivities share clear phononic signatures, while the transversal signal changes sign upon entering the low-temperature, magnetically ordered phase. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in Kitaev topological quantum spin liquids such as $\alpha \text{−}{\mathrm{RuCl}}_3$ and ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ requires disentangling phonon vs magnetic contributions, including potentially fractionalized excitations such as the expected Majorana fermions.

We show here using atomistic simulations that ${\mathrm{BiFeO}}_3$ films can be driven through a topological transition when increasing uniaxial anisotropy, which can possibly be achieved by strain. Tuning the anisotropy close to the transition, we show that individual antiferromagnetic skyrmions can reach a very large topological protection. These entities can then be excited by electric fields and spin torque and controllably driven at speeds exceeding 10 km/s.

Three-dimensional (3D) gapped topological phases with fractional excitations are divided into two subclasses: one has topological order with point-like and loop-like excitations fully mobile in the 3D space, and the other has fracton order with point-like excitations constrained in lower-dimensional subspaces. These exotic phases are often studied by exactly solvable Hamiltonians made of commuting projectors, which, however, are not capable of describing those exhibiting surface states with gapless chiral dispersion. Here we introduce a systematic way, based on cellular construction recently proposed for 3D topological phases, to construct another type of exactly solvable models in terms of coupled quantum wires with given inputs of cellular structure, two-dimensional Abelian topological order, and their gapped interfaces. We show that our models can describe both 3D topological and fracton orders (and even their hybrid) and study their universal properties such as quasiparticle statistics and topological ground-state degeneracy. We also apply this construction to two-dimensional coupled-wire models with ordinary topological orders and translation-symmetry-enriched topological orders. Our results pave the way for effective quantum field theory descriptions or microscopic model realizations of fracton orders with chiral gapless surface states.

Previous studies have suggested that high pressure may induce chemical activity in noble gases (He and Xe), resulting in the emergence of high-pressure compounds containing these elements. By using first-principles theory and crystal structural prediction methods, we propose that six unconventional stoichiometries, namely, ${\mathrm{Na}}_2\mathrm{Xe}$, NaXe, ${\mathrm{Na}}_2{\mathrm{Xe}}_3, {\mathrm{NaXe}}_2, {\mathrm{NaXe}}_3$, and ${\mathrm{NaXe}}_4$, can be stabilized at pressures ranging from 48 to 400 GPa, which is the highest pressure considered in this study. All the predicted Xe-containing compounds exhibit metallic properties with strong ionic Na–Xe bonds. Further $abinitio$ molecular dynamics simulations show that NaXe, ${\mathrm{NaXe}}_3$, and ${\mathrm{NaXe}}_4$ remain in the solid state under extreme conditions, such as those present in the Earth's mantle, at pressures up to 135 GPa and temperatures up to 5000 K. This suggests that Na–Xe compounds might be possible constituents inside the Earth's mantle. These findings not only enhance our understanding of the Earth's interior, but also provide insights into Xe chemistry.

Non-Hermitian (NH) Hamiltonians have been shown to exhibit unique signatures, including the NH skin effect and an exponential spectral sensitivity with respect to boundary conditions. Here, we investigate as to what extent these remarkable phenomena, recently predicted and observed in a broad range of settings, may also occur in closed correlated fermionic systems that are governed by a Hermitian many-body Hamiltonian. There, an effectively NH quasiparticle description naturally arises in the Green's function formalism due to interparticle scattering that represents an inherent source of dissipation. As a concrete platform we construct an extended interacting Su-Schrieffer-Heeger (SSH) model subject to varying boundary conditions, which we analyze using exact diagonalization and nonequilibrium Green's function methods. That way, we clearly identify the presence of the aforementioned NH phenomena in the quasiparticle properties of this Hermitian model system.

We investigate the time evolution of the second-order Rényi entropy (RE) for bosons in a one-dimensional optical lattice following a sudden quench of the hopping amplitude $J$. Specifically, we examine systems that are quenched into the strongly correlated Mott-insulating (MI) regime with $J/U\ll 1$ ($U$ denotes the strength of the on-site repulsive interaction) from the MI limit with $J=0$. In this regime, the low-energy excited states can be effectively described by fermionic quasiparticles known as doublons and holons. They are excited in entangled pairs through the quench dynamics. By developing an effective theory, we derive a direct relation between the RE and correlation functions associated with doublons and holons. This relation allows us to analytically calculate the RE and obtain a physical picture for the RE, both in the ground state and during time evolution through the quench dynamics, in terms of doublon-holon pairs. In particular, we show that the RE is proportional to the population of doublon-holon pairs that span the boundary of the subsystem. Our quasiparticle picture introduces some remarkable features that are absent in previous studies on the dynamics of entanglement entropy in free-fermion models. It provides with valuable insights into the dynamics of entanglement entropy in strongly-correlated systems.

In the presence of strong electronic spin correlations, the hyperfine interaction imparts long-range coupling between nuclear spins. Efficient protocols for the extraction of such complex information about electron correlations via magnetic response are not well known. Here we study how machine learning can extract material parameters and help interpret magnetic response experiments. A low-dimensional representation that classifies the strength and range of the interaction is discovered by unsupervised learning. Supervised learning generates models that predict the spatial extent of electronic correlations and the total interaction strength. Our work demonstrates the utility of artificial intelligence in the development of new probes of quantum systems, with applications to experimental studies of strongly correlated materials.

Finite simplex lattice models are used in different branches of science, e.g., in condensed-matter physics, when studying frustrated magnetic systems and non-Hermitian localization phenomena; or in chemistry, when describing experiments with mixtures. An $n$-simplex represents the simplest possible polytope in $n$ dimensions, e.g., a line segment, a triangle, and a tetrahedron in one, two, and three dimensions, respectively. In this work, we show that various fully solvable, in general non-Hermitian, $n$-simplex lattice models with open boundaries can be constructed from the high-order field-moments space of quadratic bosonic systems. Namely, we demonstrate that such $n$-simplex lattices can be formed by a dimensional reduction of highly degenerate iterated polytope chains in $(k>n)$-dimensions, which naturally emerge in the field-moments space. Our findings indicate that the field-moments space of bosonic systems provides a versatile platform for simulating real-space $n$-simplex lattices exhibiting non-Hermitian phenomena, and it yields valuable insights into the structure of many-body systems exhibiting similar complexity. Among a variety of practical applications, these simplex structures can offer a physical setting for implementing the discrete fractional Fourier transform, an indispensable tool for both quantum and classical signal processing.

We develop a self-supervised ensemble learning (SSEL) method to accurately classify distinct types of phase transitions by analyzing the fluctuation properties of machine learning outputs. Employing the 2D Potts model and the 2D Clock model as benchmarks, we demonstrate the capability of SSEL in discerning first-order, second-order, and Berezinskii-Kosterlitz-Thouless transitions, using in situ spin configurations as the input features. Furthermore, we show that the SSEL approach can also be applied to investigate quantum phase transitions in 1D Ising and 1D $XXZ$ models upon incorporating quantum sampling. We argue that the SSEL model simulates a special state function with higher-order correlations between physical quantities, and hence provides richer information than previous machine learning methods. Consequently, our SSEL method can be generally applied to the identification/classification of phase transitions even without explicit knowledge of the underlying theoretical models.

Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and condense into a new insulating ground state—the so-called excitonic insulator—when the energy of electron-hole Coulomb attraction exceeds the band gap. In the presence of electron-phonon coupling, a periodic lattice distortion often concomitantly occurs. However, a similar structural transition can also be induced by electron-phonon coupling itself, therefore hindering the clean identification of bulk excitonic insulators (e.g., which instability is the driving force of the phase transition). Using high-resolution synchrotron x-ray diffraction and angle-resolved photoemission spectroscopy, we identify key electron-phonon coupling effects in a leading excitonic insulator candidate ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. These include an extensive unidirectional lattice fluctuation and an electronic pseudogap in the normal state, as well as a negative electronic compressibility in the charge-doped broken-symmetry state. In combination with first principles and model calculations, we use the normal state electronic spectra to quantitatively determine the electron-phonon interaction vertex $g$ and interband Coulomb interaction $V$ in the minimal lattice model, the solution to which captures the experimental observations. Moreover, we show how the Coulomb and electron-phonon coupling effects can be unambiguously separated based on the solution to quantified microscopic models. Finally, we discuss how the strong lattice fluctuations enabled by low dimensionality relate to the unique electron-phonon interaction effects beyond the textbook Born-Oppenheimer approximation..

We measure electron transport through point contacts in an electron gas in AlGaAs/GaAs heterostructures and graphene for a range of temperatures, magnetic fields, and electron densities. We find a magnetoconductance peak around $B=0$. With increasing temperature, the width of the peak increases monotonically, while its amplitude first increases and then decreases. For GaAs point contacts the peak is particularly sharp at relatively low temperatures $T\approx 1.5\mathrm{K}$: the curve rounds on a scale of a few tens of microteslas, hinting at length scales of several millimeters for the corresponding scattering processes. We propose a model based on the transition between different transport regimes with increasing temperature: from ballistic transport to few electron-electron scatterings to hydrodynamic superballistic flow to hydrodynamic Poiseuille-like flow. The model is in qualitative and, in many cases, quantitative agreement with the experimental observations.