We present an analytical formulation of the thermodynamics, free energy, and entropy of any generic Bogoliubov–de Gennes model which develops exceptional point (EP) bifurcations in its complex spectrum when coupled to reservoirs. We apply our formalism to a non-Hermitian Josephson junction where, despite recent claims, the supercurrent does not exhibit any divergences at EPs. The entropy, on the contrary, shows a universal jump of $1/2ln2$, which can be linked to the emergence of Majorana zero modes at EPs. Our method allows us to obtain precise analytical boundaries for the temperatures at which such Majorana entropy steps appear. We propose a generalized Maxwell relation linking supercurrents and entropy which could pave the way towards the direct experimental observation of such steps in, e.g., quantum-dot-based minimal Kitaev chains.

Low-energy excitations of a magnetically ordered system are spin waves with magnon being their excitation quanta. Magnons are demonstrated to be useful for data processing and communication. To achieve magnon transport across extended distances, it is essential to minimize magnonic dissipation which can be accomplished by material engineering to reduce intrinsic damping or by spin torques that can counteract damping. This study introduces an alternative methodology to effectively reduce magnon dissipation based on magnonic bound states in the continuum (BIC). We demonstrate the approach for two antiferromagnetically coupled magnonic waveguides, with one waveguide being attached to a current carrying metallic layer. The current acts on the attached waveguide with a spin-orbit torque effectively amplifying the magnonic signal. The setup maps on a non-Hermitian system with coupled loss and more loss, enabling the formation of dissipationless magnon BIC. We investigate the necessary criteria for the formation of magnon BIC through electric currents. The influences of interlayer coupling constant, anisotropy constants, and applied magnetic field on the current-induced magnon BIC are analyzed. The identified effect can be integrated in the design of magnon delay lines, offering opportunities for the enhancement of magnonic devices and circuits.

We present a theory of magnonic thermal Hall transport driven by many-body skew scattering. In field-polarized chiral magnets, the Dzyaloshinskii-Moriya interaction induces three-magnon interactions that violate time-reversal symmetry and interfere with the four-magnon scattering, thereby breaking microscopic detailed balance and causing thermal Hall currents. This mechanism operates without disorder or Berry curvature and is intrinsic to interacting spin systems. We argue that it can be as significant as the band-geometric anomalous velocity, highlighting the importance of magnon interactions in thermal Hall physics.

Diamond, the hardest natural crystal, has attracted significant attention for its plasticity, which is believed to be influenced by its stacking faults. Studies have mainly focused on one-dimensional linear pathways in stacking transitions, neglecting its transverse freedom on the main slip plane. However, in an actual stacking process, stacking faults can follow curved lines along the slip plane rather than being constrained to straight lines. In this study, using ab initio calculations, we mapped the γ-surface, defined as the landscape of generalized stacking fault energies, along the weakest direction of the {111} orientation in diamond. We then applied the nudged elastic band method on diamond's γ-surface to determine the minimum energy paths for both the stacking and dislocation processes, finding significantly reduced stacking energy barriers compared to previous reports (for the glide set, our energy barrier is only one-third of that for the traditional direct path). Our calculations reveal that the glide set can round its high-energy peak, with a lower energy barrier across the entire stacking plane than the shuffle set. Our results provide alternative insights into the plasticity and stacking faults of diamond, advancing the understanding of the transition in superhard carbon materials, especially diamond under shear stress.

The investigation of ergodicity or lack thereof in isolated quantum many-body systems has conventionally focused on the description of the reduced density matrices of local subsystems in the contexts of thermalization, integrability, and localization. Recent experimental capabilities to measure the full distribution of quantum states in Hilbert space and the emergence of specific state ensembles have extended this to questions of deep thermalization, introducing the notion of the projected ensemble—ensemble of pure states of a subsystem obtained by projective measurements on its complement. While previous studies examined chaotic unitary circuits and Hamiltonian evolution in systems with or without global conserved charges, we study the projected ensemble in systems where there are an extensive number of conserved charges all of which have (quasi)local support. We employ a strongly disordered quantum spin chain that shows many-body localized dynamics over long timescales, as well as the $\ell$-bit model, a phenomenological archetype of a many-body localized system, with the charges being 1-local in the latter. In particular, we discuss the dependence of the projected ensemble on the measurement basis. Starting with random direct product states, we find that the projected ensemble constructed from time-evolved states converges to a Scrooge ensemble at late times and in the large system limit except when the measurement operator is close to the conserved charges. This is in contrast to systems with global conserved charges where the ensemble varies continuously with the measurement basis. We relate these observations to the emergence of the Porter-Thomas distribution in the probability distribution of bitstring measurement probabilities.

We conducted a study on the electron stopping power of protons in aluminum at finite electron temperatures, utilizing time-dependent density functional theory nonadiabatically coupled with molecular dynamics. Our investigation focused on protons with initial velocities ranging from 0.1 to 1.0 a.u., providing a wealth of detailed information on the electronic states involved in the stopping process, with exceptional spatial and temporal resolution. Our results show that the electron temperature can significantly effect the electron stopping power. A quantum-blocking mechanism based on a physical picture of electronic transitions in energy levels has been proposed for explaining the phenomenon of electron stopping power decreasing with the increase of target electron temperature.

The discovery of the altermagnet (AM) marks a significant advancement in magnetic materials, combining the characteristics of both ferromagnetism and antiferromagnetism. In this paper, we focus on CrSb, which has been verified to be an AM and to exhibit substantial spin splitting near the Fermi level. After successfully growing high-quality CrSb single crystals, we performed comprehensive magnetization, magnetoresistance (MR), and Hall resistivity measurements, along with the electronic structure, and Fermi-surface calculations, as well as magnetotransport property numerical simulations. An antiferromagnetic transition occurring at $T_N=712\mathrm{K}$ was reconfirmed. It was found that both experimental MR and Hall resistivity are consistent with the numerical simulation results, and exhibit obvious scaling behavior. The nonlinear Hall resistivity is due to its multiband structure, rather than an anomalous Hall effect. Especially, the scaling behavior in the Hall resistivity is first observed within an AM material. These findings demonstrate that the magnetotransport properties in CrSb originate from the intrinsic electronic structure and are dominated by the Lorentz force. Additionally, through a comparison of theory and experiment, we found that the magnetotransport properties calculated using the altermagnetic structure perfectly match the experimental data, while using a nonmagnetic structure does not align with the experimental results. This further confirms that CrSb is indeed an altermagnet.

The electronic structure of metallic ${\mathrm{Bi}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ has been investigated by a combination of soft x-ray absorption spectroscopy, x-ray emission spectroscopy (XES), and resonant inelastic x-ray scattering (RIXS) in the vicinity of the oxygen $K$ edge. The O $K$-edge RIXS spectra are found to resemble the O $K$-edge XES spectra with resonating features but with an absence of any Raman modes, revealing the highly itinerant nature of this compound. The O $K$-edge response is compared with scalar relativistic band structure calculations within the local density approximation, which captures the main spectral characteristics. The RIXS spectra either display a localized or delocalized Ir $5d$ response, depending on whether the photon energy is tuned to the pre-edge or above. Our results uncover a significant coupling of O $K$-edge RIXS via the O $2p-\mathrm{Ir} 5d$ hybridization to the partial density of states of the Ir $5d$ states. An alternative explanation of the Ir $5d-\mathrm{O} 2p$ hybridization spectral structure in the optical region of the RIXS spectra is given in terms of Ir $dd$ excitations. Our findings allow us to infer that the relative electronic correlations in the metallic ${\mathrm{Bi}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ compound with sizable spin-orbit coupling are $0.06\le U/W\le 0.22$ and thus are in the moderate-to-weak regime. We found that the $J_{\mathrm{eff}}$ model, developed to understand the existence of insulating states in various iridate perovskites, is not applicable to ${\mathrm{Bi}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, casting doubts on its applicability to pyrochlore iridates with moderate to weak electronic correlations. We argue that the strong O $2p-\mathrm{Ir} 5d-\mathrm{Bi} 6s/6p$ hybridization is responsible for the highly itinerant nature of this material. Our results establish the capability of O $K$-edge RIXS to identify the electronic ground state of $5d$ spin-orbit transition metal compounds and to qualitatively assess their degree of localization.

Low-energy excitations in correlated electron systems may show intricate behaviors and provide essential insights into the dynamics of quantum states and phase transitions. Here, we study a typical half-filled two-orbital Hubbard model featuring the so-called holon-doublon (HD) low-energy excitations in the orbital-selective Mott phase (OSMP), where the principal form of the low-energy excitations has been considered to be a HD bound state. We employ standard single-site dynamical mean-field theory (DMFT), using NORG as an improved impurity solver to calculate the spectral functions at zero temperature. We show that the HD bound state gives an incomplete or even wrong picture for the low-energy excitations. Instead, the excitations are composed of a Kondo-like state in the wide band and a doublon in the narrow band, termed as interband Kondo-like (IBK) bound states. Remarkably, we find that, as the bandwidths of the two bands approach each other, anomalous IBK bound-state excitations appear in the metallic wide band. Our study provides a new picture for the low-energy excitations in the OSMP.

Charge ordering is often found in the phase diagram of unconventional superconductors in close proximity to the superconducting state. This has led to the suggestion that fluctuations of charge order can mediate superconducting pairing. While several mechanisms can lead to charge order, one common mechanism is the long-range Coulomb interaction, resulting in a Wigner crystal charge ordered state. For an electron density of 0.5 per site, we investigate the extended Hubbard model on the two-dimensional square lattice using exact diagonalization and density matrix renormalization group methods. Our results show that the strength of pairing decreases with the nearest-neighbor Coulomb interaction strength $V$ and remains weaker than the tendency of pairing for noninteracting electrons.

We investigate the magnetic orders in a mixture of a boson (exciton) and a fermion (electron or hole) trapped in transition-metal dichalcogenides moiré superlattices. A sizable antiferromagnetic exchange interaction is found between a carrier and an interlayer exciton trapped at different high-symmetry points of the moiré supercell. This interaction, at a distance much shorter than the carrier-carrier separation, dominates the magnetic order in the Bose-Fermi mixture, where the carrier sublattice develops ferromagnetism opposite to that in the exciton sublattice. We demonstrate the possibility of increasing the Curie temperature of moiré carriers through electrical tuning of the exciton density in the ground state. In a trilayer moiré system with a $p\text{−}n\text{−}p$ type band alignment, the exciton-carrier interplay can establish a layered antiferromagnetism for holes confined in the two outer layers. We further reveal a spontaneous nematic order in the Bose-Fermi mixture, arising from the interference between the Coulomb interaction and $p$-wave interlayer tunneling dictated by the stacking registry.

Boosting the efficiency of second harmonic generation based on subwavelength lithium niobate films has long been a hot topic of concern. However, it is difficult to achieve steep sidewalls and high edge resolution during the fabrication of lithium niobate nanoresonators, leading to devices with poor performance. Here, we propose a second harmonic generation (SHG) device based on lithium niobate that is robust against fabrication errors. The SHG effect is significantly enhanced by constructing the Brillouin zone folding induced bound states in the continuum (BZF-BIC) in the lithium niobate nanoresonators. The device has a high conversion efficiency of 2.26% under only $2\text{−}\mathrm{kW}/{\mathrm{cm}}^2$ pump intensity, which, to the best of our knowledge, is the highest second harmonic generation efficiency at the lowest pump intensity. This efficiency is maintained despite fabrication errors with sidewall tilt angles of 82–86° and pore size errors of ±15 nm. Furthermore, the device demonstrates high second harmonic generation efficiency in a large range of incident angles due to the high quality factor of BZF-BIC in the entire Brillouin zone. Our approach can be applied in various fields, including nonlinear optics, sensing, and optical communications.

We theoretically demonstrate that the Chern number tunable quantum anomalous Hall effect (QAHE) and second-order topological insulators can be induced in a two-dimensional ${\mathbb{Z}}_2$ topological insulator (TI), i.e., the Kane-Mele model, by applying $d$-wave altermagnetism. When the Néel vector of altermagnetism lies in the $x\text{−}y$ plane, the ${\mathbb{Z}}_2$ TI is broken and driven into a second-order topological insulator phase, exhibiting the representative corner states at nanoflakes. When the intrinsic Rashba spin-orbit coupling is further included, the second-order TI is further driven into the QAHE phase with various Chern numbers (e.g., $\mathcal{C}=\pm 1$ or $\pm 3$). When the Néel vector is along the $z$ direction, the intrinsic Rashba spin-orbit coupling is necessary to break the mirror symmetry to allow a sequential emergence of second-order TI and QAHE along with the increase of altermagnetism strength. We also observe the QAHE with mixed chirality, i.e., there exist counterpropagating edge modes but net chiral current at the ribbon boundary. Our work shows that altermagnetism can play a crucial role in exploring a rich variety of topological phases, just like its counterparts of ferromagnetism and antiferromagnetism.

We propose a superconducting nanomechanical device that can actuate and probe an oscillating flying qubit facilitating quantum information transfer over a distance. The flying qubit is formed by a movable Cooper-pair box (CPB) consisting of a superconducting dot and a bulk superconductor, which are entangled by lifting the Coulomb blockade of Cooper-pair tunneling electrostatically. We show that flying qubit states formed on a movable CPB can be observed in electron transport to a normal electrode via Andreev reflections. The charge transfer due to periodic mechanical motion of CPB leads to the occurrence of a nonzero current at zero bias voltage, and its coherence can be identified through the oscillatory current dependence of the current on a gate voltage. The proposed device provides a building block for dissipationless quantum information circuits.

The entanglement Hamiltonian (EH) provides the most comprehensive characterization of bipartite entanglement in many-body quantum systems. Ground states of local Hamiltonians inherit this locality, resulting in EHs that are dominated by local, few-body terms. Unfortunately, in nonequilibrium situations, analytic results are rare and largely confined to continuous field theories, which fail to accurately describe microscopic models. To address this gap, we present an analytic result for the EH following a quantum quench in noninteracting fermionic models, valid in the ballistic scaling regime. The derivation adapts the celebrated quasiparticle picture to operators, providing detailed insights into its physical properties. The resulting analytic formula serves as a foundation for engineering EHs in quantum optics experiments.

Non-Hermitian systems and their topological singularities, such as exceptional points (EPs), lines, and surfaces, have recently attracted intense interest. The investigation of these exceptional constituents has led to fruitful applications. The responsivity of the eigenvalue diverges at EPs, and chiral state transfer occurs when encircling an EP. Traditionally, it was believed that these exceptional features were unique to non-Hermitian systems requiring gain, loss, or nonreciprocal hopping. Here we show that these exceptional features are also present in nonlinear Hermitian systems. We consider two coupled resonators with Kerr nonlinearity in one resonator, and no non-Hermitian terms. We identify EP-like points (ELPs) on the eigenspectra where the critical behaviors are the same as those of typical EPs. Additionally, this nonlinear Hermitian system can be mapped to linear non-Hermitian systems, with ELPs corresponding to EPs. We also demonstrate that encirclement around an ELP in the parameter space leads to unique chiral state transfer behavior.

Recent experimental work indicates that the Kitaev spin liquid may be realizable close to zero magnetic field in exfoliated $\alpha \text{−}{\mathrm{RuCl}}_3$ flakes, thus providing a more versatile setting for studying non-Abelian anyons. Here, we propose a robust nanoscale signature of the Kitaev spin liquid that results from its edge states and manifests in the low-energy spin dynamics. In particular, we highlight a singular peak in the dynamical spin structure factor of a zigzag edge whose energy scales linearly with only one component of the magnetic field. This sharp feature in the local spin dynamics directly reflects the bond-directional Kitaev spin interactions and, more generally, the projective symmetries of the Kitaev spin liquid. We demonstrate that our proposed signature survives in the presence of edge disorder as well as non-Kitaev interactions, and provide detailed guidelines for experimentally observing it using inelastic electron tunneling spectroscopy and color-center relaxometry.

Bound states in the continuum (BICs) are radiation-free states whose energy is embedded in the continuum spectrum of the radiating modes. In general, BICs in periodic structures appear at isolated points in momentum space, whereas BICs forming a line require radiative continuum designs. Here, we demonstrate theoretically and experimentally that BICs can form continuous lines in a simple phononic crystal slab without special radiative continuums. Contrary to common sense, these BICs are symmetry-protected BICs not only at the center of the Brillouin zone ($\mathrm{\Gamma }$ point) but also along high-symmetry directions of momentum space. We utilize numerical simulations, a group theory method, and a mode expansion method to comprehensively understand the formation of the BICs. The BICs lines are observed experimentally by measuring the transmittance of the phononic crystal slab. Moreover, it is revealed that the BICs lines correspond to phase singularity lines of far-field radiation, and the phase winding number can be defined as a topological index. Our findings have the potential to improve multifrequency and multiwave-vector performance for applications such as high-intensity sound sources, filters, and sensors.