Maximizing electromagnetic absorption in subwavelength structures is crucial for a wide range of engineering applications, from stealth technology to energy harvesting. Traditional approaches have focused on the design of sophisticated resonant structures, but less attention has been paid to the impact of the surrounding medium. In this work, we derive a universal and geometry-independent upper bound for the absorption cross section (ACS) of subwavelength structures immersed in arbitrary background media, quantitatively expressed as the inverse of the wavelength in the host medium. This theoretical framework further indicates that the bound diverges in the epsilon-near-zero (ENZ) limit, where the wave number of the background medium approaches zero. By leveraging waveguide-emulated plasmonics, we experimentally demonstrate an extreme ACS that surpasses the vacuum upper bound by more than tenfold via ENZ immersion. In ENZ media, the absorption can reach very large values and is accompanied by minimal local flow disturbance, suppressed scattering, and near-uniform energy distribution. ENZ immersion opens avenues for engineering applications, including ultraefficient sensors, advanced stealth coatings, and compact thermal management systems, significantly advancing the fields of microwave engineering, optics, and metamaterials.

The interplay between spin and orbital degrees of freedom gives rise to a variety of emergent phases in correlated $4d$ and $5d$ transition-metal systems. Among these materials, it is known that strong spin–orbit coupling (SOC) significantly alters Jahn-Teller (JT) effects, either by suppressing static distortions or promoting dynamic fluctuations, so that orbital polarization related to JT effects is reduced or even quenched. Here, by using a Matsubara lattice formalism, we investigate how orbital fluctuations are generated and propagated by local perturbations in spin-orbit coupled systems with no global polarization, corresponding to either orbitally disordered phases or dynamic JT regimes. By analyzing how such perturbations propagate through the correlated, spin-orbit-entangled ground state, we observe that they generate short-range orbital fluctuations that carry local polarization. We find that these dynamical fluctuations propagate to neighboring sites with a polarization orthogonal to the initial perturbation. Finally, we discuss to what extent such orbital fluctuations may be probed as excitations in spectroscopic measurements.

A puzzling phenomenon has been recently revealed in terahertz nonlinear optical response experiments: A third harmonic generation (THG) signal, identified only in the superconducting (SC) phase in a conventional BCS superconductor, is found to persist into a wide pseudogap regime in the underdoped cuprate, accompanied by a $\pi$ phase shift in the THG signal across the SC transition. In this paper, we offer a consistent understanding of such an unconventional phenomenon based on an emergent Higgs mode of the condensed holons in the doped Mott insulator. Specifically, in the lower pseudogap phase (LPP), where, although the holons still remain Bose-condensed, the SC phase coherence gets disordered by excited spinons, which induce vortexlike responses from the holon condensate. By coupling to such internal fluctuations described by a mutual Chern-Simons gauge theory, we show that an external electromagnetic field can indeed produce the optical THG response in both the SC and LPP states, which are distinguished by a $\pi$ phase shift with a substantial suppression of the THG signal in the latter regime at higher temperatures.

Symmetry and topology are essential principles in topological physics. Recently, the idea of subsymmetry-protected topology—where some of the original symmetries are broken while a remaining subset, called subsymmetries, continues to protect specific boundary states—has been developed. Here, we extend subsymmetry-protected topology to higher-order topological systems from second-order topological insulators to semimetals. By introducing a subsymmetry-protecting perturbation that acts on a single sublattice and selectively preserves specific topological boundary states, we track the evolution of these states and their topological features using numerical and analytical methods, and we show that state-resolved quadrupole moments diagnose which corner or hinge modes remain topological. As a representative example of a second-order topological insulator, we begin with the Benalcazar–Bernevig–Hughes model. We demonstrate that, under a subsymmetry-protecting perturbation, subsymmetry-protected corner states remain pinned at zero energy and maintain quantized state-resolved quadrupole moments. In contrast, corner states on subsymmetry-broken boundaries shift away from zero energy and lose their quantized character. We further extend this framework to a three-dimensional second-order topological semimetal, constructed by stacking second-order topological insulator layers, and analyze how second-order Fermi arc states—hinge-localized modes that link the projections of bulk Dirac points, in contrast to conventional surface Fermi arcs—evolve under a subsymmetry-protecting perturbation. While one second-order Fermi arc becomes dispersive and loses its quadrupolar character under a subsymmetry-breaking perturbation, the remaining second-order Fermi arcs retain chiral symmetry and preserve quantized quadrupolar characters. These findings demonstrate that subsymmetry-protected topology can manifest in both insulating and gapless phases, offering routes to engineering symmetry-resilient topological phases in electronic, photonic, and synthetic systems.

Projected branes are constituted by a small subset of sites of a higher-dimensional crystal, otherwise placed on a hyperplane oriented at an irrational or a rational slope therein, for which the effective Hamiltonian is constructed by systematically integrating out the sites of the parent lattice that fall outside such branes [Commun. Phys. 5, 230 (2022)]. Specifically, when such a brane is constructed from a square lattice, it gives rise to an aperiodic Fibonacci quasicrystal or its rational approximant (displaying emergent periodicity) in one dimension. In this work, starting from square lattice-based models for topological crystalline insulators, protected by the discrete fourfold rotational ($C_4$) symmetry, we show that the resulting one-dimensional projected topological branes encode all the salient signatures of such phases in terms of robust endpoint zero-energy modes, quantized local topological markers, and mid-gap modes bound to dislocation lattice defects, despite such linear branes lacking the $C_4$ symmetry of the original lattice. Furthermore, we show that such branes can also feature all the hallmarks of two-dimensional strong and weak topological superconductors through Majorana zero-energy bound states residing near their endpoints and at the core of dislocation lattice defects, besides possessing suitable quantized local topological markers. Finally, we showcase a successful realization of a square lattice-based second-order topological insulator with the characteristic corner-localized zero modes (protected by composites of nonspatial and crystalline symmetries) in its geometric descendant one-dimensional quasicrystalline or crystalline branes that generically feature a quantized localizer index, but support the endpoint zero-energy mode only at one of its endpoints when it passes through a corner of the parent crystal. Otherwise, topological endpoint modes at slightly higher energies (still separated from bulk states) appear at both ends of the brane, when they are slightly far from the square lattice corners. Possible engineered quantum and metamaterial-based platforms to experimentally harness our theoretically proposed topological crystalline and higher-order insulating as well as superconducting branes are discussed.

Flat photonic bands promise extreme control of light-matter interactions but are often degraded by loss and angular dispersion. We demonstrate robust polaritonic flatbands in ${\mathrm{WS}}_2$ dimer metasurfaces by strongly coupling an engineered quasibound state in the continuum ($q$-BIC) to intrinsic ${\mathrm{WS}}_2$ excitons. The photonic mode, realized via suppression of the first-order Fourier harmonic and originating from an anapolelike interference between electric dipole and magnetic quadrupole moments, forms an ultraflat band with an angle-conserved quality factor. Upon strong coupling, distinct upper and lower polariton branches emerge with a vacuum Rabi splitting of 190 meV. Crucially, both branches inherit the dispersionless character of the parent $q$-BIC, maintaining nearly constant resonance energy, linewidth, and amplitude over a wide angular range, even when anapole conditions are broken by material loss. This identifies a mechanism transition from interference-based to inheritance-based dispersion control, in which lattice-determined photonic dispersion and energy-selective coupling ensure resilience against loss. The resulting angle-invariant polaritonic response, compatible with substrate-supported implementations and external-cavity-free architectures, opens avenues for wide-angle sensing and ultralow-threshold polariton lasing, establishing a versatile platform for flatband engineering in the strong-coupling regime.

Owing to its unique characteristics, graphene is inherently susceptible to instabilities that can give rise to novel ordered phases. One representative example is the bond-density wave, which leads to Kekulé-distorted structure with alternating bond lengths, accompanied by modified electronic and phononic properties. Here, an effective distance-dependent framework is presented to analyze the associated coupling behavior between the electron and phonon degrees of freedom. Within this approach, the strength of the bond-resolved electron-phonon interaction is found to scale linearly with the electronic hopping, contributing to a uniform picture of this relationship in distorted graphene. The analysis also reveals that the Kekulé order induces a pronounced anisotropy in the coupling, leading to its nonuniform variation under bond-length modulation. These results provide microscopic insight into the electron-phonon interaction in Kekulé-ordered graphene, and suggest a potential route to tune this coupling and related properties through controlled spatial engineering.

Strain engineering is effective for tuning the electronic properties of Dirac materials via pseudomagnetic fields. However, current understanding of pseudo-Landau levels and pseudospin polarization is largely limited to graphene, leaving other complex hexagonal lattices with Dirac cones uncharted for quantum materials exploration. To address the above challenge, in this study, we develop a universal partial unitary transformation framework to derive a unified form of Dirac Hamiltonian, applicable to a wide range of complex hexagonal Dirac materials. Our results show that the strain-induced pseudomagnetic field has a universal analytical form in these lattices, while the sensitivity of its strength to strain is determined by intrinsic parameters including effective velocity and effective energy. The pseudo-Landau spectra predicted by our theory ($E\propto \sqrt{\left|n\right|}$) are in excellent agreement with numerical simulations by the tight-binding method. Furthermore, we clarify the underlying physical condition governing the sublattice polarization of the $n=0$ Landau level: the specific sublattice composition of pseudospin states. This insight explains the dramatic differences in polarization observed across various lattices. This work opens up a broader material platform for strain electronics and pseudospintronics, and provides a rich set of candidates and a firm theoretical basis for the design of future quantum devices.

We develop a general theory of radiative heat exchange between dipoles with time-modulated optical properties. This framework extends fluctuational electrodynamics beyond equilibrium by incorporating nonstationary correlations and memory effects induced by temporal modulation. Closed-form expressions for the heat currents in modulated many-body systems are obtained, together with a generalized Landauer-like formulation of the pairwise exchanges, where the transmission coefficient accounts for all inelastic frequency-conversion channels. Near-resonant modulation redistributes and amplifies thermal fluctuations across Floquet sidebands, acting as a parametric amplifier of thermal radiation and enabling active, frequency-selective control of nanoscale heat transfer.

While second-order nonlinear conductivities are extensively applied to detect the Néel vector direction of antiferromagnets, they are absent in $p$-wave magnets (PWMs), an unconventional type of magnet, due to their time ($\mathcal{T}$) reversal and $p$-wave band symmetry. Here, we extend the study of nonlinear charge transports to nonlinear spin response. We first derive a quantum formula to calculate the nonlinear spin polarization, which can be applied to a system with an alternating electric field and weak disorder. In PWMs, we find that the nonlinear spin polarization is greatly enhanced near the Dirac point where the mixed Berry curvature (BC) becomes significant due to strong interband coherence. The nonlinear spin response near the Dirac point is exclusively contributed by the $\mathcal{T}$-even component induced by mixed BC, excluding the $\mathcal{T}$-odd component induced by the mixed quantum metric, which is crucial in an altermagnet. Importantly, the nonlinear spin polarization heavily depends on the Néel vector direction and spin splitting parameter in PWMs. Thus, the $\mathcal{T}$-even nonlinear spin polarization is helpful for understanding the physic of mixed BC and further provides an alternate detection scheme of the Néel vector, rather than one based on the usual nonlinear conductivity.

Using stochastic series expansion quantum Monte Carlo and density matrix renormalization group methods, we investigate the ground-state phase diagram of the $S=1/2$ Heisenberg model on the two-dimensional square-hexagon-octagon (SHO) lattice. The model incorporates nearest-neighbor interactions $J_1$ (intrahexagon interaction) and $J_2$ (interhexagon), as well as a selected third-neighbor interaction $J_3$ along the $x$ direction. We identify five distinct phases in the parameter regime $0<{\lambda }_1=J_2/J_1<4,0<{\lambda }_2=J_3/J_1<4$: a Néel antiferromagentic phase, two dimer phases (orthogonal and ladder staggered dimers), a hexagon singlet phase, and notably a Haldane-like symmetry-protected topological (SPT) phase. The topological nature of the Haldane-like phase is confirmed by the degeneracy of the ground-state energy under open boundary conditions and the twofold degeneracy of the entanglement spectrum. Phase boundaries are accurately determined using finite-size scaling of the spin stiffness and Binder cumulant. Data collapse analysis reveals that all transitions from nonmagnetic phases to the antiferromagnetic phase belong to the three-dimensional $O\left(3\right)$ Heisenberg universality class. In addition, we investigate the robustness of the SPT phase to other interactions, such as those that could arise in experimental materials. Our work establishes a comprehensive theoretical framework for understanding magnetic and topological phases on the SHO lattice.

The Edelstein effect consists of the nonequilibrium accumulation of magnetization in response to an applied electric field in systems with broken inversion symmetry. While the spin Edelstein effect (SEE), originating from spin moments, is well established, its orbital counterpart, where magnetization arises from orbital moment, has only recently begun to attract attention. In this work, we investigate the orbital Edelstein effect (OEE) in gated monolayer transition‐metal dichalcogenides (TMDs) in the 2H structure, such as ${\mathrm{MoS}}_2$, by using first‐principles density‐functional calculations with both electron and hole doping. The gate‐induced broken mirror symmetry produces a Rashba-type chiral spin/orbital angular momentum texture, which in turn leads to the Edelstein effect in response to an applied in-plane electric field. We find that for electron doping the Edelstein response is dominated by the orbital channel, whereas for hole doping the orbital and spin contributions are comparable. For the case of hole doping, both OEE and SEE are strongly enhanced by a small amount of strain, due to strain-driven shifts between the $\mathrm{\Gamma }$ and $\mathrm{K}/{\mathrm{K}}^{\prime }$ valley energies. We derive analytical expressions for the spin and orbital Edelstein susceptibilities and evaluate their magnitudes from first‐principles. Remarkably, the predicted OEE in gated monolayer TMDs is an order of magnitude larger than values reported in previously studied systems. Our results identify TMDs as promising platforms for studying the orbital Edelstein effect and highlight their potential applications in spintronics devices.

The exchange coupling of the helical edge state of a quantum spin-Hall insulator with an easy-plane magnet has no effect on its dc electrical conductance if the magnet's anisotropy axis is aligned with the spin-quantization axis of the helical edge state [Meng et al., Phys. Rev. B 90, 205403 (2014)]. We here calculate the ac conductance $G_V\left(\omega \right)$ and the noise power $S_V\left(\omega \right)$ in the presence of a dc bias $V$. While both take the universal values $G_V\left(\omega =0\right)=e^2/h$ and $S_V(\omega =0)=4e^2{k}_{\mathrm{B}}T/h$ in the zero-frequency limit, $G_V\left(\omega \right)$ and $S_V\left(\omega \right)$ are quickly suppressed for finite $\omega$, so that low-frequency transport is effectively noiseless.

Ruddlesden-Popper (RP) nickelates are a promising class of high-temperature superconductors, with superconducting transition temperatures exceeding the boiling point of liquid nitrogen. However, oxygen nonstoichiometry remains a persistent challenge that commonly presents in all RP nickelates. Understanding the formation of oxygen vacancies, their ordering patterns, and their impact on superconductivity is crucial, especially in the newly discovered $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$. In this study, the first-principles structural calculations reveal the formation of in-plane oxygen vacancy chains in $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$, a key structural feature observed under both ambient and pressurized conditions. These vacancies induce significant lattice distortion and generate residual electrons that hybridize with the Ni ${d_z}_2$ orbital, altering the sign of the hopping integral between the ${d_z}_2$ orbitals. Furthermore, the vacancies alter the $\mathrm{N}{\mathrm{i}}^{2+}\text{/}\mathrm{N}{\mathrm{i}}^{3+}$ ratio, lower the ${d_z}_2$ orbital energy, and decrease the ${d_z}_2$ orbital density of states at the Fermi level. Interestingly, at higher vacancy concentrations, the vacancy chains tend to align diagonally along the out-of-plane direction. The phase diagram of $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ exhibits a narrow stability range at ambient pressure, which expands under applied pressure, aligning with the high-oxygen-pressure conditions required for its synthesis. Importantly, these vacancy chains broaden the optical conductivity peak, which could serve as a marker for their detection. Our findings offer valuable insights into the distribution of oxygen vacancies, their role in modifying the electronic structure, and their influence on optical conductivity in $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$.

We introduce an analytic slave-boson method for treating the finite-$U$ Anderson impurity model. Our approach introduces two bosons to track both $Q\rightleftharpoons Q\pm 1$ valence fluctuations and reduces to a single symmetric $s$ boson in the effective action, encoding all the high-energy atomic physics information in the boson's kinematics, while the low-energy part of the action remains unchanged across finite-$U$, infinite-$U$, and Kondo limits. We recover the infinite-$U$- and Kondo-limit actions from our approach and show that the Kondo resonance already develops in the normal state when the slave boson has yet to condense. We show that the slave rotor and $s$ boson have the same algebraic structure, and we establish a unified functional integral framework connecting the $s$-boson and slave-rotor representations for the single-impurity Anderson model.

Among the intrinsic ferromagnetic semiconducting lanthanide nitrides, samarium nitride (SmN) stands out for its combined ferromagnetic and superconducting behaviors at low temperatures. This has triggered an interest in developing a better understanding of the electronic structure of SmN utilizing both experimental and computational methods. In this study, we report an extensive in situ x-ray photoelectron spectroscopy and near-edge x-ray absorption study of the surface of samarium nitride compared with metallic samarium. Atomic multiplet calculations were completed to guide the interpretation of our experimental data. X-ray photoelectron spectroscopy depth-profiling provides evidence of a nonstoichiometric SmN surface, with the topmost atomic layers showing spectral features consistent with metallic Sm, including the presence of ${\mathrm{Sm}}^{2+}$. In addition, we show that samarium ions in the bulk layers of SmN remain trivalent, with no evidence of mixed valence.

We study the quantum hard-rods model and obtain compact analytical expressions for density form factors, and a semianalytical treatment for dynamic and static structure factors calculations, greatly reducing computational complexity. We identify conditions under which these form factors vanish and analyze real-space correlations, confirming the model's Tomonaga-Luttinger liquid behavior. The results reveal universal features of low energy physics of a gapless quantum fluid and its relation to Luttinger liquid theory, providing precise benchmarks for numerical simulations. This work establishes quantum hard rods as an important testbed for theories of strongly correlated one-dimensional systems.

We study the tunneling density of states (TDOS) in one-dimensional (1D) Mott insulators at energies below the charge gap. By employing nonlinear Luttinger liquid theory and density-matrix renormalization group (DMRG) simulations, we predict that in the presence of a magnetic impurity at the boundary, characteristic Fermi-edge singularity features can appear at subgap energies in the TDOS near the boundary. In contrast to the Kondo effect in a metal, these resonances are strongly asymmetric and of power-law form. The power-law exponent is universal and determined by the spinon-Kondo effect.

Using a parent Hermitian tight-binding model on a bipartite lattice with chiral symmetry, we theoretically generate non-Hermitian models for free fermions with $p$ orbitals per unit cell satisfying a complex generalization of chiral symmetry. The $p$ complex energy bands in $k$ space are given by a common $k$-dependent real factor, determined by the bands of the parent model, multiplied by the $p\mathrm{th}$ roots of unity. When the parent model is the Su-Schrieffer-Heeger (SSH) model, the single-particle energy levels are the same as those of free parafermion solutions to Baxter's non-Hermitian clock model. This construction relies on fully unidirectional hopping to create Bloch Hamiltonians with the form of generalized permutation matrices, but we also describe the effect of partial unidirectional hopping. For fully bidirectional hopping, the Bloch Hamiltonians are Hermitian and may be separated into even and odd parity blocks with respect to inversion of the orbitals within the unit cell. Partially unidirectional hopping breaks the inversion symmetry and mixes the even and odd blocks, and the real energy spectrum evolves into a complex one as the degree of unidirectionality increases, with details determined by the topology of the parent model and the number of orbitals per unit cell, $p$. We describe this process in detail for $p=3$ and 4 with the SSH model. We also apply our approach to graphene, and we show that $AA$-stacked bilayer graphene evolves into a square-root Hamiltonian of monolayer graphene with the introduction of unidirectional hopping. We show that higher-order exceptional points occur at edge states and solitons in the non-Hermitian SSH model, and at the Dirac point of non-Hermitian graphene.

The spin-boson model is a widely used model for understanding the properties of a two-level open quantum system. Accurately describing its dynamics often requires going beyond the weak system-environment coupling approximation. However, calculating the higher-order generators of such dynamics, with a system-environment coupling that is not too weak, has been known to be challenging, both numerically and analytically. This work presents the analytical derivation of the complete fourth-order time-convolutionless (TCL) generator for a generic spin-boson model, accurate up to fourth order in the system-environment coupling parameter for a broad class of environmental spectral densities. In the case of a semiconductor double-quantum-dot system, our results reveal corrections to the dynamics that may become physically significant in some parameter regimes. Within the regime of its applicability, our results provide a computational advantage over numerically exact techniques and can be applied to a wide class of nanomaterial and condensed matter systems like superconducting qubits, as well as in fields like quantum information, computing, and thermodynamics. Furthermore, we report that the widely used second-order TCL master equation tends to overestimate the non-Markovianity of a dynamics over a large parameter regime. The accuracy of the fourth-order TCL generator is rigorously benchmarked against specialized analytical calculations done for the Ohmic spectral density with Drude cutoff and against the numerically exact hierarchical equations of motion technique.