We report an ultrahigh pyroelectric response in weakly van der Waals bonded layers of two-dimensional (2D) tellurium (Te) nanosheets (thickness, $d=4$ to 6 nm) at periodic onoff temperature oscillations. A large pyroelectric coefficient, $p\sim 3\times {10}^3 \mu \mathrm{C}/\left({\mathrm{m}}^2\mathrm{K}\right)$, is observed, which is higher by eightfold than the traditional state of the art pyroelectrics, such as lead zirconate titanate, polymers, or hybrid nanocomposites. The first-principles calculations point out that the breakdown of the centrosymmetry starts from trilayers of Te atoms with a stable ground state energy and $C2$ point group ($P{3}_{1}21$ space group). It involves an angular twist in the Te-Te bonds of an exotic electronic state of 2D α-Te layers. The stimulation of the Born effective charge, in-plane piezoelectricity, and thermal expansion coefficient are shown to be the key factors that facilitate giant pyroelectricity in α-Te exfoliated thin layers. The lattice images and phonon bands (coupled to the electronic states) confirm that the α-Te is anchored in a truncated lattice (rhomboid) along the basal plane. Thus the results present a unique paradigm for a wide range of applications of infrared imaging, detection, and waste thermal energy harvesting, and many more.

Antiferromagnetic materials with van der Waals (vdW) layered structures hold great promises for developing the next generation of electronic and optoelectronic devices, providing unique combinations of terahertz spin dynamics, ultrastability against magnetic fields disturbance, and low power consumption. Here, we demonstrate unconventional nonvolatile bistable planar Hall effect (PHE) in few-layer ${\mathrm{GdTe}}_3$, a natural vdW heterostructure consisting of metallic Te layers and antiferromagnetic (AFM) GdTe bilayers. By fully unfolding the magnetic phase diagram of few-layer ${\mathrm{GdTe}}_3$ utilizing temperature- and angle-dependent magnetotransport measurements, we identify that the nonvolatile switching of bistable PHE states is rooted in the fourfold Néel vector flipping between two perpendicular in-plane magnetocrystalline anisotropy axes, each flipping reverses the signs of the PHE state. The efficient magnetic field ($H$) write-in and zero-field read out of the PHE memory pave the way for 2D AFM-based spintronics such as nonvolatile information storage and logic operations.

Accurately determining the coupling strength between polaron states is essential to describe charge-hopping transport in materials. In this paper, we revisit methodologies to extract coupling strengths using $abinitio$ approaches. Our findings underscore the critical role of incorporating anharmonic effects in the model Hamiltonian when analyzing total energy variations along reaction coordinates. Furthermore, we demonstrate that coupling strength extraction based on total energy calculated from $abinitio$ approaches is fundamentally limited owing to the stabilization of diabatic states, which do not involve coupling strength. We demonstrate that such limitation exists in both $\mathrm{DFT}+\mathrm{U}$ and HSE hybrid functional, which are widely used in the study of polaron transport. Instead, we suggest extracting coupling strength directly from the electronic structure of a system in neutral conditions. The neutral condition avoids overestimating coupling strength because of additional energy splitting between bonding and antibonding states from the charging energy. This study highlights the limitations of existing methods and introduces a robust framework for accurately extracting coupling parameters, paving the way for improved modeling of charge transport in complex materials.

Quantum information processing based on solid-state qubits like semiconductor quantum dots (QDs) have several advantages, which has attracted much attention in the past few decades. However, QDs are usually nonidentical, which hinders their effective interactions. Here, we propose a scheme to address this issue by introducing a cavity to construct a dressed resonance condition (DRC) where two nonidentical emitters can have efficient interaction. The exact numerical results demonstrate that the DRC is nearly the optimal condition for achieving the maximum population transfer when the emitter-cavity coupling strengths are restricted to a reasonable range. Quantum entanglement can also be established between two nonidentical emitters under the DRC. The results here can pave the way for the scalable quantum information processing based on inhomogeneous solid-state qubits.

Charge density wave (CDW), superconductivity, nontrivial topology, and their interplay have garnered significant attention in condensed matter physics. In this Letter, we predicted ${\alpha }_2\text{−}{\mathrm{NbSi}}_2{\mathrm{As}}_4$ as a ground-state CDW candidate, combining with superconductivity and nontrivial topology within the two-dimensional (2D) $M{A}_2{Z}_4$ family. Using first-principles calculations, we systematically investigated its electronic structure, vibrational properties, CDW instability, and superconductivity. The ${\alpha }_2\text{−}{\mathrm{NbSi}}_2{\mathrm{As}}_4$ monolayer shares a similar crystal field with $2H\text{−}{\mathrm{NbSe}}_2$, resulting in a comparable Fermi-surface topology that enables the formation of CDW. The CDW instability arises from the electron-phonon coupling between Nb-$d$ orbitals and longitudinal acoustic phonons from the Nb-$xy$ vibrational mode. Electron doping and tensile strain can suppress the CDW instability, leading to 2D superconductivity with critical temperatures of 5.73 and 5.11 K under 0.2 $e^{-}$/f.u. and 2.3% strain, respectively. The strong hybridization between Nb-$d$ and As-$p$ orbitals causes the metallic band crossing the Fermi level to intersect with lower bands, forming six topologically nontrivial Dirac points as described by a three-band tight-binding model. This nontrivial topology coexists with both CDW and superconductivity within the same metallic band. The ground-state CDW in ${\alpha }_2\text{−}{\mathrm{NbSi}}_2{\mathrm{As}}_4$ monolayer offers a potential platform for exploring the interplay of many-body instabilities and topological physics.

It is generally believed that there is a correspondence between the topological charge of nodal points or lines and the presence of Fermi arcs. Using a $\mathcal{P}\mathcal{T}$-invariant system as an example, we demonstrate that this general belief is no longer valid. When two charged nodal lines or surfaces touch, the topological charge dissipates without gap opening, yet the surfaces or hinge Fermi arcs can remain preserved. It is found that both static and Floquet semimetals can exhibit Fermi arcs, even when the nodal lines or surfaces do not carry a $Z_2$ charge from the second Stiefel-Whitney class.

Understanding multiparticle entities in a many-body system is not only of fundamental importance in condensed matter physics but also critical for many technological applications. So far, multiparticle entities in semiconductors have been studied with corresponding irreducible clusters up to the second order only. In this Letter, we theoretically predict the general existence of a different four-particle entity involving two electrons and two holes using the four-body Bethe-Salpeter equation (4B-BSE). The 4B-BSE was applied to calculate the helicity-resolved absorption for a monolayer $\mathrm{MoT}{\mathrm{e}}_2$. Surprisingly, we found a rich series of spectral peaks within an energy span of ∼40 meV below the exciton. To understand the origin of the spectral peaks, the Feynman diagrams of the 4B-BSE are recast into the cluster expansion formalism, allowing us to study the individual effects of various clusters. We found that the irreducible clusters of up to the third order and their factorized combinations cannot explain the spectral features. Importantly, we found that the fourth-order irreducible correlation, corresponding to a quadron or quadruplon, is necessary and sufficient to explain the different features. The prediction of quadruplon not only enriches our understanding of many-body correlations but also provides a different light-matter interaction mechanism for optoelectronic applications.

Semiconducting alloys, in particular SiGe, have been employed for several decades as high-temperature thermoelectric materials. Devising strategies to reduce their thermal conductivity may provide a substantial improvement in their thermoelectric performance also at lower temperatures. We have carried out an ab initio investigation of the thermal conductivity of SiGe alloys with random and spatially correlated compositional disorder employing the quasiharmonic Green-Kubo (QHGK) theory with force constants computed from density functional theory. Leveraging QHGK and the hydrodynamic extrapolation to achieve size convergence, we obtained a detailed understanding of lattice heat conduction in SiGe and demonstrated that colored disorder suppresses thermal transport across the acoustic vibrational spectrum, leading to up to a fourfold enhancement in the intrinsic thermoelectric figure of merit.

Bismuth selenide, a benchmark topological insulator, grows in a trigonal structure at ambient conditions and exhibits a number of enticing properties related to the formation of Dirac surface states. Besides this polytype, a metastable orthorhombic modification with the Pnma space group, o-${\mathrm{Bi}}_2{\mathrm{Se}}_3$, has been produced by electrodeposition and high-pressure high-temperature synthesis. It displays significant thermoelectric properties in the midtemperature range, particularly upon Sb doping. However, very little experimental information is available on the fundamental properties of this orthorhombic polytype, such as, e.g., the electronic band gap and the lattice dynamics. Here we report the temperature dependence of the Raman spectra of o-${\mathrm{Bi}}_2{\mathrm{Se}}_3$ between 10 K and 300 K, which displays an anharmonic behavior of the optical phonons that can be modeled with a two-phonon decay channel. To analyze the data, we performed ab initio calculations of the electronic band structure, the phonon frequencies at the center of the Brillouin zone, and the phonon dispersion relations along the main symmetry directions, examining the effect of spin-orbit coupling in both phonon and electronic energies. Lastly, we report cathodoluminescence experiments at 83 K that set a lower limit to the electronic band gap at 0.835 eV, pointing to an indirect nature, in agreement with our calculations. These results shed light on the essential properties of orthorhombic ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ for further understanding of the potential of this semiconductor for thermoelectrics and other relevant applications.

The potential of fidelity in accurately detecting unconventional quantum phase transitions (QPTs) remains an open question. Specific attention is directed towards high-order phase transitions, including the infinite-order Berezinskii-Kosterlitz-Thouless (BKT) transition and the Gaussian transition. In this study, we extend the concept of fidelity susceptibility to fixed-point fidelity susceptibility in the thermodynamic limit and assess this measure using tensor-network algorithms. Our results demonstrate that the fixed-point fidelity susceptibility effectively identifies the BKT and Gaussian phase transitions, a capability lacking in the traditional finite-fidelity approach. Furthermore, we establish a correlation between the fixed-point fidelity method and the entanglement method, demonstrating their equivalence. This research not only confirms the efficacy of fidelity in detecting QPTs but also enhances the understanding of fidelity-related methodologies.

Artificial spin-ice systems offer a versatile platform for exploring frustrated magnetism. Among them, the quasicrystal spin-ice system with Penrose tiling exhibits intriguing properties and convoluted spin configurations as a result of the aperiodic nature of its geometry. Due to the complex frustration behavior, the system's physical characteristics, including thermal properties, are challenging to intuitively understand, and remain unexplored. In this study, the thermal properties of the system across the entire temperature range are thoroughly investigated using various numerical approaches. We start with the ground state of this system, which is characterized by two different parts: the long-range-ordered rigid skeleton and the topologically induced emergent frustration parts. We investigate how the physical quantities (e.g., entropy, heat capacity) and spin configuration evolve throughout the entire thermalization process. At low temperatures, Schottky anomalies arise due to excitations within the frustrated spin clusters. At higher temperatures, we quantitatively analyze how the rigid skeleton breaks down under thermal fluctuations, and subsequently observe the emergence of fragmented spin clusters. Our discovery of the various excited states in the systems shows that multiple distinct magnetic states can be accessed within the system. Furthermore, the complex temperature-dependent behavior suggests the possibility of emergent phenomena in artificial quasicrystal spin-ice systems.

The discovery of superconductivity (SC) in the trilayer nickelate compound ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ under pressure has generated significant interest. In this work, we propose a trilayer two $E_g$-orbital $t\text{−}{J}_{\parallel }\text{−}{J}_{\perp }$ model to investigate the microscopic origin of SC in this system. In the strong-coupling regime, each layer is governed by a $t\text{−}{J}_{\parallel }$ model with intralayer antiferromagnetic exchange $J_{\parallel }$, while electrons are allowed to hop between layers, interacting via interlayer exchange $J_{\perp }$. The inner-layer $3d_{z^2}$-orbital electrons tend to form bonding states with those in the neighboring layers, leading to redistribution of the electron densities. The numerical simulation results indicate that SC is predominantly mediated by the $3d_{z^2}$ orbital, characterized by an intralayer extended $s$-wave pairing in the outer layers, accompanied by an interlayer pairing with opposite sign. Furthermore, we find that electron doping enhances SC, while hole doping tends to suppress it. These findings provide new insights into the SC mechanisms of ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and its sensitivity to charge doping.

We report the anomalous and topological Hall effect of the ferromagnetic Weyl semimetal ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$. We observe a significant anisotropic anomalous Hall effect due to nonzero Berry curvature in the momentum space, such that the anomalous Hall conductivity (AHC) is 965 S/cm for the $xy$ plane and 233 S/cm for the $zx$ plane of the single crystal. The band structure calculations predict that several Weyl and nodal points span across the momentum space, gapped out under the spin-orbit-coupling effect, leading to significant $k$-space Berry curvature and large AHC. Experimentally, we also demonstrate a sizable topological Hall effect that is originated by the noncoplanar chiral spin structure due to the competition between the out-of-plane uniaxial magnetocrystalline anisotropy and the dipole-dipole interaction between two Mn sublattices. This study hints at the importance of dipole-dipole interactions in producing the skyrmion lattice in ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$.

Motivated by the time-reversal symmetry-breaking signal in muon spin relaxation below a transition temperature without accompanying noticeable magnetic Bragg peaks in $5d^2$ Os double perovskites, a rare ferro-octupolar order was proposed to account for such hidden order. Here, we study the phase transitions under a magnetic field in triangular and face-centered-cubic lattices using classical Monte Carlo simulations. It is expected that higher-rank moments do not couple linearly to the magnetic field. For example, conventional Ginzburg-Landau theory expects that a field applied along the ferro-octupolar [111] axis couple in cubic order to $h_x{h}_y{h}_z$. However, we observe the emergence of antiferro-quadrupolar ordering mixed with the antiferro-octupolar order (AFQO) owing to the field-induced bond-dependent exchange interaction. This field-linear interaction among non-Kramers doublets arises via the coupling to the excited triplet states enabled by the external field. In the triangular lattice, we uncover intriguing vortex states, which could inspire future research into $5d^2$ triangular lattice systems.

Since the successful synthesis of the MoSSe monolayer, which violated the out-of-plane mirror symmetry of transition metal dichalcogenide monolayers, considerable and systematic research has been conducted on Janus monolayer materials. By systematically analyzing the LaBrI monolayer, we are able to learn more about the novel Janus material by focusing on the halogen family next to group VIA (S, Se, Te). The structural optimizations have been carried out using pseudopotential-based Quantum Espresso code. Computed structural parameters are in good agreement with literature reports. The optimized crystal structures are used for computing the effect of strain on electronic and thermoelectric properties. Dynamical stability predicts that this material can withstand up to 10% of tensile strain. Computed electronic structure reveals material to be indirect wide band-gap ferromagnet with magnetic moment $1 {\mu }_{\mathrm{B}}$. With increase in the biaxial tensile strain the band gap decreases. Furthermore, the computed magnetothermoelectric properties predict high Seebeck coefficient ∼400 µV/K and low thermal conductivity of ∼0.93 W/m K in LaBrI, which results in a high $\mathit{ZT}\sim 1.84$ for 4% strain at 800 K. The present study supports the fact that tensile strain on ferromagnetic LaBrI material can further enhance TE properties, making it a promising material for TE applications at higher temperatures.

Intercalation is a robust approach for modulating the properties of epitaxial graphene on SiC and stabilizing two-dimensional intercalant layers at the graphene/SiC interface. In this paper, we present synchrotron-based angle resolved photoelectron spectroscopy (ARPES) measurements focusing on the band structure of intercalated Pb under a single layer of epitaxial graphene. The interlayer Pb exhibits a metallic character, a ($1\times 1$) registry with respect to SiC, and free-electron-like bands to first order. Divergences from the free-electron approximation include various band splittings and gaps throughout the Pb Brillouin zone. Light polarization-dependent ARPES measurements indicate a predominant out-of-plane orbital character for the Pb bands, suggesting potential interactions between the interlayer Pb and graphene's $\pi$ orbitals that may induce proximity effects in graphene. Density functional theory calculations for a ($1\times 1$) Pb monolayer on SiC show reasonable qualitative agreement with the experimentally observed interlayer bands as well as the polarization-dependent measurements. Finally, temperature-dependent ARPES measurements reveal that the nearly charge-neutral graphene layer involves charge transfer from both the interlayer Pb and the substrate SiC.