We develop a general theory of frustration-free free-fermion systems and derive the necessary and sufficient conditions for such Hamiltonians. Assuming locality and translation invariance, we find that any band touching between the valence and the conduction bands is always quadratic or softer, which rules out the possibility of describing Dirac and Weyl semimetals using frustration-free local Hamiltonians. We further construct a frustration-free free-fermion model on the honeycomb lattice and show that its density fluctuations acquire an anomalous gap originating from the diverging quantum metric associated with the quadratic band-touching points. Nevertheless, an $O(1/L^2)$ finite-size scaling of the charge-neutral excitation gap can be verified even in the presence of interactions, consistent with the more general results we derive in an accompanying work [Masaoka et al., Phys. Rev. B 113, 195120 (2026)].

Thermodynamic phase transitions typically involve a transition in the microscopic ordering between a less and a more ordered state. After transitioning into an ordered state, localized structures known as defects may remain, which disrupt the overall macroscopic order. Kibble-Zurek theory predicts that for any second-order phase transition the density of defects that form should be determined by the scaling law for the system coherence time and the phase transition quench time. We have performed measurements of sound dissipation due to vortex mutual friction in thin channels of superfluid ${}^3\mathrm{He}$ where one spatial dimension is smaller than a characteristic length scale predicted by the Kibble-Zurek theory. In this regime, we find that the density of defects is not determined by quench time, but can be estimated by replacing the Kibble-Zurek length scale with a truncated length set by the channel size. This results in significantly higher defect densities than in a bulk system.

For electronic devices based on two-dimensional semiconductors (2DSs), few-layer transition-metal dichalcogenides offer enhanced electrical performance over their monolayer counterparts. Metal-semiconductor junctions (MSJs), which lie at the heart of such devices, incorporate not only a metal-semiconductor (M-S) interface but also multiple van der Waals semiconductor-semiconductor (S-S) interfaces when few-layer 2DSs are employed. However, the role of S-S interfaces within MSJs—particularly their interplay with the M-S interface and their potential device applications—remains poorly understood. Here, based on density-functional theory calculations, we conduct a systematic investigation of MSJs based on few-layer 2DSs ($\mathrm{Mo}{\mathrm{S}}_2$ and InSe) contacted with both two- and three-dimensional metals. We find layer-dependent band offsets between adjacent 2DS layers, which can change sign depending on the proximity to the metal contact. This spatial variation directly results in quantized band bending that deviates from classical models. These phenomena are attributed to the interplay and competition among three effects: (1) the decay of interface dipole with distance from the M-S interface, leading to layer-resolved band bending; (2) the interlayer quasibonding mixes up the energy levels between different 2DS layers, thereby affecting the band bending; and (3) screening by partially metallized 2DS layers within the junction, which can alter the direction of band bending. Leveraging these insights, we provide an understanding of the density-functional theory calculated Schottky barrier height values for few-layer 2DSs in MSJs. Additionally, we demonstrate that the interlayer band offset can suppress direct tunneling in vertical field-effect transistors based on 2DSs.

Time-reversal symmetry breaking is an important facet of controlling nonreciprocal responses. Here, we propose a method of photocontrol over superconducting-diode-like nonreciprocities, where time-reversal symmetry breaking is achieved via photon exchange with chiral cavity modes. We reveal the origin of the nonreciprocal superconducting response as the embedding of chirality in a many-body ground state through photon-induced orbital magnetization. With twisted bilayer graphene as an example, we demonstrate the general principles of photocontrol of diode responses, which are valid for a wide range of superconductors and cavity designs. The cavity control of superconducting nonreciprocities, particularly in the microwave regime, offers a noninvasive means of exploring functionalities in quantum circuits with ultrafast switching and on-chip integration. This control method can serve as an important contribution to the toolbox for nonreciprocal models in circuit quantum electrodynamics, primed to be harnessed for scalable and modular quantum devices.

We carried out nonresonant inelastic x-ray scattering measurements at the V $L_1$ edge to probe the local V $3d$ charge density in ${\mathrm{VO}}_2$ across the metal-insulator transition (MIT). Without relying on spectral calculations, we were able to integrate directly from the orientational dependence an image of the occupied $3d$ shell and extract quantitatively the orbital occupations of the V ions. We found that in the low-temperature insulating $M_1$ phase the V is highly polarized toward the $\sigma$ configuration. The orbital occupation undergoes significant redistribution upon transition into the metallic $R$ phase, leading to a more isotropic charge density. The massiveness of the orbital switching promotes the scenario in which electron correlations and lattice degrees of freedom are strongly coupled, so that the contribution of the lattice and the electronic part to the energetics must be treated on equal footing to model the MIT quantitatively.

Optical orientation enables the injection of spin-polarized electron and hole populations in III-V and group-IV semiconductors. In particular, absorption of circularly polarized light near the direct gap generates spin-oriented electrons in the conduction band with polarization up to 50%, while hole polarization, opposite to that of electrons, can reach 83%. In recent years, increasing attention has focused on the orbital angular momentum, as charge-to-orbital conversion in semiconductors has been shown to be significantly more efficient than charge-to-spin. This has opened possibilities for exerting torques on ferromagnets in a semiconductive platform and is motivating research on the generation of carrier populations with net orbital angular momentum. In this framework, here, we exploit a “full-zone” $\mathbf{k}·\mathbf{p}$ model to theoretically investigate the injection of orbital angular momentum in bulk Ge and GaAs by means of the absorption of circularly polarized photons. We show that the orbital polarization of holes considerably exceeds 100% for photon energies close to the direct gap of the semiconductors. These results offer a route for generating high orbital polarizations and suggest that semiconductors are a convenient platform for future development of orbitronic and opto-orbitronic devices.

We present a density-functional-theory (DFT) study of the effects of oxygen vacancies on the structural and electronic properties of vanadium dioxide $\left({\mathrm{VO}}_2\right)$. Our motivation is the reported suppression of the metal-insulator transition by oxygen vacancies and the lack of a clear consensus on its origin. We use the $\mathrm{DFT}+V$ method with a static intersite vanadium-vanadium interaction term, $V$, to calculate the properties of the oxygen-deficient metallic rutile and insulating monoclinic M1 phases of ${\mathrm{VO}}_2$ on the same footing. We find that oxygen vacancies induce local distortions in the M1 phase, but do not destroy the dimerization usually associated with the insulating behavior. In spite of this, we find that the M1 phase becomes metallic as a result of the partial filling of the conduction band due to a rigid-band-like doping effect.

We investigate one- and two-mode variants of the ${\mathbb{Z}}_3$-symmetric quantum Rabi model, which describe the interaction of a qutrit with one or two bosonic modes and are directly relevant for circuit-QED and spin-qudit platforms. We find a canonical transformation that allows one to obtain the spectrum of the ${\mathbb{Z}}_3$ Rabi model using perturbation theory in a magnetic field. We show that in a certain parameter regime (deep-strong coupling and a small magnetic field), the three lowest eigenstates become ${\mathbb{Z}}_3$ qutrit-boson cat states. In order to characterize these states, we introduce a joint qutrit-boson Wigner function and derive its closed-form expression for the qutrit-boson cat states. Numerical calculations across a wide range of coupling strengths show that the proposed Wigner function is a useful tool that allows one to unambiguously identify the states of the ${\mathbb{Z}}_3$ qutrit-boson cat states.

We develop a general framework for classifying fermionic dynamical systems with measurements using symmetry and topology. We introduce two complementary classification schemes based on the Altland-Zirnbauer tenfold way: (1) the many-body evolution operator (mEO) symmetry class, which classifies fermionic dynamics at the many-body level and naturally extends to interacting dynamics, and (2) the single-particle transfer matrix symmetry class, which classifies free-fermion dynamics at the single-particle level and connects to Anderson localization physics. In the free-fermion limit, we show that these two frameworks are equivalent via a dynamical bulk-boundary correspondence: The topology of the dynamical system's spacetime bulk determines the topology of the area-law-entangled steady-state ensemble living on its temporal boundary. Next, we prove that symmetry-invariant, postselection-free Gaussian measurements are realizable in only four of the ten mEO classes (A, AI, BDI, D); the remaining six require either postselection or interacting (non-Gaussian) measurements. Building on these results, we construct general postselection-free topological adaptive circuits that realize topological dynamical phases in any spatial dimension for the four admissible mEO classes. These circuits simultaneously provide a protocol for preparing and stabilizing free-fermion topological states in all ten symmetry classes. As a concrete demonstration, we construct and simulate $2+1\mathrm{d}$ adaptive circuits that realize mEO-class-A topological dynamics, steering toward a steady-state ensemble of Chern insulators in $\mathcal{O}\left(1\right)$ circuit depth. Finally, we numerically characterize topological phase transitions, dynamical domain-wall modes, and robustness to coherent noise, identifying finite error thresholds at which trajectory-resolved and trajectory-averaged quantities undergo distinct phase transitions.

We present the combination of a complex-time tensor-network impurity solver with an analytic continuation scheme based on exponential fitting as an efficient framework for single and multiorbital dynamical mean-field calculations. By performing time evolution along a complex-time contour, the approach balances computational cost with the difficulty of spectral recovery, offering greater flexibility than methods confined to the real or imaginary axis. By complementing the complex-time evolution with an exponential fitting scheme, we faithfully extract real-time information at negligible cost. The resulting method obtains high-resolution spectra at a significantly lower computational cost than real-time evolution, offering a promising tool for ab initio studies of strongly correlated materials.

Understanding interactions between excitons and correlated electronic states presents a fundamental challenge in quantum many-body physics. Here, we introduce a purely electronic model for the formation of exciton polarons in moiré lattices. Unlike conventional approaches that treat excitons as tightly bound bosonic particles, our model considers only electronic degrees of freedom, describing excitons as electron-hole bound states. Our findings reveal a pronounced renormalization of the polaron mass as a function of electron density, particularly near correlated insulators, consistent with recent transport experiments. Additionally, we predict an observable sign change in the effective polaron mass when increasing the electron density that can be measured in Hall-type experiments. Our purely electronic model provides a unified framework to investigate the formation and renormalization of exciton polarons in correlated states.

Simulation of mesoscopic nanostructures is a central challenge in condensed matter physics and device applications. First-principles methods provide accurate electronic structures but are computationally too demanding to simulate mesoscopic nanostructures. Though empirical band theories are efficient for the simulation of nanostructures, their validities are limited by the empirical parametrization that usually lacks the consideration of microscopic wavefunctions and do not hold the transferability. In this work, we propose a methodology that bridges these approaches, achieving first-principles-level reliability with computational efficiency through a tight-binding framework. Our approach starts with Wannier tight-binding (WTB) parameters from small nanostructures, which serve as training data for fitting. To remove the gauge freedom of Wannier functions that obscures size- and geometry-dependent parameter trends, we construct gauge-independent (GI) bases and transform the WTB model into a gauge-independent WTB (GI-WTB) model. This enables robust parameter fitting and prediction of parameter variations, yielding the parametrized GI-WTB (P-GI-WTB) model. Applied to ${\mathrm{MoS}}_2$ armchair-edge nanoribbons, the P-GI-WTB model shows excellent agreement with first-principles results and enables reliable simulations of sub-$\mu \mathrm{m}$-wide nanoribbons. This framework offers a scalable approach for predicting the electronic properties of realistic nanostructures at a computational cost far lower than that of conventional first-principles methods.

We investigate the dynamics of one-dimensional bright solitonlike state in exciton-polariton condensates under a parity-time (PT) symmetric periodic potential. For the static potential case, we demonstrate the formation of stable gap solitonlike state arising from the combined action of the potential and a composite Gaussian pump. Notably, these gap solitonlike state differ from those in the conventional nonlinear Schrödinger equation in two key ways. First, their existence and stability depend not only on the balance between nonlinearity and dispersion but also critically on the system's gain-loss balance. Second, their profiles (e.g., main-peak amplitude) can be directly controlled by the intensity of the composite Gaussian pump. Furthermore, when the PT-symmetric potential is designed to include a barrier with exponentially varying intensity, we observe that a stable incident bright solitonlike state exhibits controllable switching dynamics, in stark contrast to the unidirectional transmission seen in conservative systems. During this process, the bright solitonlike state transforms into a polariton wave, temporarily disappears, and subsequently re-emerges. This behavior is governed by the interaction between the PT-symmetric barrier and the intrinsic gain-loss mechanism of the system. Additionally, we propose an active control scheme based on nonresonant optical Gaussian pump pulses to precisely manipulate the disappearance time of the bright solitonlike state. Our findings establish a promising platform for soliton-based photonic devices in nonequilibrium systems.

${\mathrm{SrVO}}_3$ has long been considered a prototype for studying electronic correlations due to its simple electronic structure and coherent $t_{2g}$ quasiparticle band. However, the origin of the incoherent satellite at 1.5 eV binding energy seen in photoemission remains enigmatic, having been attributed to Hubbard physics, oxygen vacancies, or plasmonic coupling. Here, we investigate the $t_{2g}$ band and satellite feature in epitaxially strained thin films of ${\mathrm{SrVO}}_3$ using angle-resolved photoemission spectroscopy. We observe an increase in the quasiparticle effective mass due to tensile strain as a result of the narrowing of the corresponding bandwidth. While a low-energy kink at 60 meV indicates additional electron-phonon interactions, no strain-dependent behavior is detected. The increased effective mass coincides with a decrease in the plasma frequency from reported ellipsometry measurements and theoretical calculations. From this, we associate the plasma frequency with the quasiparticle-satellite energy separation that follows the same strain-induced changes. This consistency affirms the identification of the satellite feature as a replica band due to coherent coupling between quasiparticles and low-energy plasmons and emphasizes the role of dynamical screening as a key organizing principle in the electronic structure of correlated oxides.

The manipulation of single spins through spin-polarized tunneling opens pathways for quantum control at the atomic scale. We present a theoretical framework describing spin transfer, spin torques, and spin resonance in molecular quantum dots weakly coupled to magnetic electrodes. By deriving a Lindblad master equation from microscopic tunneling processes, we capture both coherent exchange interactions and dissipative spin torque effects within a unified approach. We analyze how charge transport through localized orbitals influences spin dynamics and show that modulating the tunneling rates in time can induce electron spin resonance. This framework is further extended to coupled spin systems, revealing how spin coherence and entanglement respond to local spin torques and highlighting sources of transport-driven decoherence. Our results provide a general model to interpret spin-resolved tunneling experiments and extend classical spin torque concepts into the quantum regime.

Quantum systems with strong long-range interactions are thought to robustly resist thermalization because their energy spectra organize into a discrete number of multiplets. We show that this nonthermal behavior is surprisingly fragile to an experimentally ubiquitous perturbation: a staggered magnetic field. Using self-consistent mean-field theory and exact diagonalization, we reveal that the energy spectrum, while composed of discrete subspaces or multiplets, collectively forms a dense spectrum. The equilibration time is independent of system size and depends only on the fluctuations in the initial state. For initial states at low to intermediate energies, the long-time average aligns with the $S^z$ sector-weighted microcanonical ensemble. However, for states in the middle of the spectrum, the long-time average depends on the initial state due to quantum scarlike eigenstates localized at unstable points in classical phase space. Our results can be readily tested on a range of experimental platforms, including Rydberg atoms or optical cavities.