In recent years, Born-Markov master equations based on tracing out the electromagnetic degrees of freedom have been extensively employed in the description of quantum optical phenomena originating from photon-mediated interactions in quantum emitter ensembles. The breakdown of these effective models, built upon assumptions such as ensemble spectral homogeneity, an unstructured photonic density of states, and weak light-matter coupling, has also recently attracted considerable attention. Here, we investigate the accuracy of this well-established framework beyond the most conventional, and extensively explored, spontaneous emission configuration. Specifically, we consider a system comprising two coherently driven and detuned quantum emitters, embedded within a hybrid photonic-plasmonic cavity, formed by a metallic nanorod integrated into a high-refractive-index dielectric microresonator. The local density of photonic states in this structure exhibits a complex frequency dependence, making it a compelling platform for exploring photon-mediated interactions beyond the assumptions above. We benchmark this modeling approach for the quantum dynamics of the emitter pair against exact calculations based on a macroscopic field quantization formalism, providing an illustrative assessment of its validity in significantly structured and dispersive photonic environments. Our analysis reveals four distinct regimes of laser driving and frequency splitting that lead to markedly different levels of accuracy in the effective model.

We demonstrate that the thermal radiation between subwavelength membranes of silicon nitride exhibits a minimum enhancement over the blackbody limit, for a given separation distance. Using fluctuational electrodynamics simulations, we show that this minimum appears for membranes' separation distances corresponding to the transition between the far- and near-field regimes. The observed minimum results from the weak contributions of both the evanescent and propagating electromagnetic modes, and its values become lower for higher temperatures and thicker membranes. Outside this transition, the radiative enhancement increases and saturates for sufficiently long separation distances. Notably, the difference between the minimum and saturation values diminishes as the membranes' thickness increases. The far-field plateau of radiative enhancement is primarily governed by the coupling of surface phonon-polaritons propagating along the top and bottom surfaces of sufficiently thin membranes, which expands the effective emission cross-section area beyond the membranes' geometric one. Our findings thus show that while subwavelength polar membranes enable us to significantly enhance the far-field thermal radiation, this enhancement is limited in the far-to-near field transition. This observation highlights the necessity of optimizing both the separation distance and membrane properties to enhance the radiative heat transfer between membranes.

The quantum anomalous Hall effect (QAHE), achieved through in-plane magnetization or characterized by a high Chern number, provides an opportunity to examine unconventional quantum states. In addition, it offers theoretical guidance for the development of spintronic devices with low energy consumption. Based on crystal field theory, we propose a general design principle for constructing in-plane-magnetized QAHE systems, and systematically analyze the mechanism by which chemical functionalization of two-dimensional (2D) materials induces this effect. Using first-principles calculations, we further predict that a MnSeCl monolayer can intrinsically host in-plane-magnetized QAHE, whose nonzero Berry curvature and chiral edge states unambiguously characterize its nontrivial topological nature. Moreover, our study reveals that tuning the in-plane magnetization direction enables Chern number switching under threefold rotational symmetry ($C=\pm 1$). When the magnetization is rotated out of plane, the system transitions into a high-Chern-number ($C=\pm 3$) QAH state. Wannier tight-binding calculations confirm the essential results and clarify their physical origin. In addition, chemical functionalization provides an effective route to realize in-plane QAHE in 2D systems and enables high-temperature platforms with tunable Chern numbers for low-dissipation spintronic applications.

The Kondo effect originates from the spin exchange scattering of itinerant electrons with a localized magnetic impurity. Here, we consider generalization of Weyl-type electrons with their spin locked on a spherical Fermi surface in an arbitrary way and study how such spin-momentum locking affects the Kondo effect. After introducing a suitable model Hamiltonian, a simple formula for the Kondo temperature is derived with the second-order perturbation theory, which proves to depend only on the spin averaged over the Fermi surface. In particular, the Kondo temperature is unaffected as long as the average spin vanishes, but decreases as the average spin increases in its magnitude, and eventually vanishes when the spin is completely polarized on the Fermi surface, illuminating the role of spin-momentum locking in the Kondo effect.

Altermagnets, recently identified as a third class of collinear magnetic materials, have attracted significant attention in condensed matter physics. Despite this growing interest, the realization of real Chern insulators in intrinsic altermagnetic systems has rarely been reported. In this work, based on first-principles calculations and theoretical analysis, we identify monolayer ${\mathrm{Fe}}_2{\mathrm{S}}_2\mathrm{O}$ and ${\mathrm{Fe}}_2{\mathrm{Se}}_2\mathrm{O}$ as a novel class of two-dimensional altermagnetic real Chern insulators. We demonstrate that these materials possess altermagnetic ground states and host a nontrivial mirror real Chern number, leading to the emergence of symmetry-protected zero-dimensional corner states. Notably, these corner modes are spin-polarized, giving rise to a unique spin–corner coupling effect. We further show that the real Chern insulating phases and their associated corner states remain robust against spin–orbit coupling, as well as under both uniaxial and biaxial strain. Additionally, these materials exhibit pronounced linear dichroism and strong optical absorption. Our findings uncover the novel topological character of ${\mathrm{Fe}}_2{\mathrm{S}}_2\mathrm{O}$ and ${\mathrm{Fe}}_2{\mathrm{Se}}_2\mathrm{O}$, establishing them as promising platforms for exploring real Chern insulators in altermagnetic systems.

In this work, we experimentally and theoretically study the dressed-state emission of the biexciton-exciton cascade in a semiconductor quantum dot under pulsed, resonant, two-photon excitation. Building on the well-characterized steady-state dressed emission of the four-level system, we examine its dynamic counterpart under pulsed, resonant excitation, addressing both experimental observations and theoretical modeling. Here we report several sidebands emerging from the biexciton-to-exciton transition, whose number and spectral width depend on the excitation pulse duration and the effective pulse area, while no sidebands emerge from the exciton-to-ground-state transition. Since the biexciton state population follows a nonlinear pulse area function, sidebands with a small spectral nonlinearity result. Detuning- and time-dependent measurements provide deeper insight into the emission properties of the dressed states. They show that side peak emission only occurs in the presence of the excitation pulse. Moreover, when the system is excited by a Gaussian-shaped laser pulse, side peak emission takes place sequentially.

An invisibility cloak has been realized across various physical fields underpinned by metamaterials, but is demanding for delicate microstructures and thus suffers from a sensitive working frequency. In this Letter, we propose a metamaterial-free approach to design an invisibility cloak on an elastic beam. It is realized by bonding two miniature blocks and a shell without damaging the stiffness and strength of the beam. The blocks can isolate the invisible area by completely reflecting flexural waves. Meanwhile, the shell, with a varying thickness and optimized joints, ensures a perfect redirection of waves around the cloaked region by preserving both amplitudes and phases from 4.9 to 13.1 kHz. We numerically and experimentally verify its functionality and robustness against a change of the obstacle and the cloaking shell. This work presents the possibility for wave/energy flux manipulation in mechanical systems, circumventing the need of complex artificial structures.

We investigated the Fermi-energy ($E_{\mathrm{F}}$) dependence of the photoluminescence (PL) of monolayer graphene. The steady-state PL spectra for $2|E_{\mathrm{F}}|=0.80$, 1.04, and 1.22 eV (the origin of electron energy was set at the Dirac point) showed a high-energy shift of the spectral edge (Fermi edge) with an increase in $2|E_{\mathrm{F}}|$. The PL decay curve at a PL photon energy ($E_{\mathrm{PL}}$) of 1.10 eV was observed in the $E_{\mathrm{F}}$ range of +0.30 to −0.61 eV. It was found that the $E_{\mathrm{F}}$ dependence of the maximum intensity in each PL decay curve had the largest value at $E_{\mathrm{F}}=-0.55\pm 0.06\mathrm{eV}$, and $E_{\mathrm{PL}}\sim 2\left|E_{\mathrm{F}}\right|$. The observed results are discussed in terms of the Fermi edge singularity due to many-body electron-hole interactions.

Topological properties of quantum systems at finite temperatures, described by mixed states, pose significant challenges due to the triviality of the Uhlmann bundle. By inserting the density matrix into the Chern character, we introduce the thermal Uhlmann-Chern number, a generalization of the Chern number that reduces to the pure-state value in the zero-temperature limit and vanishes at infinite temperature, providing a framework to study the temperature-dependent evolution of topological features in mixed states. We provide, for the first time, a rigorous mathematical proof that the first- and higher-order Uhlmann-Chern numbers converge to the corresponding Chern numbers in the zero-temperature limit, differing only by a factor of $1/D$ for $D$-fold degenerate ground states. We demonstrate the utility of this framework through applications to a two-level system, the coherent state model, the 2D Haldane model, and a four-band model, highlighting the temperature-dependent behavior of topological invariants. Our results establish a robust bridge between the topological properties of pure and mixed states, offering new insights into finite-temperature topological phases.

Ferromagnetic (FM) semiconductors hold great promise for spintronics. However, sustaining FM order at room temperature remains a significant challenge. Conventionally, FM coupling in semiconductors is driven by superexchange, while is suppressed by direct-exchange interactions. The superexchange interactions are relatively weak, typically leading to low-temperature ferromagnetism. Here, we propose a contrasting mechanism that direct-exchange can, instead, promote FM coupling in semiconductors. Utilizing a tight-binding dimer model, we demonstrate that the FM coupling between two magnetic ions is enhanced by direct exchange, but suppressed by superexchange interactions, when one ion has less than half-filled orbitals (e.g., ${\mathrm{d}}^{<5}$) and the other more than half filled (e.g., ${\mathrm{d}}^{>5}$). Based on this mechanism, first-principles calculations predict room-temperature ferromagnetism in a class of double transition-metal compound semiconductors, driven by ${\mathrm{d}}^{<5}\text{−}{\mathrm{d}}^{>5}$ direct-exchange interactions [e.g., ${\mathrm{d}}^3\left(\mathrm{C}{\mathrm{r}}^{3+}\right)-{\mathrm{d}}^8\left(\mathrm{C}{\mathrm{o}}^{+}\right)$ interactions in the ${\mathrm{CrCoCF}}_2$ monolayer]. These findings reveal a general mechanism for achieving room-temperature FM semiconductors, opening a promising avenue for spintronics.

We simulate the dynamical spin structure factor (DSSF) $\mathcal{S}(q,\omega )$ of the spin-1/2 Heisenberg antiferromagnetic chain using classical simulations. By employing Landau-Lifshitz Dynamics, we emulate quantum correlations through temperature-dependent corrections, including rescaling of magnetic dipoles and renormalization of exchange interactions. Our results closely match Quantum Monte-Carlo calculations for $k_{\mathrm{B}}T/J\gtrsim 1$, extending the applicability of classical dynamics to the challenging case of gapless excitations. At higher temperatures, our simulations comply with general predictions for uncorrelated paramagnetic fluctuations in the infinite temperature limit. Entanglement witnesses derived from the quantum-equivalent DSSF act as sensitive diagnostics for the quantum-to-classical crossover. Their reliability stems from their dependence on spectral features alone, enabling classical dynamics to emulate quantum thresholds without genuine entanglement. This framework also reproduces transverse spin correlations in finite magnetic fields, in agreement with quantum simulations. Together, our results establish quantum-corrected classical dynamics as a scalable and predictive tool for interpreting scattering experiments and exploring quantum correlations in strongly correlated spin systems.

Three-wave scattering is a fascinating phenomenon with many applications in various technologies. Reducing the system symmetry greatly affects three-wave scattering, which, in this case, goes beyond the simple momentum conservation law. In this study, we examine three-magnon scattering at the edge of a thin ferromagnetic film, when a bulk spin wave interacts with an edge-localized propagating spin wave upon the reflection. This creates new bulk spin waves at mixed frequencies by means of three-magnon confluence or stimulated splitting processes. Using our developed analytical theory, which has been confirmed by full micromagnetic simulations, we demonstrate that the amplitude of the wave generated in the stimulated splitting process is several times larger than that generated in the confluence process. This is primarily due to the lower group velocity of the wave formed in the splitting process than in confluence. Furthermore, the intensity of inelastically scattered waves exhibits a pronounced dependence on the incidence angle and frequency of the edge spin wave that goes beyond existing qualitative models. We show that the observed behaviors can only be explained by taking into account that the scattered waves are created by several elementary three-magnon processes involving the incident and reflected waves. The complex nature of the scattered wave creation results in a strong sensitivity of its amplitude to the phase accumulation of spin waves upon reflection.

We apply the fermionic tensor network (TN) state method to understand the strongly correlated nature in a doped Mott insulator. We conduct a comparative study of the $\sigma t\text{−}J$ model, in which the no-double-occupancy constraint remains unchanged but the quantum phase string effect associated with doped holes is precisely switched off. Thus, the ground state of the $\sigma t\text{−}J$ model can serve as a well-controlled reference state of the standard $t\text{−}J$ model. In the absence of the phase string, the spin long-range antiferromagnetic (AFM) order is found to be essentially decoupled from the doped holes, and the latter contribute to a Fermi-liquid-like compressibility and a coherent single-particle propagation with a markedly reduced pairing tendency. In contrast, our TN calculations of the $t\text{−}J$ model indicate that the AFM order decreases much faster with doping and the single-particle propagation of doped holes gets substantially suppressed, concurrently with a much stronger charge compressibility at small doping and significantly amplified Cooper pairing tendencies. These findings demonstrate that quantum many-body interference from phase strings plays a pivotal role in the $t\text{−}J$ model, mediating long-range entanglement between spin and charge degrees of freedom, which might hold the key mechanism for high-$T_c$ cuprates.

Experiments have evidenced proton pairing in Y-doped ${\mathrm{BaZrO}}_3$. However, the nature of proton pairing and its impact on conduction remain insufficiently understood theoretically. Here, through quantitative computational analysis of proton-proton interactions in Y-doped ${\mathrm{BaZrO}}_3$, we identify lattice distortion mediated elastic interaction as the key factor determining whether two protons form a stable pair or exhibit net repulsion. When a proton resides at an inward-bending distortion site induced by another proton, the resulting net repulsive interaction leads to an unstable configuration. In contrast, the proton tends to be trapped at a nearby outward-bending site that favors the formation of a stable proton pair. Moreover, the site where the two protons form the lowest-energy configuration also corresponds to a proton trapping site. By calculating the long-range diffusion pathways accessible to protons under different local environments in both single- and two-proton cases, we find that the range of rate-limiting barriers is 0.24–0.45 eV for two-proton conduction and 0.19–0.39 eV for single-proton conduction. The higher and more experimentally consistent barriers in the two-proton pathways indicate that the proton trapping effect induced by pairing hinders proton conduction. Our study elucidates the multiproton diffusion mechanism, providing a theoretical foundation for the experimental design of electrolytes with enhanced proton conductivity.

Recent studies have reported a puzzling phenomenon in non-Hermitian systems: topological zero eigenmodes (TZMs) appear and disappear as the system size varies, seemingly contradicting the inherent robustness of topology. This work demonstrates that these observations do not signify genuine topological phase transitions but instead stem from a transformation between “visible” and “hidden” zero eigenmodes. The latter are extremely long-lived excitations that are absent in the finite-system eigenspectrum yet exhibit nontrivial character and topological behavior distinct from the trivial phase. We show that the topological invariants—the winding numbers—precisely predict the number of protected zero singular values, which equals the total number of stable topological hidden and visible zero eigenmodes. We further demonstrate that two coupled nonreciprocal chains—such as coupled Su-Schrieffer-Heeger (SSH) models and nonreciprocal superconducting nanowires—provide a promising platform for detecting the hidden zero eigenmodes, differentiating them from the trivial phase, and unifying the bulk-boundary correspondence (BBC) for eigenvalues and singular values. The consistent behavior observed across these models establishes that size-induced TZMs originate from hidden zero eigenmodes, governed by a shared universal mechanism.

Similar to transitions in a range of correlated quantum materials, the valence transition exhibits a strong coupling to the crystal lattice, rendering it highly sensitive to stress tuning. In the present work, we determine the effect of uniaxial stress, which breaks the lattice symmetry, on the valence transition temperature and its crossover temperature in pure and Ag-substituted ${\mathrm{YbInCu}}_4$. Our key result is that hydrostatic stress is more effective in tuning this transition than uniaxial stress. Based on a symmetry decomposition of the stress-induced strains, we argue that this observation can be quantitatively understood, given that the valence transition is mostly sensitive to symmetric strains and thus volume changes of the lattice. These results support the notion that the valence transition can give rise to critical elasticity close to its critical endpoint.

Efficient heat dissipation across metal–insulator interfaces is critical for the performance and reliability of nanoscale electronic devices. However, the microscopic mechanisms that govern interfacial thermal resistance remain poorly understood, particularly the role of carrier nonequilibrium near the interface. This study investigates how the nonequilibrium distribution of heat carriers influences the metal-Si interfacial thermal resistance. We solve the electron-phonon coupled Boltzmann transport equations with carrier energy dispersion and scattering rates from first principles for three interfaces: Al-Si, Cu-Si, and Au-Si. The carrier nonequilibrium near the interface causes significant thermal resistance in both metal and Si leads, but with different characteristics. In the metal leads, the nonequilibrium exists in a very short distance (∼20 nm) from the interface, but the local thermal resistivity is significantly high, even comparable to those of thermal insulators (∼1 m-K/W). In Si leads, the degree of nonequilibrium is lower compared to the metals but persists over 1–2 µm due to the longer mean free paths of phonons. These factors together make both metals and Si exhibit significant thermal resistance from the carrier nonequilibrium, contributing 30–50% of the total interfacial thermal resistance. Notably, we find that the degree of phonon nonequilibrium and its relaxation length in Si is strongly influenced by the paired metal due to the varying interfacial spectral transmissivity. We further show that the nonequilibrium resistivity cannot be well described with the simple assumption that the carriers with mean free path less than a characteristic length do not contribute to thermal transport. We also evaluate the impact of electrons on interfacial thermal transport and reveal their critical role in shaping carrier nonequilibrium in metals. Lastly, our simulations of Al-Si and $\mathrm{Al}\text{−}{\mathrm{Si}}_{0.99}{\mathrm{Ge}}_{0.01}$ interfaces show substantially different resistance despite nearly identical phonon dispersions of Si and ${\mathrm{Si}}_{0.99}{\mathrm{Ge}}_{0.01}$. This confirms that the difference of experimentally measured interfacial thermal resistance for the two interfaces [R. B. Wilson et al., Nat. Commun. 5, 5075 (2014)] originates from nonequilibrium effects.

Topological excitons are superpositions of electron-hole pair states, characterized by an envelope function with finite vorticity in momentum space. This vorticity is determined by the underlying topology of the electronic bands. We derive the optical selection rules for topological excitons in flat bands, considering different topological two-band models: a family of Hamiltonians with skyrmion pseudo-spin textures, the flattened BHZ model for a single spin, and the flattened Haldane model. We derive the selection rules for these three models accounting for short-range interactions. We also consider the nonhydrogenic spectrum of excitons in the single-spin flattened BHZ model with Coulomb interactions. We show that, for the case of two flat bands with skyrmion pseudo-spin textures, all excitons are bright, and the handedness of the light that couples to them is the same as the vorticity of the pseudo-spin texture. For the single-spin flattened BHZ model, we show that bright excitons couple to circularly polarized light, regardless of the range of the interactions. In the flattened Haldane model, topological excitons couple to elliptically polarized light. We obtain the phase diagram for the polarization of light in this model as a function of microscopic parameters of the Hamiltonian. Our results demonstrate how band topology affects exciton properties, offering a framework for predicting light-matter interactions in topological materials with flat bands.