Boron forms a wide variety of compounds with alkaline earth elements because of its unique bonding characteristics. Among these, binary compounds of Be and B display particularly rich structural diversity, attributed to the small atomic size of Be. Cubic ${\mathrm{BeB}}_2$ is a particularly interesting phase, in which Be donates electrons to stabilize a diamond-like boron network under high pressure. In this work, we employ ab initio methods to conduct a detailed investigation of cubic ${\mathrm{BeB}}_2$ and its functional properties. We show that this metastable phase is dynamically stable under ambient conditions, and its lattice match to existing substrate materials suggests possible epitaxial stabilization via thin-film growth routes. Through a comprehensive characterization of its electronic, transport, and superconductivity properties, we demonstrate that cubic ${\mathrm{BeB}}_2$ exhibits high hole concentrations and high hole mobility, making it a potential candidate for efficient $p$-type transport. In addition, cubic ${\mathrm{BeB}}_2$ is found to exhibit low-temperature superconductivity at degenerate doping levels, similar to several other doped covalent semiconductors such as diamond, Si, and SiC.

The recent discovery of fractional quantum anomalous Hall states, fractional quantum Hall states realized without an external magnetic field, in twisted transition-metal dichalcogenide (TMD) bilayers represents a significant development in condensed-matter physics. These states were first observed via photoluminescence (PL) spectroscopy. Surprisingly, a general theoretical understanding of PL is not available even for the conventional fractional quantum Hall states. Our study of a fully spin-polarized system brings out the following picture. At the Jain filling factors $\nu =n/(2n\pm 1)$, when an electron is excited from the valence to the conduction band, the photoexcited electron and the hole lower their energies by turning into composite-fermion (CF) particles and holes. These are charge $\mp e/(2n\pm 1)$ anyons and, in general, form a bound CF exciton with energy below the band gap. Away from $\nu =n/(2n\pm 1)$, the CF exciton either binds with or annihilates an already present anyon (i.e., a CF in a partially filled level) to form a trion or a single CF quasiparticle, further lowering the energy. When the interaction between two electrons is independent of whether they occupy the conduction or the valence band, as in an ideal two-dimensional system, the resulting SU(2) symmetry implies that the energy of the emitted photon must equal the band gap, independent of correlations. In this case, the bound CF excitons and CF trions are dark, indicating an absence of photoemission at zero temperature, except for the exciton of the $\nu =\frac{1}{3}$ ($\nu =\frac{2}{3}$) state in the valence (conduction) band, but we show that the temperature dependence of the PL peak intensity can be used to measure their binding energies. We also study, using exact diagonalization methods, the effect of an SU(2)-symmetry-breaking term; we find that as the strength of this term is increased, the dominant peaks of the SU(2)-symmetric theory evolve continuously, while additional, low-intensity peaks appear. We discuss implications for PL experiments in semiconductor quantum wells and twisted TMD bilayers.

Strain-induced lattice mismatch leads to moiré patterns in homobilayer transition metal dichalcogenides (TMDs). We investigate the structural and electronic properties of such strain-induced moiré patterns in TMD homobilayers. These moiré patterns consist of several stacking domains separated by tensile solitons. Relaxation of these systems distributes the strain unevenly in the moiré superlattice, with the maximum strain energy concentrating at the highest-energy stackings. The order parameter distribution shows the formation of aster topological defects at the same sites. In contrast, twisted TMDs host shear solitons at the domain walls, and the order parameter distribution in these systems shows the formation of vortex defects. The strain-induced moiré systems also show the emergence of several well-separated flat bands at both the valence- and conduction-band edges, and we observe a significant reduction in the band gap. These flat bands in these strain-induced moiré superlattices provide platforms for studying the Hubbard model on a triangular lattice as well as the ionic Hubbard model on a honeycomb lattice. Furthermore, we study the localization of the wave functions corresponding to these flat bands. The wave functions localize at different stackings compared to twisted TMDs, and our results on the localization of flat bands at the conduction band edge are in excellent agreement with spectroscopic experiments.

We study nonlinear hydrodynamic electron transport driven by an AC electric field. In noncentrosymmetric conductors with broken time-reversal (TR) symmetry the nonlinear flow of such liquids is nonreciprocal, giving rise to a DC current $I^{\text{DC}}$ that is quadratic in the amplitude of the AC electric field. This is the hydrodynamic analog of the linear photogalvanic effect, which arises in bulk noncentrosymmetric materials with broken TR symmetry. The magnitude of $I^{\text{DC}}$ depends on both the properties of the electron fluid and the geometry of the flow, and may be characterized by two dimensionless parameters: the nonreciprocity number $\mathcal{N}$ and the frequency-dependent vibrational number $\mathcal{R}$. Due to nonlocality of hydrodynamic transport, at low frequencies of the AC drive, $I^{\text{DC}}$ is superextensive. The AC component of the electric current is likewise strongly affected by nonreciprocity: the hysteretic current-voltage dependence becomes skewed, which can be interpreted in terms of nonreciprocity of the memory retention time.

Van der Waals two-dimensional (2D) transition-metal dichalcogenides (TMDs) exhibit great potential in solid tribology design. Understanding the mechanism at the electronic level is essential for the design of low-friction-coefficient 2D materials. Here, via a comparative study of metallic $d^1$ and semiconducting $d^2$ TMD bilayers (BLs), we find that their frictional potential energy under normal load is correlated to their metallicity; namely, the metallic TMD has a lower frictional potential energy. The underlying reason is that the metallicity or semiconductivity in TMD energy band structures results in different interlayer quasibonding (QB) interactions. For the metallic $d^1 \mathrm{Nb}{\mathrm{S}}_2$ monolayer, there is a half-filled band; in a BL, the interlayer QB alters the half-filled band into two partial-filled bands that have different numbers of electrons, leading to a charge transfer between the two partial-filled bands. The charge transfer between bands effectively weakens the Pauli repulsion between the metallic TMD layers and ultimately leads to a significant decrease in the slope of the frictional potential energy with the increase of normal load. In contrast, for semiconducting $d^2 \mathrm{Mo}{\mathrm{S}}_2$, there is no such charge-transfer effect to reduce the interlayer Pauli repulsion, and the slope of frictional potential energy with normal load is larger than the metallic TMDs. Our findings provide an approach for designing lower friction coefficients with metallic van der Waals layered materials.

We consider a proximity effect between Dirac surface states of a topological insulator and the skyrmion phase of an insulating magnet. A single skyrmion results in the surface states having a chiral gapless mode confined to the perimeter of the skyrmion. For the lattice of skyrmions, the tunneling coupling between confined states leads to the formation of low-energy bands delocalized across the whole system. We show that the structure of these bands can be investigated with the help of the phenomenological chiral network model with a kagome lattice geometry. While the network model by itself can be in a chiral Floquet phase unattainable without external periodic driving, we show how to use a procedure known as band reconstruction to obtain the low-energy bands of the electrons on the surface of the topological insulator for which there is no external driving. We conclude that band reconstruction is essential for the broad class of network models recently introduced to describe the electronic properties of different nanostructures.

Multiferroics exhibiting simultaneous magnetic and ferroelectric orders constitute essential systems for realizing magnetoelectric coupling, thereby presenting considerable promise for technological applications. However, the development of multiferroics faces a persistent challenge: achieving the coexistence of robust magnetoelectric coupling and pronounced ferroelectric polarization. In this study, we demonstrate the emergence of spin-driven ferroelectricity in $\mathrm{C}{\mathrm{r}}_2{\mathrm{S}}_3$ through first-principles calculations. The system exhibits out-of-plane polarization significantly exceeding that of most conventional type-II multiferroics, and features a unique pathway for ferroelectric switching through spin rotation. Furthermore, two-dimensional $\mathrm{C}{\mathrm{r}}_2{\mathrm{S}}_3$ demonstrates half-metallic behavior and maintains spin-induced ferroelectric polarization. This discovery marks a substantial advancement in the design of multiferroics.

Magnetic textures such as skyrmions in thin films grown on substrates possess significant technological potential. Inhomogeneous magnetic structures can be described as homogeneous ferromagnetic order in the presence of anisotropic spin-orbit coupling (SOC). However, how this induced SOC stemming from the magnetic textures interplays with the SOC of the substrate remains unexplored. Here we show that these two contributions to SOC are, in general, not additive. We demonstrate this by employing a spintronics gauge theory. We further compute local currents which, when considered in the proper frame, match the spintronics gauge theory results. Finally, we analyze global transport quantities and show that they substantiate our previous results quantitatively. The implications for skyrmionics as well as topological superconductivity are discussed.

Two-dimensional ferroelectric materials offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, in- and out-of-plane ferroelectricity, and high photoresponse. Precise experimental determination of the electronic structure of rhombohedral (3R) $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ is needed for a better understanding of potential properties and device applications. Here, combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we demonstrate that 3R $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ phase exhibits a robust inversion of the valence band parabolicity at the $\mathrm{\Gamma }$ point forming a bow-shaped dispersion with a depth of $140\pm 10\mathrm{meV}$ between the valence band maximum along the $\mathrm{\Gamma }K$ direction of the Brillouin zone. Moreover, we find an indirect band gap of about 1.25 eV, as well as a highly electron doping of approximately $5\times {10}^{12}\mathrm{electrons}\mathrm{per}\mathrm{c}{\mathrm{m}}^2$ at the surface. This leads to surface band bending and the formation of a prominent electron accumulation layer. These findings allow a deeper understanding of the rhombohedral $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ electronic properties underlying the potential of III–VI semiconductors for electronic and photonic technologies.

Nonequilibrium dynamics in non-Hermitian systems has attracted significant interest, particularly due to the skin effect and its associated anomalous phenomena. Previous studies have primarily focused on initial states with a definite particle number. Here, we present a systematic study of nonreciprocal quench dynamics in the pairing states with indefinite particle number. Our study uncovers a range of behaviors. First, we demonstrate a universal tendency towards half filling of particle density at late times. At early times for certain initial states, we observe a chiral wavefront in both particle number distribution and charge inflow, associated with a sharp decrease in particle number. Furthermore, we find that non-Hermiticity could enhance the growth of entanglement in the initial stages of evolution. In the intermediate-time regime, the characteristic skin effect leads to particle accumulation on one side, leading to a pronounced reduction in entanglement entropy. Moreover, our results reveal the presence of the quantum Mpemba effect during the restoration of U(1) symmetry. Our findings open avenues for exploring exotic dynamic phenomena in quantum many-body systems arising from the interplay of symmetry breaking and non-Hermiticity.

We investigated the element-specific electronic structure and charge-carrier dynamics of a single-crystal ferromagnet ${\mathrm{CoS}}_2$ with complementary x-ray spectroscopy techniques. Hard x-ray photoemission (HAXPES) is used to provide crucial information on the bulk electronic structure and chemical bonding in ${\mathrm{CoS}}_2$ that is compared against the isoelectronic paramagnet ${\mathrm{CoSe}}_2$. The Co $1s$ core-level line shows several satellite features for ${\mathrm{CoS}}_2$, showing explicit charge-transfer processes and local screening of the core hole by S ligands, whereas no such features are observed in ${\mathrm{CoSe}}_2$. The satellite structures indicate the electronic configuration of divalent ${\mathrm{Co}}^{2+}$ as a combination of $d^8\mathrm{Ḻ}$ and $d^9{\mathrm{Ḻ}}^2$ in addition to the nominal ionic $d^7$ state, where $\mathrm{Ḻ}$ represents an S $3p$ hole. We employ resonant Auger spectroscopy across the S $K$-edge for ${\mathrm{CoS}}_2$ to obtain electron delocalization times to adjacent Co atomic sites. The fast carrier dynamics are attributed to strongly screened Coulomb interactions and hence a facile carrier delocalization. The strong hybridization formed between the Co $3d$ and S $3p$ states with pronounced charge-transfer character reflects a self-doped system with a finite density $n$ of holes at the sulfur site (${\mathrm{Ḻ}}^n$), in line with recent models that indicate a negative charge-transfer energy for ${\mathrm{CoS}}_2$. In addition to HAXPES data, we also report on experimental and theoretical $L$-edge x-ray absorption and x-ray magnetic circular dichroism data for ${\mathrm{CoS}}_2$ that demonstrate multiconfiguration effects in the excitation process. To enable a direct comparison of the experimental spectra, we used density functional theory calculations to obtain the projected density of states to describe the ground-state electronic structure. The existence of fast carrier dynamics and strong charge-transfer properties, demonstrated in this study, highlights the unique nature of ${\mathrm{CoS}}_2$ with a wide potential in topological spintronics applications and integration in energy-related device platforms.

Understanding non-Fermi liquids in dimensions higher than one remains one of the most formidable challenges in modern condensed matter physics. These systems, characterized by an abundance of gapless degrees of freedom and the absence of well-defined quasiparticles, defy conventional analytical frameworks. Inspired by recent work [Delacretaz et al. Phys. Rev. Res. 4, 033131 (2022)], we present a procedure for bosonizing Fermi surfaces that does not rely on the existence of sharp excitation and is thus directly applicable to non-Fermi liquids. Our method involves parametrizing the generalized fermionic distribution function through a bosonic field that describes frequency-dependent local variations of the chemical potential in momentum space. We propose an effective action that produces the collisionless quantum Boltzmann equation as its equation of motion and can be used for any dimension and Fermi surface of interest. Even at the quadratic order, this action reproduces nontrivial results obtainable only through involved analysis with alternative means. By offering an alternative method directly applicable to studying the low-energy physics of Fermi and non-Fermi liquids, our work potentially stands as an important building block in advancing the comprehension of strange metals and associated phenomena.

In condensed matter physics, the excitonic insulator (EI) state represents an exotic macroscopic quantum coherent phase, which, however, remains elusive. In this work, we reveal that bright EI is likely to be realized in $\mathrm{Tl}\mathit{X}{\left(\mathrm{C}{\mathrm{H}}_3\right)}_2$ ($\mathit{X}=\mathrm{As}$, Sb) as a quantum spin Hall insulator (QSHI), based on the first-principles many-body perturbation theory. In particular, we highlight that the weakened dielectric screening due to the strong spin-orbit coupling (SOC) enhances the exciton binding energy ($E_{\mathrm{b}}$), and the anomalous inverse quasiparticle (QP) correction suppresses the band gap ($E_{\mathrm{g}}$) broadening. In the light of these two factors, the band dispersion can be dramatically altered, and the linear scaling relation between $E_{\mathrm{b}}$ and $E_{\mathrm{g}}$ can be successfully broken, paving the way for the excitonic instability and the possible formation of EI. It is of importance that ground-state bright excitons can provide direct spectroscopic evidence for excitonic condensation. Our results present a paradigm for EI formation that directly couples to light and enables ways for the experimental detection of EI.

The Bethe-Salpeter equation (BSE) is the workhorse method to study excitons in materials. The BSE Hamiltonian size, which depends on how many valence-to-conduction band transitions are considered, needs to be chosen to be sufficiently large to converge excitons' energies and wavefunctions, but small enough to make calculations tractable, as BSE calculations are expensive and scale with the number of atoms as $\mathcal{O}\left(N_{\mathrm{atoms}}^6\right)$. In particular, in the case of supercell (SC) calculations composed of $N_{\mathrm{rep}}$ replicas of a primitive cell (PC), a natural choice to build this BSE Hamiltonian is to include all transitions derived from PC calculations by zone folding. However, this leads to a very large BSE Hamiltonian, as the number of matrix elements in it is ${\left(N_k{N}_c{N}_v\right)}^2$, where $N_k$ is the number of $k$-points and $N_{c\left(v\right)}$ is the number of conduction (valence) states used. When creating a SC, the number of $k$-points decreases by a factor $N_{\mathrm{rep}}$ but both the number of conduction and valence states increase by the same factor, therefore the number of matrix elements in the BSE Hamiltonian increases by a factor $N_{\mathrm{rep}}^2$, making exactly corresponding calculations prohibitive. Here, we provide a workflow to decide how many transitions are necessary to achieve comparable results, based on only PC results. With our method, we show that to converge the first exciton binding energy of a LiF SC composed of 64 PCs, to an energy tolerance of 0.15 eV, we only need 12% of the valence-to-conduction matrix elements that result from zone folding with a minimal set of bands. As an example, we use the number of bands from our method to obtain the absorption spectrum of LiF with a ${\mathrm{V}}_k$-like defect. The procedure in our work helps evaluate excitonic properties in large SC calculations, such as moiré patterns, charge density waves, defects, self-trapped excitons, polarons, interfaces, and other kinds of disorder, and can be used within any code that builds the BSE Hamiltonian.

In this work, the plasmonic oscillations in two-dimensional charge-coupled semiconductor layers, including those biased by an external current, have been thoroughly investigated by the hydrodynamic model. The relative amplitudes of the charge-density oscillations and their phase shift in the two coupled layers have been calculated. For the long wavelengths important in experimental studies, it has been found that the electron relaxation radically modifies the mode types and dispersion, providing linear portions of the wave-vector dependencies followed by a square root branching. We have shown that the hydrodynamic approach does not predict any instability for drifting electrons in two coupled identical layers, while plasmonic instability is possible for the layers with distinct characteristics. For these cases, there exists a narrow $(\omega ,k)$ region with strong (resonant) interlayer interaction, where the real parts of $\omega \left(k\right)$ of the plasmon modes are identical and linear in $k$ (a second linear portion). The imaginary parts are different and one of them can change sign, leading to the plasmonic instability effect. This instability develops as the two-stream convective instability, whose threshold and frequency critically depend on the velocity of drifting electrons. Under negligibly weak electron relaxation, there is no drift velocity threshold for the revealed instability, while electron relaxation originates the finite velocity threshold. The numerical estimations of the instability critical parameters are provided for the GaAs-based heterostructures. We suggest that this investigation of charge-coupled current-biased semiconductor layers brings the new knowledge to solid-state plasmonic physics and can be used in the practical search of terahertz electric instabilities and sources of terahertz emission.

Artificial Kitaev chains (AKCs), formed of quantum dot–superconductor linear arrays, provide a promising platform for hosting Majorana bound states (MBSs) and implementing topological quantum computing. The main challenges along this research direction include the tuning of AKCs for hosting MBSs and the readout of the parity of the chains. In this work, we present a step-by-step procedure for tuning a three-site AKC to its sweet spots based on the spectra of a transmon circuit which is integrated with the chain for the purpose of reading out the parity of the chain. The signatures of the transmon's plasma modes in each step, in particular those related to the appearance of MBSs in the chain, will be given. We find that the sweet spots in a three-site AKC can be classified into three types based on the relative strengths of the elastic cotunneling (ECT) and crossed Andreev reflection (CAR): ECT-dominated sweet spots, genuine sweet spots, and CAR-dominated sweet spots. We show that the ECT-dominated and CAR-dominated sweet spots can be more conveniently accessed and utilized in transmon-based measurements.

Electrides, distinguished by non-nuclear attractors at interstitial sites, exhibit superconducting properties driven by enhanced electron–phonon coupling (EPC). Through crystal structure predictions and first-principles calculations, we identify $I4/mmm-{\mathrm{Li}}_4\mathrm{Co}$ as a high-pressure electride stabilized at pressures above 413 GPa, with a record-high superconducting critical temperature $(T_{\mathrm{c}}=123.5 \mathrm{K})$ among known electrides. This exceptional $T_{\mathrm{c}}$ arises from the combined effects of high-density satellite interstitial electrons (SIEs) and large, closed Fermi surfaces, which together strengthen EPC. A comparative analysis of the $I4/mmm-A_{4}B$ structure family $(A=\mathrm{Li}, \mathrm{Na}; B=\mathrm{Co}, \mathrm{Rh}, \mathrm{Ir}, \mathrm{Pd}, \mathrm{Ru})$ reveals two key design principles for high-$T_{\mathrm{c}}$ electrides: (1) a high density of states of SIEs and SIE-related atoms (SSADOS) near the Fermi level, and (2) a minimal interstitial space volume for SIEs (IAS). We introduce a compliance parameter, $Q=\frac{\mathrm{SSADOS}}{e^{0.2\sum \mathrm{IAS}}}$, that quantitatively integrates these key factors and shows a linear correlation with the EPC strength. These findings provide critical insights into the design of high-$T_{\mathrm{c}}$ electrides and offer a roadmap for optimizing their electronic structures and superconducting properties.

We present a class of one-dimensional, generic, spinless fermion lattice Hamiltonians that express quasi-Fermi liquid physics, manifesting both Luttinger and Fermi liquid features solely due to irrelevant interactions. Using infinite matrix product state techniques, we unveil its universal structure by calculating static and dynamic responses. Key features include a finite discontinuity in the momentum distribution at the Fermi level, despite power-law singularities in the spectral function protected by particle-hole symmetry. Away from half filling, Landau quasiparticles emerge. Charge dynamics show either high-energy bound states or a concentration of spectral weight within the continuum for attractive or repulsive interactions, respectively. These universal features are realized across multiple models and energy scales, thus reifying the quasi-Fermi liquid as a unique paradigm for one-dimensional fermions.

Tuning the electronic structures of two-dimensional (2D) material-based heterostructures is of crucial importance for their use in functional next-generation electronics. Here, through angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nano-ARPES), we systematically track the evolution of the near-Fermi-level electronic structure of bilayer ${\mathrm{WSe}}_2$ over a large range of twist angle. While the momentum positioning of the valence-band maxima (VBM) is independent of twist angle, we find that the energetic separation between the hole bands at the $K$ point of the Brillouin zone and the higher binding-energy hole band at $\mathrm{\Gamma }$ can be varied in excess of 100 meV. We explore the mechanisms underpinning this evolution and discuss the implications for tuning both the size of the band gaps, and the efficiency of the spin-dependent electron-phonon coupling channels in homobilayer transition-metal dichalcogenide devices.