We experimentally investigate a solid-state quantum emitter—a B center in hexagonal boron nitride (hBN)—that has lifetime-limited coherence at short times, and experiences inhomogeneous broadening due to spectral diffusion at longer times. By making use of power broadening in resonant laser excitation, we explore the crossover between the inhomogeneous and the homogeneous broadening regimes. With the support of numerical simulations, we show that the line shape, count rate, second-order correlations, and long-time photon statistics evolve from a regime where they are dictated by spectral diffusion to a regime where they are simply given by the homogeneous response of the emitter, yielding restored Lorentzian shape and Poissonian photon statistics. Saturation of the count rate and line broadening occur not at the onset of the Rabi oscillations, but when the power-broadened homogeneous response becomes comparable with the inhomogeneous linewidth. Moreover, we identify specific signatures in both the second-order correlations and long-time photon statistics that are well explained by a microscopic spectral diffusion model based on discrete jumps at timescales of micro- to milliseconds. Our work provides an extensive description of the photophysics of B centers under resonant excitation, and can be readily extended to a wide variety of solid-state quantum emitters.

The fractional quantum Hall state (FQHS), an incompressible liquid state hosting anyonic excitations with fractional charge and statistics, represents a compelling many-body phase observed in clean two-dimensional (2D) carrier systems. The expected non-Abelian nature of the FQHSs at even-denominator Landau level (LL) fillings has particularly sparked considerable recent interest. At sufficiently small fillings, another exotic phase, namely a quantum Wigner crystal (WC) state, dominates. Here, we report magnetotransport measurements in an ultrahigh-quality GaAs 2D hole system where the large hole effective mass leads to a significant LL mixing (LLM) even at very high magnetic fields and affects the many-body states at very small fillings. We observe numerous developing FQHSs at both even- and odd-denominator fillings, deep in the insulating regime at $\nu \lesssim 1/3$ where WC states dominate. The FQHSs we observe at odd-denominator fillings on the flanks of $\nu =1/4$ and 1/6 are consistent with the Abelian Jain sequence of four-flux and six-flux composite fermions, while the ones at even-denominator fillings $\nu =1/4$ and 1/6 are likely non-Abelian states emerging from the pairing of these quasiparticles induced by severe LLM. Our results demonstrate that the competition between the FQHSs and WC phases is close at very small fillings even in the presence of severe LLM. We also measure activation energies of WC states near $\nu =1/6$, and find that they are substantially larger than what has been reported for ultrahigh-quality GaAs 2D electrons. A moderate LLM is believed to lower the activation energy associated to the formation of WC intrinsic defects. The surprisingly large activation energy for our 2DHS with significant LLM is therefore puzzling, and may suggest a different type of intrinsic WC defect compared to that in 2D electrons.

We report the observation of a current-induced anomalous Hall effect (AHE) and a nonlinear magnetoelectric effect in few-layer $\mathrm{WT}{\mathrm{e}}_2$ Weyl semimetal devices, achieved through independent magnetization generation and Hall measurements conducted in the absence of an external magnetic field. A direct current (dc) is used to induce out-of-plane magnetization via the magnetoelectric effect, breaking time-reversal symmetry, while an alternating current (ac) is applied for Hall measurements. The Hall resistance exhibits a quadratic dependence on the applied dc, indicating that the current induces a quadratic increase in magnetization, a phenomenon not observed in previous experiments. By employing multiple electrode pairs arranged in a circular configuration, we reveal significant anisotropy in the magnetoelectric response, which aligns with the third-order nonlinear Hall effect and is attributed to Berry connection polarizability (BCP) tensors. These findings deepen our understanding of BCP-driven magnetoelectric effects and transport phenomena in quantum materials.

According to Boltzmann-Gibbs (BG) statistical mechanics, the thermodynamic response, such as the isothermal susceptibility, at critical points (CPs) presents a divergent-like behavior. An appropriate parameter to probe both classical and quantum CPs is the so-called Grüneisen ratio $\mathrm{\Gamma }$. Motivated by the results reported in [Phys. Rev. B 108, L140403 (2023)], we extend the quantum version of $\mathrm{\Gamma }$ to the nonadditive $q$-entropy $S_q$. Our findings indicate that using $S_q$ at the unique value of $q$ restoring the extensivity of the entropy, $\mathrm{\Gamma }$ is universally nondiverging at CPs. We unprecedentedly introduce $\mathrm{\Gamma }$ in terms of $S_q$, being BG recovered for $q\to 1$. We thus solve a long-standing problem related to the illusory diverging susceptibilities at CPs.

The superconductor-insulator transition is an archetypal example of a quantum phase transition that plays a crucial role in physics and statistical mechanics. Belonging to different universality classes, the classical and quantum percolative transitions usually manifest in different systems and are studied separately. Here, we fabricate thickness-gradient ${\mathrm{Fe}}_y{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ (FTS) films and observe the existence of these two transitions. Variation in the film thickness, which controls substrate misfit and leads to a modification of Fe concentration, results in a superconducting quantum percolative transition. By exposing FTS films to air, we observe a shift in electrical resistivity behavior and a superconducting classical percolative transition. FTS films subjected to the two treatments exhibit very similar phase diagrams, except for different critical exponents. Remarkably, we identify a two-step superconductor-insulator transition at low temperature, where the appearance of superconductive zero resistivity coincides with a sign change in the temperature derivative of electrical resistivity in the normal state. Our analysis can provide a universal scheme to categorize the superconductor-insulator transition in various copper-oxide and iron-based superconductors.

We investigate the intrinsic nonlinear Hall effect (INHE) in $\mathcal{PT}$-symmetric antiferromagnetic nodal-line semimetals, highlighting a significant enhancement characterized by a logarithmic divergence in the INH conductivity as the Fermi energy intersects the nodal line within a finite energy range. Through analytical derivations and numerical calculations, we demonstrate that the INH conductivity logarithmically depends on the gap opening at the nodal line. We further reveal that current-induced spin-orbit torque, which rotates the Néel vector and opens a gap at the nodal line, results in a nonanalytical transverse response, where the amplitude of high-harmonic generation decreases as the inverse cube of the harmonic order. Our finding not only serves as a pathway to an enhanced nonlinear Hall response without the necessity of fine tuning the chemical potential but also suggests a different nonanalytical response that may contribute to the experiment identification of topological nodal-line semimetals and potential applications in ultrafast nonlinear devices.

We investigate the electronic structures of noncentrosymmetric Weyl semimetals $R\mathrm{AlSi}$ ($R$ = Ce and Nd) using soft x-ray angle-resolved photoemission spectroscopy. We find that the photoelectron intensity distribution observed in the momentum-resolved electronic bands is highly sensitive to the probed Brillouin zone (BZ) owing to the zone-selection effect arising from the nonsymmorphic crystal structure of $R\mathrm{AlSi}$. Our data reconstruct the photoelectron distributions varied according to the zone-selection effect and reveal comprehensive information about the electronic band structures predicted by band calculations. This detailed information enables us to experimentally trace the Weyl-cone dispersion throughout three-dimensional momentum space, providing valuable insights into the unique properties of $R\mathrm{AlSi}$.

We investigate the persistent current in a correlated trimer nanoring comprising three magnetic atoms, which sits on a metallic host and encloses a magnetic flux. In the molecular-orbital regime, charge fluctuations can reverse the aromaticity of the trimer molecule by driving quantum phase transitions between many-body states. It is shown that in frustrated trimers the superexchange-induced current as a function of interatom hopping is enhanced by the competition of conflicting magnetic orders, with an asymmetric peak at the quantum criticality separating the ferromagnetic and antiferromagnetic Kondo regimes. Interestingly, the critical current undergoes an anomalous rise with temperature before decaying, signaling the suppression of the Kondo bound state at finite temperature. Our results demonstrate that the coherent current response to external flux indeed conveys important information on the states of strongly correlated systems.

Three-dimensional (3D) third-order topological insulators (TIs) have zero-dimensional corner states, which are three dimensions lower than bulk. Here we investigate third-order TIs on breathing pyrochlore lattices with $p$-orbital freedom. The tight-binding Hamiltonian is derived for the $p$-orbital model, for which we find that the two orthogonal $\pi$-type (transverse) hoppings are the key to open a band gap and obtain higher-order topological corner states. We introduce the $Z_4$ Berry phase to characterize the bulk topology and analyze the phase diagram. The corner states, demonstrated in a finite structure of a regular tetrahedron, exhibit rich 3D orbital configurations. Furthermore, we design an acoustic system to introduce the necessary $\pi$-type hopping and successfully observe the orbital corner states. Our work extends topological orbital corner states to third order, which enriches the contents of orbital physics and may lead to applications in novel topological acoustic devices.

We consider the common spin-1/2 XX model in one dimension with open boundary conditions and a large but finite number of spins. The system is in thermal equilibrium at times $t<0$, and is subject to a weak local perturbation (quantum quench) at $t=0$. Focusing mainly on single-spin perturbations and observables, we show that the system rethermalizes for sufficiently large times $t>0$ without invoking any unproven assumptions besides the basic laws of quantum mechanics. Moreover, the time-dependent relaxation behavior is obtained in quantitative detail and is found to exhibit a wealth of interesting features.

We report on how the deposition of a nonmagnetic metal on the surface of a ferromagnetic insulator can provide a sensitive, albeit indirect, means of measuring the magnon Hall effect. This is inferred from the thermoelectric potential that develops across the metal layer as a consequence of the transverse thermal Hall gradient. Moreover, a manifestation of magnon surface states is observed via the drag which the surface spin currents of the ferromagnet impose on the conduction electrons of the adjacent metallic layer. This electron-magnon interaction operates across the interface, and the polarity of the resultant drag voltage is determined by the spin Nernst angle of the metal layer.

Deconfined quantum critical points (DQCPs) are proposed as unconventional second-order phase transitions beyond the Landau-Ginzburg-Wilson paradigm. The nature and experimental realizations of DQCPs are crucial issues of importance. We illustrate the potential for DQCPs between the valence bond solid state and the antiferromagnetic phase to arise in optical lattices containing frustrated dipolar bosons subject to hard-core constraints. The emergence of DQCPs is comprehended through the fusion of two Berezinskii-Kosterlitz-Thouless (BKT) transitions. The DQCPs and the BKTs are confirmed by the scaling of ground-state fidelity susceptibilities in finite systems and the analysis of order parameters obtained from infinite systems. The numerical analysis reveals varying critical exponents of the correlation length in DQCPs and the logarithmic scaling in BKTs, respectively. In this paper, we offer a promising platform for realizing DQCPs and provide valuable insights into their nature within the framework of topological phase transitions.

This study explores using magnetically levitated force sensors to measure Casimir forces between a gold-coated ferromagnetic sphere and a superconducting niobium plate. We model the Casimir and Meissner magnetic forces in a self-consistent way for real materials in the plane-sphere geometry at a finite temperature. Our findings reveal that magnetic levitation, combined with Casimir forces and gravitational pull, achieves a stable equilibrium modifiable by an external magnetic field. The sensors demonstrated sufficient sensitivity to detect Casimir forces under the studied conditions, with thermal fluctuations of the sphere's motion having minimal impact on measurements. This highlights the potential of magnetically levitated sensors for precise force measurements in advanced applications.

Nonlinear lattice models can support “discrete breather” excitations that stay localized in space for all time. By contrast, the localized Wannier states of linear lattice models are dynamically unstable. Nevertheless, symmetric and exponentially localized Wannier states are a central tool in the classification of band structures with crystalline symmetries. Moreover, the quantized transport observed in nonlinear Thouless pumps relies on the fact that—at least in a specific model—discrete breathers recover Wannier states in the limit of vanishing nonlinearity. Motivated by these observations, we investigate the correspondence between nonlinear breathers and exponentially localized Wannier states for a family of discrete nonlinear Schrödinger equations with crystalline symmetries. We develop a formalism to analytically predict the breathers' spectrum, center of mass and symmetry data, and apply this to nonlinear generalizations of the Su-Schrieffer-Heeger chain and the breathing kagome lattice.

Hybrid magnetic skyrmions, characterized by helicities intermediate between Bloch- and Néel-type textures, have emerged as promising spintronic quasiparticles due to their enhanced mobility and reduced skyrmion Hall effect. Here, we present a universal strategy to achieve the $C_3$ point-group symmetry in two-dimensional heterostructures by stacking monolayers, thereby enabling the formation of a hybrid skyrmion phase. Guided by symmetry analysis and a density functional theory-based spin model, we construct a phase diagram revealing that the in-plane Dzyaloshinskii-Moriya interaction components critically modulate skyrmion helicity. First-principles calculations validate our approach, identifying the $\mathrm{CrMo}{\mathrm{Ge}}_2{\mathrm{Te}}_6/\mathrm{InSiT}{\mathrm{e}}_3$ heterostructure as a host for hybrid skyrmions with a helicity of ${78}^{\circ }$. Moreover, we demonstrate electrical tunability of helicity, reducing it to ${71}^{\circ }$ under an applied electric field of $0.20\mathrm{V}/\AA$. These findings establish a robust design platform for exploring hybrid skyrmions and controlling their helicity, fostering advances in skyrmion-based spintronic devices.

Interacting bosons on a single plaquette threaded by a $\pi$ flux can spontaneously break time-reversal symmetry, resulting in a chiral loop current. Connecting such bosonic $\pi$-flux plaquettes in a dispersive configuration was recently shown to lead to long-range chiral order. Here, instead, we design a chain of $\pi$-flux plaquettes that exhibits an all-flat-bands single-particle energy spectrum and an extensive set of local symmetries. Using Elitzur's theorem, we show that these local symmetries prevent the emergence of long-range chiral order. Moreover, projecting the dynamics to a Creutz ladder model with an effective intrarung interaction allows one to derive simple spin Hamiltonians capturing the ground-state degeneracy and the low-energy excitations, and to confirm the absence of chiral order. Nevertheless, we show how to obtain gauge-invariant information from a mean-field approach, which explicitly breaks gauge-invariance. Finally, we observe an impurity self-pinning phenomenon, when an extra boson is added on top of a ground state at integer filling, resulting in a nondispersive density peak. Exact diagonalization benchmarks are also provided, and experimental perspectives are discussed.

The exotic electronic states in the charge loop current (cLC) phase, in which the permanent charge current breaks the time-reversal symmetry, have been attracting increasing attention in recently discovered kagome metals ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{Cs}, \mathrm{Rb}, \mathrm{K})$. Interestingly, the cLC state is sensitively controlled by applying a small magnetic field as well as a tiny uniaxial strain. In addition, many experiments indicate that the cLC state is sensitive to the small number of impurities. To understand the impurity effects on the cLC electronic states accurately, we analyze the giant unit-cell (up to 1200 sites) kagome lattice model with single impurity potential. The loop current is found to be strongly suppressed within the current correlation length ${\xi }_J$ centered on the impurity site, where ${\xi }_J$ increases as the cLC order parameter $\eta$ decreases. (The cLC order is the pure imaginary hopping integral modulation $\delta t_{i,j}=\pm i\eta$.) In addition, both the uniform orbital magnetization $M_{\mathrm{orb}}$ and the anomalous Hall conductivity ${\sigma }_{xy}$ are drastically suppressed by dilute impurities. Especially, the suppression ratio $R=-\mathrm{\Delta }{M}_{\mathrm{orb}}/M_{\mathrm{orb}}^0$ can exceed $50\%$ with the introduction of $\sim 1\%$ impurities. Unexpectedly, the ratio $R$ is qualitatively insensitive to $\eta$, in high contrast to a naive expectation that $R$ is proportional to the current suppression area $\pi {\xi }_J^2$. The resulting giant impurity effect of $M_{\mathrm{orb}}$ originates from the nonlocal contribution of the itinerant circulation of electrons. The present study gives a natural explanation of why the cLC electronic states in kagome metals are sensitive to dilute impurities.

This paper uncovers the formation of topological edge polaritons that are protected by the presence of quantum vacuum. Such quantum-vacuum-protected edge polaritons could be achieved in a system of neutral atomic lattice under appropriate interaction with a single photonic mode. In the absence of the light-matter coupling, the system is shown to be topologically trivial, which consequently does not support edge modes. By employing Floquet theory, the system is also found to be topologically trivial in the classical light limit, i.e., at very small light-matter coupling but a very large number of photons. On the other hand, by treating both the atomic and photonic degrees of freedom quantum mechanically, the system becomes topologically nontrivial in the full (atomic and photonic) Hilbert space, which manifests itself as a pair of topological (almost) zero energy eigenstates localized near each lattice's edge and with very small mean photon number. Finally, the robustness of such quantum-vacuum-protected edge polaritons against spatial disorder and counterrotating coupling effect is explicitly demonstrated.

Surface-enhanced Raman scattering (SERS) enhances molecules' vibrational signals for single-molecule detection which correlates with light-induced polarizability variations. The involved charge-transfer process impacts molecular polarizability, offering efficient modulation methods for SERS. Herein, the effect of ambient molecules $({\mathrm{H}}_2\mathrm{O},$ ${\mathrm{CO}}_2, \text{and} {\mathrm{N}}_2)$ on the SERS spectra of a metal-pyridine (Py) system is explored. It is found that the relative intensity changes differently between vibrational modes when adsorbing different ambient molecules. Especially, ${\mathrm{CO}}_2$ adsorption alters the intensity ratios between ring-breathing and ring-stretching modes in Py molecule. Further polarizability analysis shows that ${\mathrm{CO}}_2$'s larger polarizability contribution and adsorption configuration lead to the dominating ring-breathing mode. Additionally, SERS enhancement reduces with increasing ambient molecules, due to weakened electron exchange between Py and metal substrate. This study has the potential to provide an insight into detecting relative intensity changes in SERS and promote the development of related applications.