We employ polarization-resolved Raman spectroscopy combined with first-principles calculations to study the sodium-ion lattice dynamics in a sodium zigzag ordered cobaltate compound ${\mathrm{Na}}_{0.5}{\mathrm{CoO}}_2$. We detect two sodium phonon modes at 129 and $157{\mathrm{cm}}^{-1}$ for the first time, and their mode frequencies are consistent with first-principles phonon calculations based on an orthorhombic unit cell. We find that they appear below around $T^{*}\sim 300\pm 50\mathrm{K}$ with large linewidth broadening, much lower than the sodium zigzag ordering temperature $T_{\text{S}}\sim 460\mathrm{K}$, and then narrow at lower temperatures. We interpret the sodium-phonon anomalies occurring at $T^{*}$ as a dynamical-to-static crossover involving mainly the motion of sodium ions. Our results suggest that the gradual freezing of the sodium ions and the well-defined static sodium-zigzag order below $T^{*}$ set the stage for the emergent electronic and magnetic orders in the ${\mathrm{CoO}}_2$ layer of ${\mathrm{Na}}_{0.5}{\mathrm{CoO}}_2$.

We present an exact solution of the two-electron problem on the Lieb lattice and analyze how the combination of a flat band, sublattice structure, and local interactions shapes the two-particle excitation spectrum. The separable form of the on-site interaction reduces the problem to a $3\times 3$ secular equation whose roots yield a triplet of discrete modes. At the $\mathrm{\Gamma }$ point these modes form bonding, nonbonding, and antibonding combinations of sublattice amplitudes, while at the $M$ point they become strictly site selective due to the vanishing of intersublattice correlations. Along high-symmetry lines, destructive interference suppresses the bandwidth of one branch to order ${\left(\right|t|/U)}^4$, providing a clear signature of flat-band locality. The interaction-induced redistribution of spectral weight is reflected in the two-electron density of states, where resonances evolve into discrete bound states that constitute the low-density precursors of the upper Hubbard band. The $\omega =0$ flat-band peak remains pinned by symmetry, but virtual transitions into the dispersive bands deplete its intensity and generate an asymmetric infrared continuum with a logarithmic divergence. Together, these results provide a complete and analytically controlled description of two-electron physics on the Lieb lattice and illustrate how flat-band singularities and local interactions conspire to produce discrete excitations, anomalously weak dispersion, and marginal infrared behavior.

Interaction effects are dramatically enhanced in flat-band systems due to quenched kinetics, facilitating the binding of single excitations into composite quasiparticles. In this work, we present a comprehensive study of spin polarons over the entire momentum and energy space within the Mielke-Tasaki model using projected exact diagonalization. We identify distinct low-energy spin polarons at momenta $q=0$ and $q=\pi$, and also find multiple high-energy branches of a spin polaron. It is demonstrated that the interaction-induced Hartree dispersion plays a decisive role in determining the momentum sector of low-energy spin polarons. Furthermore, by introducing a finite bandwidth, we unravel the underlying binding mechanisms: the formation of low-energy spin polarons is governed by the conventional virtual exchange mechanism, whereas the high-energy spin polarons arise from a joint effect of the effective attraction and virtual exchange. Our results suggest promising avenues for realizing spin polaron crystals and exploring novel superconducting pairing mechanisms in moiré materials such as twisted ${\text{WSe}}_2$ and ${\text{MoTe}}_2$.

Quasisymmetry (QS) provides a novel route to understand and control near-degeneracies, Berry curvature, optical selection rules, and symmetry-protected phenomena in quantum materials. Here we give physical interpretations of the emergence of QS operators across multiple material families. Using density functional theory and the $\mathit{k}·\mathit{p}$ formalism, we identify QS subspaces and calculate their representation matrices, quantifying the quasisymmetry via a metric $\epsilon$ that measures subspace invariance. For Sn/SiC and transition-metal dichalcogenide monolayers, QS corresponds to an emergent mirror symmetry, whereas in wurtzite crystals it manifests as an emergent spatial inversion. By contrast, for AgLa the QS appearing in avoided crossings is inherited from a nearby high-symmetry point rather than being an emergent lattice symmetry. Combining group-theoretical analysis and $\mathit{k}·\mathit{p}$ modeling, our results establish concrete physical pictures for QS and provide practical criteria to diagnose it in first-principles calculations.

Van der Waals heterostructures incorporating graphene have been an active area of research, both theoretical and experimental, due to their potential to yield devices with a wide variety of applications. In this paper, first-principles calculations are employed to investigate ${\mathrm{C}}_6{\mathrm{N}}_6/\text{graphene}$, $\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4/\text{graphene}$, and ${\mathrm{C}}_6{\mathrm{N}}_6/\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ two-dimensional van der Waals heterostructures. A systematic analysis of structural and thermodynamic stability, electronic, mechanical, and optical properties of semiconductor/metal (${\mathrm{C}}_6{\mathrm{N}}_6/\text{graphene}$ and $\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4/\text{graphene}$) and semiconductor/semiconductor $({\mathrm{C}}_6{\mathrm{N}}_6/\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4)$ interfaces is performed. Both semiconductor/metal heterostructures form $n$-type Schottky contacts, which can be converted into $p$-type Schottky or Ohmic contacts by tuning the external perpendicular electric field and the interlayer coupling. In the semiconductor/semiconductor ${\mathrm{C}}_6{\mathrm{N}}_6/\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ heterostructure, the valence and conduction band edges originate from distinct layers, resulting in a type-II band alignment that promotes efficient electron-hole separation. Furthermore, the band alignment can be effectively tuned between type-I and type-II by applying an external electric field and varying the interlayer distance. From the optical absorption spectra of the heterostructures, we concluded that the ${\mathrm{C}}_6{\mathrm{N}}_6/\text{graphene}$ and $\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4/\text{graphene}$ heterostructures exhibit an optical response across a wide frequency range, whereas the ${\mathrm{C}}_6{\mathrm{N}}_6/\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ heterostructure shows prominent activity primarily in the ultraviolet region. Using the $G_0{W}_0+\mathrm{BSE}$ approach, the exciton binding energies are also calculated for the gapped systems, namely, ${\mathrm{C}}_6{\mathrm{N}}_6$, $\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ monolayers, and their heterostructure $({\mathrm{C}}_6{\mathrm{N}}_6/\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4)$, yielding values of 1.01, 1.14, and 1.18 eV, respectively, highlighting strong electron-hole (e-h) interactions. Moreover, the band-edge analysis of ${\mathrm{C}}_6{\mathrm{N}}_6/\mathrm{hg}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ heterostructure further favors pronounced interlayer e-h coupling. These findings provide valuable insights into the design and tuning of two-dimensional van der Waals heterostructures, paving the way for advanced applications in nanoelectronics, optoelectronics, and photocatalysis.

Proximity effects between graphene and hybrid ferromagnetic and heavy metal structures offer a promising road for modifying graphene's electronic properties. Such systems have been proposed for spintronic applications due to their perpendicular magnetic anisotropy and significant Dzyaloshinskii-Moriya interaction. Here, we present experimental and theoretical study on electronic and magnetic properties of graphene intercalated by cobalt on Ir(111) single crystal. We have synthesized well-oriented graphene without rotational domains on a Co/Ir(111) system and analyzed the influence of the Co film thickness and the mutual arrangement of C and Co atoms on magnetism and spin-orbit coupling effects. We find that two types of graphene/[3 ML Co]/Ir(111) structures can be realized, exhibiting tilted (TMS) and nontilted (NTMS) magnetic configurations. For the TMS structure, spin-resolved ARPES measurements and density functional theory calculations reveal complex spin structure, consisting predominantly of an in-plane $\left(S_y\right)$ component with a smaller out-of-plane $\left(S_z\right)$ contribution. In contrast, the NTMS structure exhibited spin polarization dominated by the $S_z$ component, indicating out-of-plane magnetization. Our results demonstrate ferrimagnetic ordering of the carbon atoms' magnetic moments along with a tilt of the Co atoms' magnetic moments relative to the surface normal in the graphene/Co/Ir(111) system. Moreover, spin-orbit coupling and exchange interaction lead to valley-dependent widths of local band gaps and differences in the spin splitting of the hybridized C $2p$ and Co $3d$ states at the $\overline{K}$ and ${\overline{K}}^{\prime }$ points. These interactions play a crucial role in the formation of the observed electronic structure in graphene/Co/Ir(111).

Negatively charged silicon vacancy centers (${\mathrm{SiV}}^{-}$) in diamond exhibit excellent optical properties and, below $100\mathrm{m}\mathrm{K}$, excellent spin coherence, making them promising candidates for quantum technologies. However, the strain-induced inhomogeneous distribution of optical transition frequencies poses a challenge for scalability. We demonstrate electrical tuning of the ${\mathrm{SiV}}^{-}$ center zero-phonon lines using in-plane contacts to apply moderate electric fields up to $45\mathrm{M}\mathrm{V}/\mathrm{m}$. The second-order Stark shift exceeds $10\mathrm{G}\mathrm{Hz}$, which is of the same order of magnitude as the 15-GHz inhomogeneous distribution of ${\mathrm{SiV}}^{-}$ observed in emitters embedded in optical nanostructures such as photonic crystal nanocavities. Analysis of individual ${\mathrm{SiV}}^{-}$ centers shows significant variation in effective polarizabilities between defects indicating that the effective polarizability strongly depends on local parameters like strain. The observed effective polarizabilities are $\approx 3–25$ times larger than those of tin vacancy centers, which we attribute to valence band resonances that delocalize the $e_u$ wave functions. Photoluminescence excitation measurements reveal that optical linewidths increase moderately with applied electric field strength. Our results demonstrate that large electrical Stark shifts can overcome the inhomogeneous distribution of transition frequencies, representing a significant step toward scalable ${\mathrm{SiV}}^{-}$-based quantum technologies such as quantum repeaters.

We investigate the adsorption geometry, binding energetics, and electronic structure of the polycyclic aromatic hydrocarbons benzene, naphthalene, and anthracene on a monolayer of ${\mathrm{MoS}}_2$ using first-principles calculations. Structural relaxations are performed within density functional theory (DFT) including corrections for van der Waals interactions. Quasiparticle electronic structures are subsequently obtained using many-body perturbation theory employing the $\mathit{GW}$ approximation. All three molecules preferentially adsorb flat above sulfur sites of the ${\mathrm{MoS}}_2$ surface and form type-I heterojunctions with ${\mathrm{MoS}}_2$. Only anthracene exhibits weak hybridization with the ${\mathrm{MoS}}_2$ valence band maximum (VBM) and VBM-1, leading to a slight reduction of the ${\mathrm{MoS}}_2$ valence band splitting. The HOMO-LUMO (highest occupied molecular orbital, lowest unoccupied molecular orbital) gaps of the molecules are substantially reduced by the dielectric screening of the ${\mathrm{MoS}}_2$ substrate. These results highlight the asymmetric role of screening and hybridization in ${\mathrm{MoS}}_2$-organic hybrid systems and provide insight into band alignment and electronic structure engineering for molecularly functionalized two-dimensional semiconductors.

We theoretically study microwave absorption spectroscopy of fractional quantum Hall droplets in the presence of quasiparticle tunneling across a quantum point contact. This contact-free probe provides access to collective edge dynamics beyond conventional transport measurements. We develop a nonperturbative path-integral Monte Carlo approach that enables computation of the frequency-dependent response at finite temperature and for arbitrary droplet geometries and benchmark the method against analytical results in the weak-tunneling regime. We find that tunneling produces measurable shifts and broadening of resonance peaks, with systematic dependence on tunneling strength and device geometry. Such shifts and broadenings are not obtained in perturbative treatments acting directly on the response function but emerge when interaction-kernel effects are properly incorporated. Our results indicate experimentally accessible signatures of edge-mode interference and tunneling-induced renormalization of collective excitations and support the use of microwave spectroscopy as a quantitative probe of quasiparticle dynamics in mesoscopic quantum Hall structures.

Non-Hermitian skin effects (NHSEs) enable dramatic boundary accumulation of waves, yet their experimental realization typically demands engineered nonreciprocity or spatially patterned loss. Here we demonstrate theoretically and experimentally that uniform loss provides a simple and previously overlooked mechanism for enforcing unidirectional-like edge transport in photonic crystals that breaks time-reversal symmetry in the presence of nonchiral edge states. Using a core-cladding geometry where domains share identical Chern numbers but possess distinct bulk polarizations, we show that uniform loss activates NHSEs that reshape the spectral topology of edge bands, giving rise to point-gap windings that dictate a one-way propagation. Near-field measurements confirm that loss converts intrinsically bidirectional interface states into a unidirectional-like circulation around the entire domain wall, showing excellent agreement with theory. Our results establish uniform loss as a universal and structurally simple route for achieving unidirectional-like wave transport.

The surface phonon Hall viscosity (PHV)—an acoustic analog of axion electrodynamics—emerges from the strain response of magnetic topological insulators and gives rise to novel acoustic phenomena. In this work, we propose a previously unexplored effect: a phonon polarization-filter mechanism induced by the surface PHV, which generates an interface phonon mode with its frequency below the bulk mode frequency. This interface mode possesses a specific circular polarization and therefore acts as a polarization filter, confining only phonons with the matching polarization at the interface. Magnetic topological insulators can thus selectively transmit one type of circularly polarized phonon mode, enabling the manipulation of phonon polarization and angular momentum. In addition, we further develop a generalized scattering framework to study the effect of an injected acoustic wave from a trivial insulator to a magnetic topological insulator with both normal and oblique incidence and discuss the phenomena of surface acoustic Faraday rotation and longitudinal-transverse mode conversion. Our results establish surface Hall viscosity as a powerful mechanism for engineering axial phonon states and open new avenues for topological phononic devices based on phonon angular momentum.

We investigate bias-point engineering of a quantum thermal transistor using a minimal set of thermal baths. A hot bath at temperature $T_1$ injects heat into a metallic conduction bar coupled at its far end to a qubit, while both the bar and the qubit exchange energy with an environment at temperature $T_e<T_1$. From a microscopic transport description of the bar, we derive a compact steady-state energy-balance model by identifying temperatures as node potentials and heat flows as branch currents. The resulting formulation gives closed-form expressions for the steady-state heat current and temperature drops across the bar-qubit network and determines the conditions for proper thermal biasing. We further show that conduction bars and baths can be arranged to function as tunable thermal potential dividers, known as quantum thermal links, enabling transistorlike control of the heat current delivered to the qubit terminal. Linearization about the bias point yields small-signal thermal analogs of Kirchhoff's current and voltage laws. These results provide a first step toward a circuit-based framework for the small-signal analysis and design of thermotronic devices.

We compare two complementary theoretical approaches to compute and interpret Floquet sidebands in periodically driven quantum materials: a first-order perturbative approach (first-order perturbative Born approximation, PB1) and time-dependent nonequilibrium Green's functions (tdNEGF). Using graphene as a model Dirac system, we disentangle in pump-probe setups Floquet-dressed initial states, Volkov-dressed final states (also known as laser-assisted photoelectric effect), and their interference. We quantify how photoemission matrix elements, polarization, incidence angle, and near-surface screening shape the momentum-resolved sideband intensity observed in tr-ARPES. PB1 yields an analytical expression for the momentum-dependent sideband intensity and for graphene it captures the correct symmetry trends, such as the magnitude of the intensities when considering the interference between the Floquet and the Volkov states and photoemission matrix elements. tdNEGF reproduces the full energy-momentum-resolved spectra, including hybridization gaps and spectral-weight redistribution. We find qualitative agreement between PB1 and tdNEGF once matrix elements are included; quantitative differences arise near hybridization regions and at specific angles where higher-order processes and self-energies are essential. Thus, for systems with simple band structures and away from these regions, the two approaches can be used in a complementary way.

Twist angle, interlayer coupling, and the dielectric environment affect electron-hole interactions in moiré transition metal dichalcogenide heterojunctions, thereby shaping exciton properties such as binding energy, peak position, and line shape. As a result, isolating the individual contributions of these factors to the excitonic response remains difficult. To address this problem, we combine first-principles calculations with physics-aware machine learning. We use monolayer ${\mathrm{MoS}}_2$ as a reference system to test whether screening corrections can be reliably transferred to excitonic spectra and then investigate how the dielectric environment and interaction scale affect exciton properties in the ${\mathrm{MoS}}_2/{\mathrm{MoSe}}_2$ moiré heterojunction. In monolayer ${\mathrm{MoS}}_2$, the screening-related intermediate quantity is predicted with an ${\mathrm{R}}^2$ of 0.998. Under a low-fidelity Bethe-Salpeter equation setting, the deviation of the main exciton peak from the reference is reduced to 9 meV. Prediction time for a single spectrum is reduced from 9 h to 1.5 s, corresponding to a speedup of approximately $2.1\times {10}^4$. We then apply the framework to the ${\mathrm{MoS}}_2/{\mathrm{MoSe}}_2$ moiré heterojunction. The mean absolute error of exciton binding energy prediction is ${10}^{-3}$ eV. A physics-based hybrid model reduces this error by 35% compared with a purely physical power-law baseline. These results clarify the roles of dielectric screening, interlayer coupling, and moiré potential in exciton spectra and give guidance for parameter exploration and structural design in moiré optoelectronic devices.

We investigate ballistic electron transport through a one-dimensional Rashba quantum ring coupled to two quantum point contacts and driven by an off-resonant high-frequency electric field. Based on the Floquet effective Hamiltonian and the scattering matrix formalism, we propose an all-optical spin modulator that operates without external magnetic fields. We demonstrate that the synergy between Floquet engineering and the resonant geometry of the ring enables a full spin-flip transition at significantly reduced driving intensities. This resonant amplification provides an efficient alternative to traditional spin-control methods. Analytical conditions for the transmission peaks are derived, showing that the high-frequency field acts as an “optical flux” that can double the spin conductance magnitude compared to conventional magnetic tuning. Our findings establish the Floquet-Rashba ring as a scalable, intensity-efficient building block for next-generation spintronic circuits. Notably, while the core spin-modulating and switching features operate efficiently at zero external magnetic field, we show that a weak static magnetic flux can be employed as a complementary diagnostic tool to tune and map the coexisting Floquet, geometric, and Aharonov-Bohm interference phases.

We report a well-resolved 0.7 conductance anomaly at $G=0.7\times (2e^2/h)$ in bilayer $\mathrm{graphene}/{\mathrm{WSe}}_2$ quantum point contacts. Proximity-enhanced spin-orbit coupling splits the fourfold ground state of bilayer graphene into well-separated spin-valley locked Kramers doublets. The anomaly emerges between these opposite spin-valley states. Despite fundamentally different band structure and wave function characteristics, the temperature and bias phenomenology closely mirror GaAs systems. In contrast, the parallel magnetic field response differs significantly, confirming the central role of valley degrees of freedom. This opens new pathways to study valley-exchange correlation physics in regimes inaccessible to conventional semiconductors.

Conventional wisdom holds that static disorder is indispensable for the integer quantum Hall effect, underpinning both quantized plateaus and the plateau-plateau transition. We show that pure dephasing, without elastic disorder, is sufficient to generate the same $\theta$-driven criticality. Starting from a Keldysh formulation, we derive an open-system nonlinear $\sigma$ model for class A with a topological $\theta$ term but no cooperon sector, and we demonstrate that nonperturbative instantons still govern a two-parameter flow of $({\sigma }_{xx},{\sigma }_{xy})$. Evaluating $\theta$ in a dephasing quantum anomalous Hall setting, we predict a quantum Hall critical point at ${\sigma }_{xy}=1/2$ with finite ${\sigma }_{xx}$—the hallmark of the integer quantum Hall universality class realized without Anderson localization. Boundary-driven simulations of the Qi-Wu-Zhang model with local dephasing confirm this prediction and provide an experimentally aligned protocol to extract the Hall ratio $\lambda ={\sigma }_{xy}/{\sigma }_{xx}$ from Hall potential maps. By establishing dephasing as a self-contained route to Hall criticality, our framework reframes plateau physics in open solid-state and cold-atom platforms, offering a practical diagnostic for topological transport in nonunitary matter.