High Nb-containing TiAl alloys exhibit exceptional high-temperature strength and room-temperature ductility, making them widely used in hot-section components of automotive and aerospace engines. However, the lack of accurate interatomic interaction potential for large-scale modeling severely hampers a comprehensive understanding of the failure mechanism of Ti-Al-Nb alloys and the development of strategies to enhance the mechanical properties. Here, we develop a general-purpose machine-learned potential (MLP) for the Ti-Al-Nb ternary system by combining the neural evolution potential framework with an active learning scheme. The developed MLP, trained on extensive first-principles datasets, demonstrates remarkable accuracy in predicting various lattice and defect properties as well as high-temperature characteristics such as thermal expansion and melting point for TiAl systems. Notably, this potential can effectively describe the key effect of Nb doping on stacking fault energies and formation energies. Of practical importance is that our MLP enables large-scale molecular dynamics simulations involving tens of millions of atoms with ab initio accuracy, achieving an outstanding balance between computational speed and accuracy. These results pave the way for elucidating micromechanical behaviors and failure mechanism TiAl lamellar structures and developing high-performance TiAl alloys towards applications at elevated temperatures.

Strong disorder may significantly slow down or even completely hinder the thermalization of quantum many-body systems due to many-body localization (MBL). A sufficiently deep quasiperiodic potential may also inhibit thermalization. In this paper, we numerically demonstrate direct differences in the behavior of standard ergodicity-breaking indicators at the MBL crossover in random and quasiperiodic systems. Our key finding is the exponential increase in the sharpness of the MBL crossover with system size for quasiperiodic systems, a trend that is only linear in disordered systems. The strong tendency towards nonanalytic behavior in quasiperiodic systems is consistent with the existence of dynamical regimes with sharply defined boundaries or an MBL phase transition. It highlights the importance of quasiperiodic systems for our understanding of many-body dynamics.

In this paper, we investigate the Anderson model on the Bethe lattice, focusing on the localized regime. Using the cavity approach, we derive compact expressions for the inverse participation ratios (IPRs) that are equivalent to those obtained using the supersymmetric formalism and naturally facilitate a highly efficient computational scheme. This method yields numerical results with unprecedented accuracy, even very close to the localization threshold. Our approach allows for high-precision validation of all theoretical predictions from the analytical solution, including the finite jump of the IPRs at the transition. Additionally, we reveal a singular behavior of the IPRs near the critical point that has not been previously reported in the literature. This singular behavior is further confirmed by the numerical solution of the nonlinear $\sigma$ model on the Bethe lattice, which provides an effective description of Anderson localization.

The discovery of ferromagnetism in van der Waals materials attracts intense research interest and holds profound implications for two-dimensional spintronic devices. However, in most cases the Curie temperature of van der Waals ferromagnets is much lower than room temperature, hindering their potential for device applications. In this study we report the discovery of room-temperature ferromagnetism in layered ${\mathrm{Ta}}_{0.67}{\mathrm{V}}_{0.33}{\mathrm{Se}}_2$. The single crystal is synthesized through the partial replacement of tantalum with vanadium. The crystal structure of ${\mathrm{Ta}}_{0.67}{\mathrm{V}}_{0.33}{\mathrm{Se}}_2$ closely resembles that of both $1\text{T-VS}{\mathrm{e}}_2$ and $1\text{T-TaS}{\mathrm{e}}_2$. The resultant ${\mathrm{Ta}}_{0.67}{\mathrm{V}}_{0.33}{\mathrm{Se}}_2$ exhibits a Hall sign reversal at around $60\mathrm{K}$, with the dominant carrier changing from electron type at higher temperatures to hole type at lower temperatures. The anomalous peak is observed in the longitudinal resistivity near the critical temperature, which is ascribed to the temperature-induced Lifshitz transition. Despite the fact that bulk $1\text{T-VS}{\mathrm{e}}_2$ and $1\text{T-TaS}{\mathrm{e}}_2$ are paramagnetic, ${\mathrm{Ta}}_{0.67}{\mathrm{V}}_{0.33}{\mathrm{Se}}_2$ displays room-temperature ferromagnetism, as evidenced by the hysteresis behavior observed in the field-dependent magnetization. Collective anomalies are observed at about $60\mathrm{K}$ in both magnetization and transport measurements, indicating a strong correlation between electric and magnetic degrees of freedom. Moreover, room-temperature ferromagnetism is confirmed in few-layer ${\mathrm{Ta}}_{0.67}{\mathrm{V}}_{0.33}{\mathrm{Se}}_2$ through magneto-optic Kerr measurements. Our work provides a strategy for accessing two-dimensional high-Curie-temperature magnets, which hold promise for potential applications in spintronic devices.

We investigate the non-Hermitian (NH) attractive Fermi-Hubbard model with asymmetric hopping and complex-valued interactions, which can be realized by collective one-body loss and two-body loss. By means of the NH BCS theory, we find that the weak asymmetry of the hopping does not affect the BCS superfluidity since it only affects the imaginary part of the eigenvalues of the BdG Hamiltonian. Systematic analysis in the $d$-dimensional hypercubic lattices clarifies that the singularity in the density of states affects the phase boundary between the normal and dissipation-induced superfluid states. Our results can be tested in ultracold atoms by using the photoassociation techniques and a nonlocal Rabi coupling with local losses and postselecting null measurement outcomes with the use of the quantum-gas microscope.

We investigate the layer-resolved magnetotransport properties of the antiferromagnetic/paramagnetic superlattices based on one-band half-filled Hubbard model in three dimensions. In our set up, the correlated layers (with on-site repulsion strength $U\ne$ 0) are intercalated between the uncorrelated ($U=0$) layers. Our calculations, based on the semiclassical Monte Carlo technique, show that the magnetic moments are induced in the thin uncorrelated layers at low temperatures due to kinetic hopping of the carriers across the interface. The average induced magnetic moment in the uncorrelated layers varies nonmonotonically with the $U$ values of the correlated layers. Interestingly, the induced magnetic moments are antiferromagnetically arranged in the uncorrelated layers and mediate antiferromagnetic ordering between the correlated layers. As a result, the whole superlattice system turns out to be antiferromagnetic insulating at low temperatures. At the same time, for $U\sim$ bandwidth, the local moments in the correlated planes increase as a function of the distance from the interface. Expectedly, our in-plane resistivity calculations show that the metal-insulator transition temperature of the central plane is larger than the edge planes in the correlated layers. On the other hand, although the induced moments in uncorrelated planes decrease considerably as move from the edge planes to the center planes, the metal-insulator transition temperature remains more or less the same for all planes. The induced moments in the uncorrelated layers gradually dissipate with increasing the thickness of the uncorrelated layer, and as a result, the long-range antiferromagnetic ordering vanishes in the superlattices, similar to the experiments.

We introduce a new framework to study the dynamics of open quantum systems with linearly coupled Gaussian baths. Our approach replaces the continuous bath with an auxiliary discrete set of pseudomodes with dissipative dynamics, but we further relax the complete positivity requirement in the Lindblad master equation and formulate a quasi-Lindblad pseudomode theory. We show that this quasi-Lindblad pseudomode formulation directly leads to a representation of the bath correlation function in terms of a complex weighted sum of complex exponentials, an expansion that is known to be rapidly convergent in practice and thus leads to a compact set of pseudomodes. The pseudomode representation is not unique and can differ by a gauge choice. When the global dynamics can be simulated exactly, the system dynamics is unique and independent of the specific pseudomode representation. However, the gauge choice may affect the stability of the global dynamics, and we provide an analysis of why and when the global dynamics can retain stability despite losing positivity. We showcase the performance of this formulation across various spectral densities in both bosonic and fermionic problems, finding significant improvements over conventional pseudomode formulations.

We study the nonlinear $\sigma$-model in $(d+1)$-dimensional space-time with connected target space $K$ and show that, at energy scales below singular field configurations (such as vortices), it has an emergent noninvertible higher symmetry. The symmetry defects of the emergent symmetry are described by the $d$-representations of a discrete $d$-group ${\mathbb{G}}^{\left(d\right)}$ (i.e., the emergent symmetry is the dual of the invertible $d$-group ${\mathbb{G}}^{\left(d\right)}$ symmetry). The $d$-group ${\mathbb{G}}^{\left(d\right)}$ is determined such that its classifying space $B{\mathbb{G}}^{\left(d\right)}$ is given by the $d\mathrm{th}$ Postnikov stage of $K$. In $(2+1)$ dimensions and for finite ${\mathbb{G}}^{\left(2\right)}$, this symmetry is always holo-equivalent to an invertible 0-form—ordinary—symmetry with a potential 't Hooft anomaly. The singularity-free disordered phase of the nonlinear $\sigma$ model spontaneously breaks this symmetry, and when ${\mathbb{G}}^{\left(d\right)}$ is finite, it is described by the deconfined phase of ${\mathbb{G}}^{\left(d\right)}$ higher gauge theory. We consider examples of such disordered phases. We focus on a singularity-free $S^2$ nonlinear $\sigma$ model in $(3+1)$ dimensions and show that it has an emergent noninvertible higher symmetry. As a result, its disordered phase is described by axion electrodynamics and has two gapless modes corresponding to a photon and a massless axion. Notably, this nonperturbative result is different from the results obtained using the $S^N$ and $\mathbb{C}{P}^{N-1}$ nonlinear $\sigma$ models in the large-$N$ limit.

Two-dimensional (2D) silicon carbide stands out among 2D materials, primarily due to its notable band gap, unlike its carbon-based counterparts. However, the binary nature and nonlayered structure of bulk SiC present challenges in fabricating its 2D counterpart. Recent advancements in technology have led to the successful synthesis of atomically thin, large-scale epitaxial monolayers of hexagonal SiC [C. M. Polley et al., Phys. Rev. Lett. 130, 076203 (2023)] and ${\mathrm{Si}}_9{\mathrm{C}}_{15}$ [Z.-Y. Gao et al. Adv. Mater. 34, 2204779 (2022)], marking a significant milestone in semiconductor research. Inspired by these advancements, we have computationally designed another stable phase of 2D SiC in the popular biphenylene network, termed SiC biphenylene. This structure is characterized by interconnected polygons of octagons, hexagons, and tetragons arranged periodically. The dynamical and thermal stability has been confirmed through ab initio phonon dispersion and molecular dynamics simulations. The structure demonstrates a high melting point of approximately 3475 K and a “direct” band gap of 2.16 eV using the HSE06 functional. Upon considering many-body effects, the quasiparticle band gap widens to 2.89 eV at the $G_0{W}_0$ level, indicating pronounced electron correlation effects within the material. Our analysis further reveals that the effective mass of charge carriers exhibits higher values along the $\mathrm{\Gamma }\to Y$ compared to the $\mathrm{\Gamma }\to X$ direction. Moreover, the optical spectrum obtained from solving the Bethe-Salpeter equation $(G_0{W}_0+\text{BSE})$ identifies the first optically active exciton peak at 2.07 eV, corresponding to a strongly bound exciton with a binding energy of 0.82 eV. The effective mass and Bohr radius of the exciton are calculated to be 1.01 $m_0$ and 2.14 Å, respectively, demonstrating the characteristic of a Frenkel exciton. Furthermore, the investigation into stable bilayer structures across various stacking configurations (AA, ${\mathrm{AA}}^{\prime }$, and AB stacked) highlights the impact of stacking patterns on excitonic binding energies, with the AA-stacked configuration indicating the presence of a Mott-Wannier exciton. Our investigation extends to identifying the stable bulk phase of SiC biphenylene, revealing lower self-energy corrections compared to monolayer and bilayer structures, attributed to increased electron delocalization in bulk structures.

Charge-transfer excitons are studied by magneto-optical spectroscopy on (Ga,In)As/GaAs/Ga(As,Sb) asymmetric type-II double quantum well heterostructures grown by metal-organic vapor phase epitaxy. Several charge-transfer magnetoexciton resonances are clearly resolved in the optical absorption spectra. The dependence of the energies and the oscillator strengths of the exciton ground state and excited exciton states on the magnetic field are studied. For small magnetic fields, a diagmagnetic shift proportional to $B^2$ dominates, which for larger magnetic fields continuously changes into a linear dependence due to Landau quantization. The measured spectral shifts and the increase in oscillator strengths with magnetic field are in very good agreement with microscopic theoretical simulations. Our results thus demonstrate that magnetoexcitons exist in type II quantum well structures, where the electrons and holes are spatially separated and localized in different quantum wells, and we analyze their properties and tunability over a broad range of field strength. Thereby, we provide a detailed analysis of the properties of magnetoexcitons in type-II quantum well structures, where the electrons and holes are spatially separated and localized in different quantum wells, and demonstrate their tunability over a broad range of field strengths.

The imaginary part of the quantum geometric tensor is the Berry curvature, while the real part is the quantum metric. Dirac fermions derived from a tight-binding model naturally contains a mass term $m\left(k\right)$ with parabolic dispersion, $m\left(k\right)=m+u{k}^2$. However, in the Chern insulator based on Dirac fermions, only the sign of the mass $m$ is relevant. Recently, it was reported that the quantum metric is observable by means of the optical conductivity, which is significantly affected by the parabolic coefficient $u$. We analytically obtain the quantum metric and the optical conductivity in the Dirac Hamiltonian in arbitrary dimensions, where the Dirac mass has parabolic dispersion. The optical conductivity at the band-edge frequency significantly depends on the dimensions. We also undertake an analytical study of the quantum metric and the optical conductivity in the Su-Schrieffer-Heeger model, the Qi-Wu-Zhang model, and the Haldane model.

Recent studies have revealed that the Holstein model on a honeycomb lattice generates charge density wave (CDW) order at half-filling if the electron-phonon interaction strength surpasses a critical threshold and this transition occurs at a finite temperature. Here we consider the honeycomb-lattice Holstein model away from half-filling and employ the determinant quantum Monte Carlo method to explore the fate of CDW and possible superconductivity. We find that CDW order persists under light doping conditions, albeit with a reduced CDW region compared to the half-filling scenario. Similar to half-filling, the transition from semimetal order to CDW order also occurs at a finite temperature. Additionally, we investigate antiferromagnetic order and $s$-wave pairing order in the presence of doping.

Based on first-principles calculations combined with symmetry analyses, we theoretically investigate the topological phase transitions in noncentrosymmetric compound ${\mathrm{Ga}}_2\mathrm{BiAs}$. Without spin-orbital coupling (SOC), ${\mathrm{Ga}}_2\mathrm{BiAs}$ is a topological semimetal with a pair of triple degenerate points located in the $k_z$ axis of the Brillouin zone. When SOC is included, the triple degenerate points evolve into Dirac points due to the protection of crystal symmetry. To understand the constraints of crystal symmetry on degenerate points, the $\mathit{k}·\mathit{p}$ effective models are constructed. Furthermore, we demonstrate the topological translations of ${\mathrm{Ga}}_2\mathrm{BiAs}$ from the Dirac semimetal phase to the topological insulator and Weyl semimetal phases through the selective breaking of symmetry. Our work not only reveals the bulk-boundary correspondence of ${\mathrm{Ga}}_2\mathrm{BiAs}$ but also promotes the study of topological phase transitions.

We employ a neural-network architecture based on the vision transformer (ViT) architecture to find the ground states of quantum long-range models, specifically the transverse-field Ising model for spin-1/2 chains across different interaction regimes. Harnessing the transformer's capacity to capture long-range correlations, we compute the full phase diagram and critical properties of the model in both the ferromagnetic and antiferromagnetic cases. Our findings show that the ViT maintains high accuracy across the full phase diagram. We compare these results with previous numerical studies in the literature and, in particular, show that the ViT has a superior performance than a restricted-Boltzmann-machine-like ansatz.

We investigate nitrogen-vacancy center (NV) ensembles in diamond under the influence of strongly correlated electron-spin baths. We thoroughly calculate the decoherence properties of the NV central spin for bath concentrations of $0.1\text{–}100$ ppm using the cluster-correlation expansion (CCE) method. We systematically analyze possible origins of the significant deviations in the values of the $T_2$ coherence time reported in the literature. We demonstrate that significant variations can originate from the choice of averaging and fitting procedures used for the ensemble average and we point out the respective aspects that need to be considered when comparing the various theoretical studies. Our study may inspire readers to perform reliable simulations on the central spin problem. It provides an understanding and interpretation of the outcome parameters describing the dynamics of the local bath spins.

It was recently shown that, for central spin-1/2 and central spin-1, the XX central spin model is integrable in the presence of a magnetic field oriented perpendicular to the XY plane in which the coupling exists. In the spin-1/2 case, it was also shown, through an appropriate limit of the non-skew symmetric XXZ Richardson-Gaudin models, that it remained integrable even when the magnetic field is tilted to contain an in-plane component. Although the model has not yet been shown to explicitly belong to a known class of Richardson-Gaudin models, we show, in this work, that the spin-1 case also remains integrable in a tilted magnetic field. We do so by writing explicitly the complete set of conserved charges, then showing that these operators obey polynomial relations. It is finally demonstrated numerically that dark states, for which the central spin is completely unentangled with the bath, can emerge at strong enough coupling just as they do in the central spin-1/2 model in an arbitrarily oriented magnetic field.

We predict large regions of the charge stability diagram using a multiband and multielectron configuration interaction model of a double quantum dot system. We account for many-body interactions within each quantum dot using full configuration interaction and solve for single-particle density operators. This allows charge states to be predicted more accurately than the extensively used classical capacitance model or the single-band Hubbard model. The resulting single-particle mixed states then serve as inputs into an atomic orbital picture that allows for the explicit calculation of the underlying Hubbard model parameters by performing the appropriate integrals. This numerical approach allows for arbitrary choices of electrostatic potential and gate geometry. A common assumption when calculating charge stability diagrams from the Hubbard model is that the charge stability diagrams are periodic, but we find that the tunnel couplings for valence electrons in dots with $N=3$ electrons are significantly enhanced when compared to single-electron dots. This difference is apparent in the charge stability diagram for higher occupancy Coulomb diamonds. We also quantitatively explore how the barrier gate strength and dot pitch impact this behavior. Our work should help improve the future realistic modeling of semiconductor-dot-based quantum circuits.

One of the most pronounced non-Hermitian phenomena is the non-Hermitian skin effect, which refers to the exponential localization of bulk eigenstates near the boundaries of non-Hermitian systems. Whereas non-Bloch band theory has been developed to describe the non-Hermitian skin effect in lattice systems, its counterpart in continuum systems still lacks a quantitative characterization. Here, we generalize the non-Bloch band theory to non-Hermitian continuum systems. In contrast to lattice systems for which the bulk Hamiltonian alone determines the non-Hermitian skin effect and energy spectrum, we find for continuum systems that the number of boundary conditions, i.e., the number of independent differential equations satisfied by wave functions at two boundaries, must also be included as essential information. We show that the appropriate discretization of continuum systems into lattice models requires matching the hopping range of the latter with the number of boundary conditions in the former. Furthermore, in periodic non-Hermitian continuum systems, we highlight the application of the transfer matrix in determining the generalized Brillouin zone. Our theory serves as a useful toolbox for investigating the rich non-Bloch physics in non-Hermitian continuum systems, such as photonic crystals, elastic media, and certain cold-atom systems.

We propose an approach to study the topological properties of matter. In this approach, fermionic atoms are placed in an external magnetic field and in a two-dimensional spin-dependent optical lattice (SDOL) created by intersecting laser beams with a superposition of polarizations. To demonstrate the utility of the SDOL-based technique we compute the topological invariants (Chern numbers) for the SDOL bands as a function of an external magnetic field and show the existence of a rich topology of the energy bands for this system which does not have parity-time-reversal symmetry. We explicitly consider ${}^6\mathrm{Li}F=1/2$ atoms. Using a projection matrix method we observe topological phase transitions among an ordinary insulator, an Abelian topological insulator, and a non-Abelian topological insulator as the external magnetic field strength is varied. On introducing edges for the SDOL we find topological edge states (that are correlated with the band Chern numbers) that simultaneously exhibit nontrivial density and spin currents with both a rotational flow contribution and flow along the edge of the SDOL.