Artificial atoms in graphene hosting a series of quasibound states can serve as an excellent platform to explore atomic collapse and become a basis to design novel graphene nanodevices. We theoretically study behaviors of massless Dirac fermions in singular potentials with a general form of $1/r^{\gamma }$. Different from the Coulomb potential that demands a supercritical charge $Z>Z_c$, a high-order singular potential ($\gamma >1$) is found to, in principle, induce atomic collapse with an infinitesimal charge $Z$. The energies of atomic collapse states (ACSs) within these potentials are arranged roughly as a power sequence. We also show that some special ACSs can exist even above the bulk Dirac point, which cannot appear in the Coulomb potential. These findings uncover the anomalies of massless Dirac fermions in diverse charge potentials and provide guidance for further experiments and graphene nanodevice applications.

The entanglement entropy (EE) represents an operationally defined entanglement measure which has been widely applied in Hermitian systems such as black holes and quantum many-body systems. Recently, interest has also shifted to the entanglement properties of non-Hermitian systems. It has been theoretically predicted that negative EE scaling can enigmatically arise in non-Hermitian free-fermion lattices hosting exceptional points. However, experimental simulation of such a theoretical prediction remains a challenge. Here, we report an experimental simulation of negative EE scaling using an electrical circuit platform, where the requisite nonreciprocal long-range couplings can be engineered precisely. By measuring resonant voltage profiles, the corresponding simulated negative EE scaling can be demonstrated. Our work paves the way for simulating analogs of probability for nonconserving negative entanglement processes in classical systems.

Many driven binary systems can exhibit laning transitions when the two species have different mobilities, such as colloidal particles with opposite charges in electric fields. Another example is pedestrian or active-matter systems, where particles moving in opposite directions form a phase-separated state that enhances the overall mobility. In this work, we use atomistic simulations to demonstrate that mixtures of skyrmions and skyrmioniums also exhibit pattern formation and laning transitions. Skyrmions move more slowly and at a finite skyrmion Hall angle compared to skyrmioniums, which move faster and without a skyrmion Hall effect. At low drives, the system forms a partially jammed phase where the skyrmioniums are dragged by the surrounding skyrmions, resulting in a finite angle of motion for the skyrmioniums. At higher drives, the system transitions into a laned state, but unlike colloidal systems, the lanes in the skyrmion-skyrmionium mixture are tilted relative to the driving direction due to the intrinsic skyrmion Hall angle. In the laned state, the skyrmionium angle of motion is reversed when it aligns with the tilted lane structure. At even higher drives, the skyrmioniums collapse into skyrmions. Below a critical skyrmion density, both textures can move independently with few collisions, but above this density, the laning state disappears entirely, and the system transitions to a skyrmion-only state. We map out the velocity and Hall responses of the different textures and identify three distinct phases: partially jammed, laned, and skyrmion-only moving crystal states. We compare our results to recent observations of tilted laning phases in pedestrian flows, where chiral symmetry breaking in the particle interactions leads to similar behavior.

The linear magnetoelectric (ME) coupling effect is of particular interest due to its role in hosting fascinating off-diagonal magnetoelectricity and toroidal moments. In this work we present a systematic investigation of successive metamagnetic transitions by high-field study up to 55 T and switching of the off-diagonal ME state of the spin-chain antiferromagnet $\mathrm{CoS}{\mathrm{e}}_2{\mathrm{O}}_5$ single crystal. When the magnetic field is applied along the easy $a$-axis, in addition to the low-field spin-flop transition at $H_{a1}=6.1$ T, two other metamagnetic transitions occur at $H_{a2}=8.8$ T and $H_{a3}=11.2$ T, corresponding to the appearance of 1/2 and 4/5-like plateaus, respectively. For the hard $c$-axis, there are two successive upturns in the magnetization at $H_{c1}=10.7$ T and $H_{c2}=13.9$ T, corresponding to 1/2 and 4/5 of the saturation magnetization. In contrast, only one metamagnetic magnetic transition is revealed for $b$-axis magnetization, highlighting a strongly anisotropic feature. Furthermore, the selection and switching between two magnetoelectric states have been realized via the combination of electric and magnetic fields. The switching field of the ME state exhibits a temperature dependence that can be explained by a phenomenological model for ME antiferromagnets. We corroborated our experimental results with density-functional theory calculations, thereby determining the magnetic ground-state and field-induced electric polarization, both of which align well with our experimental results. The strong hybridization between O-$p$ and Co-$d$ orbitals, coupled with the reduced symmetry of the octahedral environment, may act as one of the spin-driven mechanisms underlying the observed ME properties. Furthermore, the calculation shows that the net ionic displacement along the $b$ axis is extremely small, demonstrating that the ferroelectric polarization in $\mathrm{CoS}{\mathrm{e}}_2{\mathrm{O}}_5$ is primarily driven by electronic rather than ionic contributions. Our results suggest that $\mathrm{CoS}{\mathrm{e}}_2{\mathrm{O}}_5$ is a compelling candidate for investigating potential toroidicity and collinear spins, with the toroidal moment expected to be parallel to spins.

Quantum systems are powerful detectors with wide-ranging applications from scanning probe microscopy of materials to biomedical imaging. Nitrogen vacancy (NV) centers in diamond, for instance, can be operated as qubits for sensing of magnetic field, temperature, or related signals. By well-designed application of pulse sequences, experiments can filter this signal from environmental noise, allowing extremely sensitive measurements with single NV centers. Recently, optimal control has been used to further improve sensitivity by modification of the pulse sequence, most notably by optimal placement of $\pi$ pulses. Here we consider extending beyond $\pi$ pulses, exploring optimization of a continuous, time-dependent control field. We show that the difficulty of optimizing these protocols can be mapped to the difficulty of finding minimum free energy in a classical frustrated spin system. While most optimizations we consider show autocorrelations of the sensing protocol that grow as a power law, similar to an Ising spin glass, the continuous control shows slower logarithmic growth, suggestive of a harder Heisenberg-type glassy landscape.

Topologically protected magnetic states in condensed matter physics, particularly antiferromagnetic (AFM) skyrmions (Sks) and bimerons (Bms), offer promising prospects for terahertz dynamics and sustained current-induced motion, thanks to their compensating multiple sublattice structure. However, nucleating AFM Sks and Bms is challenging due to the lack of net magnetization. Previous attempts to imprint predefined Sks and Bms in a ferromagnet and transfer them to an antiferromagnet using interfacial exchange bias in ferromagnetic (FM)/AFM heterostructures have been hindered by complex multilayers with discontinuities, polycrystallinity, or multipartite chiral antiferromagnets. Employing atomistic spin simulations, we demonstrate the viability of texture imprinting for nucleating Sks and Bms in antiferromagnets, using a prototypical bipartite AFM layer in a multilayer structure free from discontinuities. Such imprinting is a crucial step towards understanding the static and dynamic properties of natural AFM textures and their unique properties.

Nonanalytic features are interesting in physics as they carry valuable information about the physical properties of the system. In this work, we use a stationary-point analysis to deduce the nonanalytic features of dynamical correlation functions that are relevant to inelastic light scattering from superconductors with an anisotropic order parameter. We show that nodal regions of the order parameters are, quite generally, associated with linear-in-frequency scaling at low frequencies, the minima points of the order parameters are associated with step jumps, while the maxima points are associated with $ln$ singularities. Despite this general association, we show that, depending on the anisotropy of the light-scattering vertex, these features can manifest themselves as various power laws, and even not remain singular at all. We are also able to demonstrate that the association of these nonanalytic features with the extrema and nodal points of the order parameter is, in fact, derived from a more universal behavior of functions near parabolic-like and saddle-like stationary points.

At zero temperature, the Pauli potential—the functional derivative of the Pauli kinetic energy density functional—is the key to the accuracy of the orbital-free density functional theory (OF-DFT) as it is supposed to capture all the effects associated with the Pauli exclusion principle. Here, we extend this concept to finite temperature by defining Pauli free energy and the modified Pauli free energy, both representing the natural generalizations of the Pauli term from zero-$T$ to finite-$T$. We discuss their physical interpretation, the mathematical nuances, and the applicability, arguing that the modified Pauli potential should be used as an extension of the zero-$T$ counterpart within the OF-DFT framework. Through analytical and numerical methods, we then analyze some of the exact properties concerning the modified Pauli free- and kinetic-energy terms and examine the temperature dependence of the modified Pauli potential.

Classical polarizable approaches have become the gold standard for simulating complex systems and processes in the condensed phase. These methods describe intrinsically dissipative polarizable media, requiring a formal definition within the framework of open quantum systems. We present a Hamiltonian formulation for the quantum dynamics of polarizable sources based on a generalized theory of the damped harmonic oscillator, using pseudoboson theory to characterize their coherent state dynamics. We then apply our theory to the study of the optical response of two plasmonic systems. Furthermore, by exploiting the phase-space formulation of quantum mechanics and the integrability of quadratic Hamiltonians, we derive a self-consistent relation for the emitted electric field of the polarizable medium under the semiclassical approximation, based on exact formulas for medium polarization. Finally, we derive the master equation describing the open dynamics of a quantum system interacting with the quantum polarizable medium, along with analytical expressions for correlation functions calculated over arbitrary Gaussian states.

We discuss electron redistribution during the semiconductor-to-Dirac semimetal transition in Na-Sb-Bi alloys using x-ray Compton scattering experiments combined with first-principles electronic structure modeling. A robust signature of the semiconductor-to-Dirac semimetal transition is identified in the spherically averaged Compton profile. We demonstrate how the number of electrons involved in this transition can be estimated to provide a novel descriptor for quantifying the strength of spin-orbit coupling responsible for driving the transition. The associated theoretical deviation of the Born charge of Na in ${\mathrm{Na}}_3\mathrm{Bi}$ from the expected ionic charge of $+1$ is found to be consistent with the corresponding experimental value of about 10%. Our study also shows the sensitivity of the Compton scattering technique toward capturing the spillover of Bi $6\mathrm{p}$ relativistic states onto Na sites.

Quasiphase matching (QPM) serves as an effective strategy to achieve second harmonic generation (SHG) with high conversion efficiency, which compensates for phase mismatch through periodic poling of nonlinear coefficients. Unlike traditional materials that rely on high-voltage electric fields or complex fabrication for periodic poling, two-dimensional (2D) layered materials enable QPM through nanoscale thickness control and tunable twist angles. In addition, the larger nonlinear susceptibility of 2D materials enables the realization of high-power coherent light sources using SHG technology. By combining the nonlinear transfer matrix method with experimental validation, we demonstrate that reverse stacking of ${\mathrm{NbOBr}}_2$ with optimized thickness achieved a four to five times SHG enhancement in both reflection and transmission configurations. Notably, the transmission SHG attained a conversion efficiency of $5.8\times {10}^{-6}\%$ under an ultralow power density of $10\mathrm{MW}/\mathrm{c}{\mathrm{m}}^2$. Our experiments confirm that ${\mathrm{NbOBr}}_2$ exhibits low dispersion and a large SHG response in the visible range, making it an ideal 2D nonlinear crystal for phase-matching enhancement through reverse stacking.

Anderson localization and the non-Hermitian skin effect are two distinct confinement phenomena of the eigenfunctions that are driven, respectively, by disorder and nonreciprocity. Understanding their interplay within a unified framework offers valuable insights into the localization properties of low-dimensional systems. To this end, we investigate a non-Hermitian version of the celebrated Aubry-André model, which serves as an ideal platform due to its unique self-dual properties and ability to demonstrate a delocalization-localization transition in one dimension. Interestingly, in our setting, the competition between Anderson localization and the skin effect can be precisely controlled via the complex phase of the quasiperiodic disorder. Additionally, by analyzing the time evolution, we demonstrate that quantum jumps between the skin states and the Anderson-localized states occur in the theoretical model. Further, to gain support for our theoretical predictions in an experimental platform, we propose a topolectrical circuit featuring an interface that separates two distinct electrical circuit networks. The voltage profile of the circuit exhibits confinement at the interface, analogous to the skin effect, while the phenomenon of Anderson localization in the circuit can be perceived via a predicted localization behavior near the excitation node, rather than exhibiting sudden non-Hermitian jumps, as observed in the tight-binding framework. This interplay leads to a spatially tunable localization of the output voltage of the circuit. Our findings provide deeper insights into the controlled confinement of the eigenstates of the non-Hermitian Aubry-André model by designing analogous features in topolectrical circuits, opening avenues in the fabrication of advanced electronic systems such as highly sensitive sensors and efficient devices for information transfer and communication.

Metasurfaces are efficient and versatile electromagnetic structures that have already enabled the implementation of a wide range of microwave and photonic wave shaping applications. Despite the extensive research into metasurfaces, a rigorous and comprehensive understanding of their angular dispersion remains largely underexplored. Here, we use the generalized sheet transition conditions (GSTCs) to model and analyze the angular dispersive properties of metasurfaces. Based on this theoretical framework, we demonstrate that a metasurface may exhibit either partial or complete co- and cross-polarized transmission and reflection coefficients that are angle-invariant, meaning that their amplitude, phase, or both remain unchanged with varying incidence angles. We show that these angle-independent responses exist only when specific conditions, given in terms of the metasurface effective susceptibilities, are met. Using the GSTCs formalism, we derive several of these conditions and illustrate their scattering properties. Among other findings, this analysis reveals that, contrary to common assumptions, nonlocality (spatial dispersion) does not only increase the angular dispersion of a metasurface, but may also be used to achieve complete angle-invariant scattering. Additionally, this work demonstrates that fully efficient extrinsic chirality is possible with a pseudochiral metasurface in a partially angle-invariant fashion. We expect our work to provide a general strategy for eliminating, or at least reducing, angular-dependent scattering responses of metasurfaces, which may prove instrumental for applications that are highly sensitive to the detrimental effects of angular dispersion.

Quasicrystals represent a category of rarely structured solids that challenge traditional periodicity in crystal materials. Recent advancements in the synthesis of two-dimensional (2D) van der Waals materials have paved the way for exploring the unique physical properties of these systems. Here, we report on the synthesis of 2D quasicrystals featuring ${30}^{\circ }$ alternating twist angles between multiple graphene layers, using chemical vapor deposition (CVD). Strikingly, we observe periodic Moiré patterns in the quasicrystal. The Moiré periodicity, varying with the parity of the total constituent layers, aligns with the theoretical predictions that suggest a stress cancellation mechanism. The emergence of Moiré fringes is attributed to the spontaneous mismatched lattice constant in the oriented graphene layers, proving the existence of atomic relaxation. This phenomenon, which has been largely understudied in graphene systems with large twist angles, has now been validated through our use of scanning transmission electron microscopy. Our CVD-grown Moiré quasicrystal provides an ideal platform for exploring the unusual physical properties that arise from Moiré periodicity within quasicrystals.

Quantum many-body scars (QMBSs) represent a weak ergodicity-breaking phenomenon that defies the common scenario of thermalization in closed quantum systems. They are often regarded as a many-body analog of quantum scars (QSs)—a single-particle phenomenon in quantum chaos—due to their superficial similarities. However, unlike QSs, a clear connection between QMBSs and classical chaos has remained elusive. It has nevertheless been speculated that in an appropriate semiclassical limit, QMBSs should have a correspondence to weakly unstable periodic orbits. In this paper, I present a counterexample to this conjecture by studying a bosonic model with a large number of flavors. The dynamics of out-of-time-ordered correlators (OTOCs) suggest that QMBS do not display chaotic behavior in the semiclassical limit. In contrast, chaotic dynamics are expected for initial states not associated with QMBS. Interestingly, the anomalous OTOC dynamics persist even under weak perturbations that eliminate the scarred eigenstates, suggesting a certain robustness in the phenomenon.

Motivated by recent studies on topologically nontrivial moiré bands in twisted bilayer transition metal dichalcogenides (TMDs), we study ${\mathrm{MoTe}}_2$ bilayer systems subject to pressure, which is applied perpendicular to the material surface. We start our investigation by first considering an untwisted bilayer systems with an arbitrary relative shift between layers; a symmetry analysis for this case permits us to obtain a simplified effective low-energy Hamiltonian valid near the important $\mathbf{K}$ valley region of the Brillouin zone. Ab initio density functional theory (DFT) was then employed to obtain relaxed geometric structures for pressures within the range of 0.0–3.5 GPa and corresponding band structures. The DFT data were then fitted to the low-energy Hamiltonian to obtain a pressure-dependent Hamiltonian. We then applied our model to a twisted system by treating the twist as a position-dependent shift between layers—here, we assume rigid layers, which is a crucial simplification. In summary, this approach allowed us to obtain the explicit analytical expressions for a Hamiltonian that describes a twisted ${\mathrm{MoTe}}_2$ bilayer under pressure. Our Hamiltonian then permitted us to study the impact of pressure on the band topology of the twisted system. As a result, we identified many pressure-induced topological phase transitions as indicated by changes in valley Chern numbers. Moreover, we found that pressure could be employed to flatten bands in some of the cases we considered.

The current-voltage characteristics of Josephson junctions exhibit a subharmonic gap structure (SGS), denoting jumps at specific voltages. While the prevalent multiple Andreev reflection theory matches the experimentally observed SGS, it is limited to a dc voltage bias. For a dc current bias, existing theories are restricted to low-transparency junctions and fail to capture the full SGS. We introduce a microscopic Floquet approach applicable for arbitrary transparencies, and recover the correct SGS for a dc current bias. We provide a comprehensive understanding of SGS for a dc current bias, which entails two-quasiparticle tunneling processes absent in existing theories, via two complementary perspectives: in the frequency domain, as generalized Andreev reflections absorbing multiple energies, and in the time domain, as the interference of nonequilibrium current pulses.

We propose a generalized Shastry-Sutherland (SS) model which bridges the SS model and the $J_1\text{−}{J}_2$ Heisenberg model. By employing large-scale density matrix renormalization group and fully augmented matrix product state calculations, combined with careful finite-size scaling, we find the phase transition between the plaquette valence bond state (pVBS) and Néel antiferromagnetic (AFM) phase in the pure SS model is a weak first one. This result indicates the existence of an exotic tricritical point in the pVBS to AFM transition line in the phase diagram, as the transition in the $J_1\text{−}{J}_2$ Heisenberg model was previously determined to be continuous. We determine the location of the tricritical point in the phase diagram at which the first-order transition turns continuous. Our generalized SS model provides not only a valuable platform to explore exotic phases and phase transitions but also more realistic description of SS materials like ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$.

We perform deep variational free-energy calculations to investigate the dense hydrogen system at 1200 K and high pressures. In this computational framework, neural networks are used to model the free energy through the proton Boltzmann distribution and the electron wave function. By directly minimizing the free energy, our results reveal the emergence of a crystalline order associated with the center of mass of hydrogen molecules at approximately 180 GPa. This transition from atomic liquid to a molecular solid is marked by discontinuities in both the pressure and thermal entropy. Additionally, we discuss the broader implications and limitations of these findings in the context of recent studies of dense hydrogen under similar conditions.