We present a microscopic quantum theory for the nonlinear optical phenomena in semiconductor quantum well heterostructures operating in the regime of ultrastrong light-matter coupling regime. This work extends the Power-Zienau-Wooley (PZW) formulation of quantum electrodynamics to account for nonlinear interactions based on a fully fermionic approach, without resorting to any bosonization approximation. It provides a unified description of the microcavity and the local field enhancement effects on the nonlinear optical response, thus encompassing the phenomena known as epsilon near zero (ENZ) effect. In particular, our theory describes the impact of the light-matter coupled states on the high-frequency generation process, relevant for recent experimental investigations with polaritonic metasurfaces. We unveil the limitations of traditional single-particle approaches and propose novel design principles to optimize nonlinear conversion efficiencies in dense, microcavity-coupled electronic systems. The theoretical framework developed here provides an efficient tool for the development of advanced quantum optical applications in the midinfrared and terahertz spectral domains. Furthermore, it establishes a foundation for exploring the quantum properties of the ultrastrong light-matter regime through frequency-converted polariton states.

In multiband systems, how to annihilate topological degeneracies with the same charges is a challenge and plays a crucial role in opening band gaps because their existence and evolution are constrained by the conservation law of topological charge. For photonic crystal slabs, both degeneracies and bound states in the continuum (BICs) are polarization singularities carrying nonzero topological charges, which unveil opportunities for addressing the challenge. Here, we demonstrate that two Dirac points (DPs) with identical charges can be annihilated by merging them with one BIC. It results in the emergence of a band gap between the originally gapless bands as well as the redistributions of BICs between the bands. Meanwhile, three bulk Fermi arcs spawning from the DPs carrying different charges could merge into one arc and subsequently split into three arcs again, which is accompanied by the annihilation and generation of two pairs of exceptional points between them. The physical mechanisms underlying the merging of DPs with BICs and bulk Fermi arcs are elucidated by two-level Hamiltonian models. Without violating the conservation law of global topological charges carried by polarization singularities within each band, the merging of DPs with BICs paves the way for manipulating the band structures of non-Hermitian multiband systems.

The quantum anomalous Hall (QAH) effect is typically observed in magnetically doped topological insulator thin films and odd-layered intrinsic magnetic topological insulators. Recent studies reported that chiral spin textures can generate QAH states even in the absence of net magnetization or spin-orbit coupling, although their transport properties remain poorly understood. Here, we investigate the topological characteristics and transport behavior of breathing kagome lattices modulated by the breathing parameter $\delta$ and the polar angle $\theta$ of on-site spin magnetic moments. In the topologically nontrivial regime, the valley index of edge states is governed by the edge geometry. Consequently, under disorder, the linear conductance remains nearly quantized even through the valley polarization is not perfect due to disorder. In contrast, in the topologically trivial regime, the Berry curvature displays valley-contrasting behavior. Trivial edge states associated with specific edge profiles are highly susceptible to disorder and edge deformation. Notably, kink states emerge at interfaces between two topologically trivial materials with opposite valley-contrasting properties. These kink states are topologically trivial but slightly more robust than trivial edge states. Finally, we demonstrate that in the topologically trivial regime, the polar angle $\theta$ enables continuous tuning of valley and spin polarization of bulk state transmission across a wide parameter range.

We propose a scheme to realize time-reversal symmetry-broken quantum spin Hall insulators using bilayer honeycomb lattices, combining intrinsic spin-orbit coupling, $C$-type antiferromagnetic ordering, and staggered potentials. The $C$-type antiferromagnetic order emerges from the interplay between intralayer antiferromagnetism and interlayer ferromagnetism. The system's topological properties are characterized by the spin Chern number. We present the topological phase diagram of the bilayer honeycomb lattices, providing detailed insight into the stability and tunability of the quantum spin Hall effect in this system. The presence of helical edge states is confirmed by the measurement of quantized longitudinal resistance values of $3/2(h/e^2)$ and $1/2(h/e^2)$ in a six-terminal Hall-bar device. Remarkably, this quantum spin Hall insulator phase is protected by interlayer parity-time ($\mathcal{PT}$) symmetry and spin $U\left(1\right)$ symmetry, despite the breaking of time-reversal symmetry.

We provide a comprehensive study of the capabilities of modulated electron wavefunctions for the preparation and readout of the quantum state of the quantum emitters (QEs) they interact with. First, we consider perfectly periodic electron combs, which do not produce QE-electron entanglement, preserving the purity of the QE state while inducing Rabi-like dynamics in it. We extend our findings to more realistic, nonideally modulated electron wave packets, showing that the phenomenology persists, and exploring their use to prepare the emitter in a desired quantum state. Thus, we establish the balance that electron comb size, emitter radiative decay, and electron-emitter coupling strength must fulfill in order to implement our findings in experimentally feasible platforms. Then, moving into the limit of small electron combs, we reveal that these wavefunctions allow for quantum state tomography of their target, providing access not only to the populations, but also the coherences of the QE density matrix. Finally, we explore the realistic range of coupling strength values and the limit of validity of our treatment in the case of free electron-QE interaction being mediated by free space and in inverse-designed waveguide structures, demonstrating that our treatment remains valid for large coupling strengths in these systems. We believe that our theoretical results showcase modulated free electrons as very promising tools for quantum technologies based on light-matter coupling.

Using density functional theory calculations, we predict that several monolayer chromium-based materials exhibit a triple-$Q$ tetrahedral magnetic insulating ground state. By studying the effect of biaxial strain on monolayer $\mathrm{CrSi}{}_2\mathrm{P}{}_4$ under various on-site Coulomb interactions, we reveal that this magnetic insulating state, sandwiched between the itinerant ${120}^{\circ }$ coplanar noncollinear antiferromagnetic and ferromagnetic states, originates from the competition between antiferromagnetic exchange and double-exchange interactions of Cr $3d$ electrons which can also be applied to account for the ground states in other chromium-based materials. Remarkably, an anomalous valley Hall effect with giant valley splitting is discovered in the magnetic states of these inversion-asymmetric systems without requiring spin-orbit coupling or net magnetization. Our findings open another avenue towards exploring monolayer materials for valleytronics.

We report a direct measurement of the temperature and density of a metal along its liquid-vapor coexistence (L-V) curve. By shocking platinum to a high-pressure liquid, we imparted sufficient heat for subsequent isentropic release to place it in a state on the boundary between the liquid and vapor phases. Released material in the liquid phase acted as a high velocity flyer pinned to the L-V curve. We measured velocity and radiant emission of the flyer as well as the interface motion and transiting shock states induced in a downstream window material by its impact. We used these measurements to calculate temperature and density of the L-V curve state which we compare to density functional theory predictions.

Monitored quantum system have sparked great interest in recent years due to the possibility of observing measurement-induced phase transitions (MIPTs) in the full-counting statistics of quantum trajectories. Here, we investigate the dynamics of the Lipkin-Meshkov-Glick model, composed of $N$ all-to-all interacting spins $1/2$, under a weak external monitoring. In the thermodynamic limit, we derive a set of semiclassical stochastic equations describing the evolution of the expectation values of global spin observables. Our results show that the limit $N\to \infty$ does not commute with the long-time limit: while for any finite $N$ the average over trajectories is expected to converge towards a trivial steady state, in the thermodynamic limit a MIPT appears. The transition is not affected by postselection issues, as it is already visible at the level of ensemble averages. We derive a quantitative theoretical picture explaining the nature of the transition within our semiclassical approach.

The superconducting properties of uniformly hole-doped perovskite hydride ${\mathrm{KMgH}}_3$ with varying doping concentration and lattice parameter corresponding to different pressures were investigated from first principles. The superconducting transition temperature ($T_{\mathrm{c}}$) was predicted from the density functional theory for superconductors (SCDFT), where the effects of lattice anharmonicity and spin-fluctuation were considered and examined. Although lattice anharmonicity tends to suppress superconductivity around the edge of dynamical stability, where the phase is stabilized due to anharmonic effects, $T_{\mathrm{c}}$ is enhanced. In the hole-doped ${\mathrm{KMgH}}_3$, substantial spin-fluctuation (SF) effects were discovered, which counter the phonon-mediated pairing and decrease $T_{\mathrm{c}}$. Such anomalously strong SF is evaluated for similar hydrides, where the hydrogen $1s$ bands are isolated at the Fermi level, and its correlation with the electronics density of states was explored.

We present a theory of SNS junctions, where a normal metal is in contact with two superconductors, along the crossover from the BCS to the BEC regime. Such systems can be realized experimentally by gate-controlled doping of thin film superconductors. We show that Andreev levels exist even in the BEC regime, as long as there is sufficient density of states in the normal metal. We derive the Josephson current through one-dimensional Sharvin contacts. We find that the Josephson current at zero temperature has no peak along the BCS-BEC crossover at $T=0\text{K}$ unless the order paramter $\mathrm{\Delta }$ is peaked, as it is the case for superconductors with one-dimensional density of states. At finite temperature, however, we find that the Josephson current is maximal at the BCS-BEC crossover for any dimension. We identify the mechanism for this phenomenon to be a decrease in the number of Andreev levels, accompanied by an increase in excitation energy to the unoccupied levels, along the BCS-BEC crossover. This makes it less likely that these states are thermally occupied. As a consequence, the Josephson current is more strongly reduced on the BCS side of the crossover, resulting in a maximal Josephson current at the BCS-BEC crossover. Furthermore, we find that the current phase relation changes along the BCE-BEC crossover with a slope which has a pronounced maximum on the BEC-side of the BCS-BEC crossover. This maximum becomes larger and sharper as the temperature is lowered.

The high-pressure strength of solid platinum is studied at the National Ignition Facility via Rayleigh-Taylor ripple growth under ramp compression. Laser-driven experiments reached pressures up to 430 GPa, collecting velocimetry and radiography data. Despite no phase transformation, the observed ripple growth aligns with hydrodynamic simulations using an amplified Steinberg-Guinan model, resulting in a 200-fold increase in strength. Molecular dynamics simulations of platinum ramp compression yield similar von Mises stress at comparable high pressure and high strain rate conditions. These findings represent the highest-pressure strength measurements ever achieved for platinum and offer critical insight into its significant strengthening behavior under high pressure and strain rate conditions.

The high-pressure behavior of silicon telluride (${\mathrm{Si}}_2{\mathrm{Te}}_3$), a two-dimensional (2D) layered material, was investigated using synchrotron x-ray powder diffraction in a diamond anvil cell to $11.5\mathrm{GPa}$ coupled with first-principles theory. ${\mathrm{Si}}_2{\mathrm{Te}}_3$ undergoes a phase transition at $<1\mathrm{GPa}$ from a trigonal to a hexagonal crystal structure. At higher pressures ($>8.5\mathrm{GPa}$), x-ray diffraction showed the appearance of new peaks possibly coincident with a new phase transition, though we suspect ${\mathrm{Si}}_2{\mathrm{Te}}_3$ retains a hexagonal structure. Density functional theory calculations of the band structure reveal metallization above $9.1\mathrm{GPa}$ consistent with previous measurements of the Raman spectra and disappearance of color and transparency at pressure. The theoretical Raman spectra reproduce the prominent features of the experiment, though a deeper analysis suggests that the orientation of Si dimers dramatically influences the vibrational response. Given the complex structure of ${\mathrm{Si}}_2{\mathrm{Te}}_3$, simulation of the resulting high-pressure phase is complicated by disordered vacancies and the initial orientations of Si–Si dimers in the crushed layered phase.

Transition metal oxides, with their wide range of electronic and magnetic properties, offer a remarkable platform for developing future electronics based on unconventional quantum phenomena, such as topological phases. The formation of topologically nontrivial states is linked to crystalline symmetry, spin-orbit coupling, and magnetic ordering. Here, by employing angle-resolved photoemission spectroscopy (ARPES), supported by density functional theory (DFT) calculations, we demonstrated that intrinsic octahedral rotations in ${\mathrm{SrNbO}}_3$ films drive the emergence of non-trivial band topology. Specifically, ARPES reveals and diffraction data confirm the presence of in-phase $a^0{a}^0{c}^{+}$ octahedral rotation, leading to the formation of topologically protected Dirac band crossings, giving rise to massless fermions in this system. Our study underscores the pivotal role of structural distortions in transition metal oxides, illustrating how they can be strategically harnessed to unlock and stabilize quantum topological states. This approach contributes to the broader understanding of quantum materials and their promising applications in advanced technologies.

Understanding individual electronic orbitals involved in chemical bonding through scanning transmission electron microscopy and electron energy loss spectroscopy is essential for advancing materials science. Although previous theoretical and experimental results show the anisotropic image contrast of the corresponding particular final-state, the interpretation of these contrasts remain unclear. Here, we demonstrate the experimental anisotropic atomic contrast in the mapping of the fine structure of the O $K$- and Ti $L_{2,3}$-edges of rutile $\mathrm{Ti}{\mathrm{O}}_2$, and propose a qualitative theoretical explanation. Our results revealed that the contrast does not directly reflect the electron density of the final state orbital, but rather the direction in which the selected transition matrix element is reflected. And its direction sometimes corresponds to that of selected orbital but not always so. The proposed interpretation method would be used for the analysis of future experimental data.

We investigate the spin-valley polarization and tunnel magnetoresistance (TMR) in periodic magnetic ${\mathrm{MoS}}_2$ superlattices in the presence of an electrostatic potential. These superlattices could be the basis for devices with spin-valley polarization and magnetoresistive capabilities or versatile devices. The periodic magnetic structure is generated through regions A and B, with each region constituting a ferromagnetic strip and a normal region. Region A plays the role of a hard region with a fixed magnetization, while region B is the soft region with variable magnetization. An electrostatic potential is also applied in the ferromagnetic regions. In the case of symmetric magnetic ${\mathrm{MoS}}_2$ superlattices, we find two well-defined spin-valley polarization states that are accessible by modulating the electrostatic potential when the magnetization between regions A and B is antiparallel. We also find perfect positive TMR in the same energy region as the two well-defined spin-valley polarization states. By inducing structural asymmetry, two well-defined spin-valley polarization states that are accessible by switching the magnetization direction and appropriately tuning the electrostatic potential are possible. Asymmetric magnetic ${\mathrm{MoS}}_2$ superlattices also present regions of perfect positive and negative TMR compatible with the regions of spin-valley polarization.

In recent years, analog quantum simulators have reached unprecedented quality, both in qubit numbers and coherence times. Most of these simulators natively implement Ising-type Hamiltonians, which limits the class of models that can be simulated efficiently. We propose a method to overcome this limitation and simulate the time evolution of a large class of spinless fermionic systems in one dimension (1D) using simple Ising-type Hamiltonians with local transverse fields. Our method is based on domain wall encoding, which is implemented via strong (anti)ferromagnetic couplings $\left|J\right|$. We show that in the limit of strong $\left|J\right|$, the domain walls behave like spinless fermions in 1D. The Ising Hamiltonians are one-dimensional chains with nearest-neighbor and, optionally, next-nearest-neighbor interactions. As a proof of concept, we perform numerical simulations of various 1D fermionic systems using domain wall evolution and accurately reproduce the systems' properties, such as topological edge states, Anderson localization, quantum chaotic time evolution, and time-reversal symmetry breaking via Floquet engineering. Our approach makes the simulation of a large class of fermionic many-body systems feasible on analog quantum hardware that natively implements Ising-type Hamiltonians with transverse fields.

The electronic structure of pure niobium and the niobium-titanium alloy ${\mathrm{Nb}}_{0.44}{\mathrm{Ti}}_{0.56}$ in the bcc phase at pressures up to $250\mathrm{G}\mathrm{Pa}$ is investigated, to reveal possible factors conducive toward the robust superconductivity reported for Ti-doped niobium upon a considerable volume reduction. We model the structural disorder using the coherent potential approximation, and the electronic correlations are taken into account using dynamical mean-field theory. At high pressure, a significant change in the topology of the Fermi surface is observed, while electronic correlations weaken with increasing pressure. Thus, the normal state of ${\mathrm{Nb}}_{0.44}{\mathrm{Ti}}_{0.56}$ is found to be a Fermi liquid with a well-defined Fermi surface and well-defined quasiparticles near it. The systematic study of the impact of disorder upon the Fermi surface at such ultrahigh pressures allows notable insights into the nature of the electronic states near the Fermi level. Furthermore, our results clearly indicate the necessity of further experimental Fermi surface explorations.

This study explored the changes in the electronic structure due to ferroelectric phase transition behavior of ${\mathrm{BaTiO}}_3$. The temperature variations in the electronic structure of a ferroelectric ${\mathrm{BaTiO}}_3$ crystal were investigated via hard x-ray photoemission spectroscopy (HAXPES) using linearly polarized x rays and density functional theory calculations. The observed valence band HAXPES spectra exhibited distinct temperature-dependent shapes owing to the crystalline phase transitions from a high-temperature paraelectric cubic phase to low-temperature ferroelectric phases with tetragonal, orthorhombic, and rhombohedral symmetries. The changes in the valence band spectra agreed with the simulated HAXPES spectra derived from the projected densities of states in the crystalline phases multiplied by the photoionization cross sections. This result suggests that the ferroelectric mechanism in ${\mathrm{BaTiO}}_3$ is of the displacive type, which involves structural phase transformations.