We determine the ground-state phase diagram for the $1/r$-Hubbard model with repulsive nearest-neighbor interaction at half band filling using the density-matrix renormalization group method. Due to the absence of Umklapp scattering, the phase diagram displays finite regions for the three generic phases, namely, a Luttinger liquid metal for weak interactions, a Mott-Hubbard insulator for dominant Hubbard interactions, and a charge-density-wave insulator for dominant nearest-neighbor interactions. Up to moderate interaction strengths, the quantum phase transitions between the metallic and insulating phases are continuous, i.e., the gap opens continuously as a function of the interaction strength. We conclude that generic short-range interactions do not change the nature of the Mott transition qualitatively.

Excitons in van der Waals heterostructures having interlayer or intralayer types are responsible for their optical absorption properties. Here, we systematically investigate the band alignment and excitons in the ${\mathrm{MoSi}}_2{\mathrm{N}}_4/{\mathrm{WSi}}_2{\mathrm{N}}_4$ heterostructure using ab initio $GW$ calculations and the Bethe-Salpeter equation. The quasiparticle energy spectrum shows that the valence band maximum and conduction band minimum are from different layers, and hence the band alignment is type II in this heterostructure. However, unlike normal type-II heterostructures with interlayer excitons at the lowest energy, the ${\mathrm{MoSi}}_2{\mathrm{N}}_4/{\mathrm{WSi}}_2{\mathrm{N}}_4$ heterostructure behaves like a type-I heterostructure, in which the optical excitation at the lowest energy is the intralayer exciton while the interlayer exciton is higher in energy. We develop a macroscopic dielectric model explaining the atypical exciton feature as a result of the large layer distance and the small exciton Bohr radius in this heterostructure. In addition, we find that the interlayer exciton is dark due to the absence of band hybridizations between the two layers, and the energy and oscillator strength of the intralayer exciton are almost independent of the stacking configuration. These results enrich the exciton mechanism in 2D layer materials and are beneficial for the design of optoelectronic devices based on moiré heterostructures.

${\mathrm{GdTe}}_3$, a van der Waals–type antiferromagnetic (AFM) metal with high mobility, is gaining a lot of attention for its potential use in high-speed spintronic devices as well as for fundamental physics research. Due to the magnetocrystalline anisotropy of ${\mathrm{GdTe}}_3$, exotic effects are envisaged, when the magnetic configurations interact with an external magnetic field. In this work, a magnetic-field-induced Weyl state in ${\mathrm{GdTe}}_3$ is revealed. In the AFM state, ${\mathrm{GdTe}}_3$ is topologically trivial. However, when an external magnetic field exceeding $\sim 20$ T aligns all spins, band splitting occurs, and then a topological transition is induced, i.e., from a trivial metallic state to a topological Weyl metallic state. In addition, a topological change of Fermi surfaces, i.e., a field-induced Lifshitz transition, is uncovered, which may also be rooted in band splitting. Moreover, high-pressure electrical transport measurements reveal a peculiar superconducting transition with a nearly invariant superconducting transition temperature ($T_c\sim 4.2$ K) spanning a wide range of pressure up to 48 GPa. These findings imply that ${\mathrm{GdTe}}_3$ provides a unique platform for investigating not only the interactions of charge-density-wave fluctuations and superconductivity but also the interplay between magnetism and topology.

We report the electronic properties of the metallic ${\mathrm{PdSn}}_2$ single crystals by means of electrical resistivity, Hall effect, angle-resolved photoemission spectroscopy (ARPES) measurements and band-structure calculations. A simultaneous two-band analysis is conducted on magnetoresistance (MR) and Hall effect data, revealing hole-carrier dominated transport behavior. ARPES results clearly indicate the multiband nature of ${\mathrm{PdSn}}_2$ with the presence of shallow hole and electron pockets at the Fermi level, supported by first-principles calculations. From the MR data, a breakdown of Kohler's rule in the 300–80 K range is observed. We consider a possible explanation for this phenomenon, which is associated with the temperature-induced band shift near the Fermi surface.

Chromium-based nitrides are used in hard, resilient coatings and show promise for thermoelectric applications due to their combination of structural, thermal, and electronic properties. Here, we investigate the electronic structures and chemical bonding correlated to the thermoelectric properties of epitaxially grown chromium-based multicomponent nitride $\mathrm{Cr}\left(\mathrm{Mo},\mathrm{V}\right){\mathrm{N}}_x$ thin films. The small amount of N vacancies causes Cr $3d$ and N $2p$ states to appear at the Fermi level and reduces the band gap in ${\mathrm{Cr}}_{0.51}{\mathrm{N}}_{0.49}$. Incorporating holes by alloying of V in N-deficient CrN results in an enhanced thermoelectric power factor with marginal change in the charge transfer of Cr to N compared with ${\mathrm{Cr}}_{0.51}{\mathrm{N}}_{0.49}$. Further alloying of Mo, isoelectronic to Cr, increases the density of states at the Fermi level due to hybridization of the (Cr, V) $3d$ and Mo $4d$-N $2p$ states in $\mathrm{Cr}\left(\mathrm{Mo},\mathrm{V}\right){\mathrm{N}}_x$. This hybridization and N off-stoichiometry result in more metal-like electrical resistivity and reduction in Seebeck coefficient. The N deficiency in $\mathrm{Cr}\left(\mathrm{Mo},\mathrm{V}\right){\mathrm{N}}_x$ also depicts a critical role in reduction of the charge transfer from metal to N site compared with ${\mathrm{Cr}}_{0.51}{\mathrm{N}}_{0.49}$ and ${\mathrm{Cr}}_{0.50}{\mathrm{V}}_{0.03}{\mathrm{N}}_{0.47}$. In this paper, we envisage ways for enhancing thermoelectric properties through electronic band engineering by alloying and competing effects of N vacancies.

We investigate the behavior of magnetic impurities placed on the surface of superconductor thin films with spin-orbit coupling. Our study reveals long-range interactions between the impurities, which decay according to a power law, mediated by the interplay between the superfluid response described by the London equations and the anomalous supercurrents due to magnetoelectric effects. Importantly, these interactions possess a ferromagnetic component when considering the influence of the electromagnetic field, leading to the parallel alignment of the magnetic moments in the case of two impurities. In a Bravais lattice of magnetic impurities, superconductivity facilitates the establishment of ferromagnetic order within specific parameter ranges. These findings challenge the conventional understanding that ferromagnetism and superconductivity are mutually exclusive phenomena. Our theoretical framework provides a plausible explanation for the recently observed remanent flux and transport signature of ferromagnetism in iron-based superconductors, particularly Fe(Se,Te).

When two solids are separated by a vacuum gap of thickness smaller than the wavelength of acoustic phonons, the latter can tunnel across the gap thanks to van der Waals forces or electrostatic interactions. Here we show that these mechanical vibration modes can also contribute significantly, at the atomic scale, to the nonlocal radiative response of polar materials. By combining molecular-dynamics simulations with fluctuational-electrodynamics theory, we investigate the near-field radiative heat transfer between two slabs due to this optomechanical coupling and highlight its dominant role at cryogenic temperatures. These results pave the way to exciting avenues for the control of heat flux and the development of cooling strategies at the atomic scale.

Phase diagrams serve as a highly informative tool for materials design, encapsulating information about the phases that a material can manifest under specific conditions. In this work, we develop a method in which Bayesian inference is employed to combine thermodynamic data from molecular dynamics (MD), melting point simulations, and phonon calculations, process these data, and yield a temperature-concentration phase diagram. The employed Bayesian framework yields not only the free energies of different phases as functions of temperature and concentration but also the uncertainties of these free energies originating from statistical errors inherent to finite-length MD trajectories. Furthermore, it extrapolates the results of the finite-atom calculations to the infinite-atom limit and facilitates the choice of temperature, chemical potentials, and the number of atoms conducting the next simulation with which will be the most efficient in reducing the uncertainty of the phase diagram. The developed algorithm was successfully tested on two binary systems, Ge-Si and K-Na, in the full range of concentrations and temperatures.

We investigate the dynamics of a particle in a binary lattice with staggered on-site energies. An additional static force is introduced which further adjusts the on-site energies. The binary lattice appears to be unrelated to the semiclassical Rabi model, which describes a periodically driven two-level system. However, in a certain parity subspace, the Floquet Hamiltonian of the semiclassical Rabi model can be exactly mapped to that of the binary lattice. These connections provide a different perspective for analyzing lattice systems. At resonance, namely that the mismatch of on-site energies between adjacent sites is nearly multiple of the strength of the static force, the level anticrossing occurs. This phenomenon is closely related to the Bloch-Siegert shift in the semiclassical Rabi model. At the $n\mathrm{th}$ order resonance, an initially localized particle exhibits periodic jumps between site 0 and site $(2n+1)$, rather than continuous hopping between adjacent sites. The binary lattice with a static force serves as a bridge linking condensed matter physics and quantum optics, due to its connection with the semiclassical Rabi model.

The process of finding activated transitions in localized spin systems with continuous degrees of freedom is developed based on a magnetic variant of the activation-relaxation technique. In addition to the description of the method and the relevant local properties of the magnetic energy landscape, a criterion to efficiently recognize failed attempts and an expression for the step magnitude to control the convergence are proposed irrespective of the physical system under study. The present implementation is validated on two translational symmetric systems with isotropic exchange interactions. Then, in one example, diffusion processes of a skyrmion vacancy and a skyrmion interstitial are revealed for a skyrmion system on a square spin lattice. In another example, the set of activation events about a metastable state of a two-dimensional dipolar spin glass is investigated and the corresponding energy barrier distribution is found. Detailed inspection of the transition states reveals the participation of nearest-neighbor pairs affording a simplified analytical understanding.

Chiral superconductors spontaneously break time-reversal symmetry and host topologically protected edge modes, supposedly generating chiral edge currents which are typically taken as a characteristic fingerprint of chiral superconductivity. However, recent studies have shown that the total edge current in two dimensions (2D) often vanishes for all chiral superconductors except for chiral $p$-wave, especially at low temperatures, thus severely impeding potential experimental verification and characterization of these superconductors. In this work, we use the quasiclassical theory of superconductivity to study mesoscopic disk-schaped chiral $d$-wave superconductors. We find that mesoscopic finite-size effects cause a dramatic enhancement of the total charge current and orbital magnetic moment (OMM), even at low temperatures. We study how these quantities scale with temperature, spontaneous Meissner screening, and system radius $\mathcal{R}\in [5,200]{\xi }_0$ with superconducting coherence length ${\xi }_0$. We find a general $1/\mathcal{R}$ scaling in the total charge current and OMM for sufficiently large systems, but this breaks down in small systems, instead producing a local maximum at $\mathcal{R}\approx 10–20{\xi }_0$ due to mesoscopic finite-size effects. These effects also cause a spontaneous charge-current reversal opposite to the chirality below $\mathcal{R}<10{\xi }_0$. Our work highlights mesoscopic systems as a route to experimentally verify chiral $d$-wave superconductivity, measurable with magnetometry.

Pair density waves (PDWs) are superconducting states formed by Cooper pairs of electrons containing a nonzero center-of-mass momentum. They are characterized by a spatially modulated order parameter and may occur in a variety of emerging quantum materials such as cuprates, transition-metal dichalcogenides (TMDs), and Kagome metals. Despite extensive theoretical and numerical studies seeking PDWs in a variety of lattices and interacting settings, there is currently no exact mechanism that spontaneously favors a modulated solution of the superconducting order parameter. Here, we study the problem of two electrons subject to an anisotropic attractive potential. We solve the two-body Schrödinger wave equation exactly to determine the pair binding energy as a function of the center-of-mass momentum. We find that a modulated (finite momentum) pair is favored over a homogeneous (zero momentum) solution above a critical, intermediate interaction strength. Hence our exact result justifies previous mean-field approximations that obtain modulated ground states at finite but large interactions. Using this insight from the exact two-body solution, we construct a variational many-body wave function and show that the conclusions of the two-body problem are robust in the many-body limit. Our results thus lay the theoretical and microscopic foundation for the existence of PDWs.

We introduce a family of one-dimensional aperiodic tight-binding models with linearly varying patches of $A$-type sites with on-site energies ${\epsilon }_A=0$ connected by single $B$-type sites with ${\epsilon }_B=W$. We analytically show such structures have strong spatial correlations. We theoretically find states are extended at resonance levels in the vicinity of $E_M^{\kappa }=-2cos\frac{\kappa \pi }{M}$ if they are allowed energies, where $M=md$ are the size differences of patches, $d$ is the variation rate of patch sizes, $m\in {\mathcal{N}}_{+}$, and $\kappa =1,2,...,M-1$. Related delocalization-localization transitions are explored. Numerical evidence is in excellent quantitative agreement with theoretical predictions.

In this paper, we theoretically investigate neuronlike spiking dynamics in an elliptic ferromagnet (FM)/heavy metal bilayer-based spin Hall nano-oscillator (SHNO) in a bias field free condition, very suitable for practical realization of brain-inspired computing schemes. We demonstrate regular periodic spiking with tunable frequency as well as the leaky integrate-and-fire (LIF) behavior in a single SHNO by manipulating the pulse features of input current. The frequency of regular periodic spiking is tunable in a range of 0.5–0.96 GHz (460 MHz bandwidth) through adjusting the magnitude of constant input DC current density. We further demonstrate the reconfigurability of spiking dynamics in response to a time-varying input accomplished by continuously increasing the input current density as a linear function of time. Macrospin theory and micromagnetic simulation provide insight into the origin of bias field free auto-oscillation and the spiking phenomena in our SHNO. In addition, we discuss how the shape anisotropy of the elliptic FM influence the bias field free auto-oscillation characteristics, including threshold current, frequency, and transition from in-plane to out-of-plane precession. The SHNO operates $<{10}^{12}\mathrm{A}/{\mathrm{m}}^2$ input current density and exhibits a large auto-oscillation amplitude, ensuring high output power. We show that the threshold current density can be reduced by decreasing the ellipticity of the FM layer as well as enhancing the perpendicular magnetic anisotropy. These findings highlight the potential of bias field free elliptic SHNO in designing power-efficient spiking neuron-based neuromorphic hardware.

Antiferromagnetic (AFM) thin films have been proposed as a promising material for manipulating perpendicular magnetic anisotropy (PMA) in ferromagnetic (FM) thin films. In this work, a series of epitaxially grown AFM/Co/Fe structures are investigated, in which the AFM layer is composed of fcc ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ and vertically expanded face-centered-tetragonal Mn films with distinct three-dimensional quadratic-type and two-dimensional layered spin structures, respectively. Our findings demonstrate that an individual AFM film in the composite AFM layer not only can enhance the long-range AFM ordering in its adjacent AFM neighbor but also can exert control over the neighbor's AFM spin structure; these modulation mechanisms subsequently induce PMA in an adjacent FM film. The research sheds light on the AFM proximity effects within the AFM composite layers and their profound influence on PMA induction in adjacent FM layers, offering essential insights for improving control over PMA with AFM layers.

We study the Andreev and normal reflection processes—retro as well as specular—in a bilayer graphene–superconductor junction where equal and opposite displacement fields are applied for the top and bottom layers to induce a band gap. By employing the Dirac-Bogoliubov–de Gennes equation for the gapped bilayer graphene–superconductor junction, we calculate the reflection probabilities within the scattering theory approach. The subgap conductance, calculated in the framework of Blonder-Tinkham-Klapwijk formalism, shows the contribution from the Andreev retro reflection (specular reflection) when the applied bias voltage is below (above) the Fermi energy. Notably, both retro and specular reflections are modified in the presence of the displacement field, and the retro-to-specular crossover gets amplified when the displacement field is relatively small. They can be further tuned to either specular or retro Andreev reflection by adjusting the Fermi energy. Furthermore, our study reveals the simultaneous existence of double Andreev reflections and double normal reflections when the displacement field becomes comparable to the interlayer coupling strength. The existence of the normal retro-reflection process in a bilayer graphene–superconductor junction is a finding which shows a distinctive feature in the conductance that can be experimentally verified.

A neural network is a powerful tool that can uncover hidden laws beyond human intuition. However, it often appears as a black box due to its complicated nonlinear structures. By drawing upon the Gutzwiller mean-field theory, we can showcase a principle of sign rules for ordered states in qubit lattice models. We introduce a shallow feed-forward neural network with a single hidden neuron to present these sign rules. We conduct systematical benchmarks in various models, including the generalized Ising, spin-$1/2$ XY, (frustrated) Heisenberg rings, triangular XY antiferromagnet on a torus, and the Fermi-Hubbard ring at an arbitrary filling. These benchmarks show that all the leading-order sign rule characteristics can be visualized in classical forms, such as pitch angles. Besides, quantum fluctuations can result in an imperfect accuracy rate quantitatively.

By utilizing time-dependent tensor-network algorithms in the infinite matrix-product-state representation, we theoretically investigate the pump-probe spectroscopy of the one-dimensional extended Hubbard model at half filling. Our focus lies on nonequilibrium optical conductivity and single-particle excitation spectra. In the spin-density-wave (SDW) phase, we identify an in-gap state in the nonequilibrium optical conductivity due to the formation of excitons (or doublon-holon pairs), generated by the pulse through nonlocal interactions. In the strong-coupling regime, we discern additional multiple in-gap and out-of-gap states. In the charge-density-wave (CDW) phase, we detect not only an in-gap state but also a finite Drude weight, which results from the dissolution of charge order by photoexcitation. Analyzing time-dependent single-particle excitation spectra directly in the thermodynamic limit confirms the origin of these new states in the SDW and CDW phases as the excitation of newly emerged dispersions. We illustrate that the pump-probe spectroscopy simulations in the thermodynamic limit furnish unambiguous spectral structures that allow for direct comparison with experimental results, and the integration of nonequilibrium optical conductivity and time- and angle-resolved photoemission spectroscopy provides comprehensive insights into nonequilibrium states.

Three-dimensional higher-order topological semimetals in crystalline systems exhibit higher-order Fermi arcs on one-dimensional hinges, challenging the conventional bulk-boundary correspondence. However, the existence of higher-order Fermi arc states in aperiodic quasicrystalline systems remains uncertain. In this paper, we present the emergence of three-dimensional quasicrystalline second-order topological semimetal phases by vertically stacking two-dimensional quasicrystalline second-order topological insulators. These quasicrystalline topological semimetal phases are protected by rotational symmetries forbidden in crystals, and are characterized by topological hinge Fermi arcs connecting fourfold degenerate Dirac-like points in the spectrum. Our findings reveal an intriguing class of higher-order topological phases in quasicrystalline systems, shedding light on their unique properties.

The existence of a curved graphene sheet with the geometry of a Bour surface $B_n$ is supposed, such as the catenoid (or helicoid), $B_0$, and the classical Enneper surface, $B_2$, among others. In particular, in this paper, the propagation of the electronic degrees of freedom on these surfaces is studied based on the Dirac model coupled to a non-Abelian gauge field that captures topological defects present on each of the surfaces. As a consequence of the polar geometry of $B_n$, it is found that the geometry of the surface causes the Dirac fermions to move as if they would be subjected to an external potential coupled to a spin-orbit term. The geometry-induced potential is interpreted as a barrier potential, which is asymptotically zero. Furthermore, the behavior of asymptotic Dirac states and scattering states are studied through the Lippmann-Schwinger formalism. It is found that for surfaces $B_0$ and $B_1$, the total transmission phenomenon is found for sufficiently large values of energy, while for surfaces $B_n$, with $n\ge 2$, it is shown that there is an energy point $E_K$ where Klein's tunneling is realized, while for energy values $E\gg E_K$ it is found that the conductance of the hypothetical material is completely suppressed. In addition, for a large number, $N$, of topological defects the transmission decays as $N^{-2}$ as far as energy values are different than $E_K$.