We study charge and heat transport in the square lattice Hubbard model at strong coupling using the finite-temperature Lanczos method. We construct the diffusion matrix and estimate the effect of thermoelectric terms on diffusive and hydrodynamic time evolution. The thermoelectric terms prevent the interpretation of the diffusion in terms of a single time scale. We discuss our results in relation to cold-atom experiments and measurements of heat conductivity based on the measurements of heat diffusion.

In this work, we introduce a self-consistent scheme based on the one-body reduced density matrix ($\gamma$) formalism. A significant feature of this methodology is the utilization of an optimal unitary transformation of the Hamiltonian, determined through a self-consistently determined, unitary reflection $\mathbf{R}\left[\gamma \right]$. This enables the extraction of all reduced properties of the system from a smaller, accurately solved embedding cluster, and the systematic reconstruction of the reduced density matrix of the system. This process ensures that both extended and embedded systems satisfy the local virial-like relation, providing quantitative insight into the correspondence between the fragment in the extended system and its embedded analog. The performance and convergence of the method, as well as the N-representability of the resulting correlated density matrix, are evaluated and discussed within the context of the one-dimensional Hubbard model, which provides exact results for a comprehensive comparison.

Layered materials have garnered significant attention for their ability to exhibit tunable physical properties through stacking, twisted angles, and interlayer coupling. The interlayer vibrations in these materials are highly sensitive to, and can be controlled by, their thickness. However, the layer-dependent interlayer vibration behavior under high pressure remains unclear. Here, we investigate the layer-dependent high-pressure Raman spectroscopy of 1–5L and bulk $\mathrm{Mo}{\mathrm{Te}}_2$ up to 14-GPa pressure, and demonstrate a pressure-induced thickness-dependent interlayer vibration behavior. We observe the pressure-induced blueshift rates of the breathing (LB) and shear (S) modes exhibit opposite strong layer-dependent behaviors, which arise from thickness-dependent interlayer coupling and restoring forces, respectively. Furthermore, we propose a pressure-dependent linear chain model to characterize the force constants under pressure and employ a bond-polarization model to explain the intensity changeover between the S and LB modes, as well as between the $A_1\prime /A_{1g}^2$ and $E^{\prime }/E{{}_g}^1$ modes, which is attributed to the increase in interlayer Te–Te bond angle and intralayer distance between Mo and Te atomic layers, respectively. Our findings elucidate the robust thickness-dependent interlayer vibrations in $\mathrm{Mo}{\mathrm{Te}}_2$ and provide a firm foundation for exploring and characterizing interlayer coupling through pressure engineering in van der Waals materials.

We theoretically study the spin current between two polarized Fermi gases with repulsive interactions near the itinerant ferromagnetic phase transition. We consider a two-terminal model where the left reservoir is fixed to be fully polarized while the polarization of the right reservoir is tuned through a fictitious magnetic field defined by the chemical-potential difference between different atomic hyperfine states. We calculate the spectra of the spin-flip susceptibility function, which displays a magnon dispersion emerging from the Stoner continuum at low momentum in the ferromagnetic phase. Based on the spin-flip susceptibility and using Keldysh Green's function formalism, we investigate the spin current induced by quasiparticle and spin-flip tunneling processes, respectively, and show their dependence on the polarization bias between two reservoirs. The one-body (quasiparticle) tunneling demonstrates a linear dependence with respect to the polarization bias. In contrast, the spin-flip process manifests a predominantly cubic dependence on the bias. While indicating an enhanced magnon tunneling in the strong-coupling regime, our results also demonstrate a characteristic behavior around the critical repulsive strength for ferromagnetic phase transition at low temperatures.

Stacking two layers of two-dimensional materials slightly twisted relative to each other causes significant alternations of the physical properties of the resulting bilayer. For graphene, at the right twist angle, the electronic band structure features a flat band at the Fermi level that gives rise to interesting many-body physics such as correlated insulators or superconducting states. Likewise, a finite twist angle modifies the phonon band structure. A reciprocal space continuum model including lattice reconstruction due to relaxation allows us to investigate the continuous evolution of the phonon band structure with twist angle. At intermediate angles, we find a complicated structure of the phonon density of states around the frequency of the layer breathing mode, that is substantially broadened by the moiré-induced interaction with the acoustic phonon branches. We infer optical activities and suggest Raman experiments to validate our predictions. Our results suggest that suitably twisting structures may manipulate both phonon and electron properties of such a system, and thus set the stage to test electron-phonon contributions to the observed correlated states.

Antiferrodistortive (AFD) motions in ${ABX}_3$ perovskites play an important role in determining, tuning, and creating functionalities. Here, through first-principles calculations, we predict that the AFD motions in $R{\mathrm{NiO}}_2$ (where $R$ denotes rare-earth ion), which recently gained significant interest as superconductor systems, show very similar behaviors with perovskites in various aspects, indicating an intrinsic property. The origin of AFD motions is rationalized by chemical bond valence theory, and we reveal that the undercoordination of the $R$ cation is the driving force for the appearance of rotation, analogous to perovskites. Going further, the temperature dependence of local deformations is addressed by developing a second-principles model, and we suggest that the structural phase transitions should be classified as displacive. Eventually, a direct rotation-electron-spin connection is established, which opens a pathway to purposefully tune the Fermi surface, magnetic coupling strength, and magnetic dimensionality. Given that superconducting properties are associated with electronic and magnetic properties, our results provide the possibility and feasibility of the practical control of behaviors in nickelate superconductors through rotation engineering.

The complete phase diagram of the Kitaev model with a magnetic field remains elusive, as do the experimental results in the candidate material $\alpha -{\mathrm{RuCl}}_3$. Here, we study the Kitaev model on a one-dimensional ladder setting within the density-matrix renormalization group method in the presence of a magnetic field at zero temperature. We find five distinct phases with increasing magnetic field, which are characterized by a homogeneous flux phase, the $Z_2$ vortex gas, solid and emergent glass phase, and finally, a spin-polarized phase. The emergent glassiness is confirmed by calculating correlation functions showing quasi-long-range behavior and ground state fidelity, showing a plethora of energetically accessible orthogonal saddle points corresponding to different flux configurations. This glassy behavior seems to arise from the slow dynamics of the $Z_2$ fluxes, which is a consequence of the local constraints present in the underlying Hilbert space. This phenomenon can also be explored in other spin-liquid systems where the corresponding low-energy excitations are similarly retarded due to constraints.

We present general properties of the optical activity in noncentrosymmetric materials, including superconductors. We derive a sum rule of the optical activity in general electric states and show that the summation of the spectrum is zero, which is independent of the details of electric states. The optical activity has a $\delta$-function singularity that vanishes in normal phases. However, the singularity emerges in superconducting phases, corresponding to the Meissner effect in the optical conductivity. The spectrum decreases by the superconducting gap and has a missing area compared to the normal phase. This area is exactly equivalent to the coefficient of the $\delta$-function singularity due to the universal sum rule. Furthermore, the coefficient is exactly equivalent to the superconducting Edelstein effect, which has not yet been observed in experiments. Thus this measurement of the missing area offers an alternative way to observe the superconducting Edelstein effect.

Nitinol (NiTi) is the most commonly used and most extensively studied shape memory alloy (SMA). However, it remains a mystery that a small increase of Ni concentration $\left(x_{\mathrm{Ni}}\right)$ from 50% to 51% in NiTi results in a dramatic drop of about 110 K in the martensitic transition temperature (MTT), while a similar increase of Ti barely changes MTT. This weird behavior drastically affects both the manufacturing and applications of the NiTi-based SMAs, which represent the majority of ongoing scientific research efforts in this field. Using the most accurate first-principles methods, we find that this trend in MTT can be well reproduced theoretically. Furthermore, the physical origin of this striking asymmetry in MTT is revealed to be the fact that the atomic size of Ni is much larger than that of Ti in NiTi; though for free atoms, Ti is larger than Ni. Replacing smaller Ti atoms with bigger Ni atoms in NiTi leads to a much larger deviation from the ideal cubic B2 structure in the austenite phase than does the other way around.

The Su-Schrieffer-Heeger (SSH) model, with bond phonons modulating electron tunneling, is a paradigmatic electron-phonon model that hosts an antiferromagnetic order to bond order transition at half-filling. In the presence of a repulsive Hubbard interaction, the antiferromagnetic phase is enhanced, but the phase transition remains first order. Here, we explore the physics of the SSH model with an attractive Hubbard interaction, which hosts an interesting interplay among charge order, $s$-wave pairing, and bond order. Using the numerically exact determinant quantum Monte Carlo method, we show that both charge order, present at weak electron-phonon coupling, and bond order, at large coupling, give way to $s$-wave pairing when the system is doped. Furthermore, we demonstrate that the SSH electron-phonon interaction competes with the attractive Hubbard interaction and reduces the $s$-wave pairing correlation.

In contrast to the Mott transition found in $R\mathrm{Ni}{\mathrm{O}}_3$ ($R$= rare earths), the metal-insulator transition temperature in the perovskite $\mathrm{NaOs}{\mathrm{O}}_3$ is not sensitive to pressure. The peculiarity may be correlated to how the crystal structure of $\mathrm{NaOs}{\mathrm{O}}_3$ responds to high pressure, which has been rarely studied so far. The pressure-induced bond-length shrinking can increase the orbital overlap integral and therefore the electron bandwidth. However, in the orthorhombic perovskite structure, the pressure-induced bending in the bond angle Os-O-Os may compensate for the bandwidth broadening due to the bond-length shrinking in some circumstances. A recent structural study on polycrystalline $\mathrm{NaOs}{\mathrm{O}}_3$ indicated that orthorhombic distortion is enlarged under high pressure. But, how the local structure changes under pressure remains unknown. Moreover, a highly unusual phase transition from the orthorhombic phase (Pbnm) to a polar phase ($Pbn{2}_1$) occurs at around 18 GPa [Sereika et al., npj Quantum Mater. 5, 66 (2020)]. Motivated by these concerns, we have done a more comprehensive structural study on $\mathrm{NaOs}{\mathrm{O}}_3$ using single-crystal diffraction with synchrotron radiation at high pressures up to 41 GPa. Diffraction patterns over the entire pressure range can be refined well with the Pbnm structural model. Moreover, the refinement results reveal in detail how the local structures change under pressure corresponding to the enhanced orthorhombic distortion from the lattice parameters. We have carried out a systematic study for understanding the pressure effect on the orthorhombic perovskites in the context of the influences of the charge distributions in the $AB{\mathrm{O}}_3$ formula, i.e., $A^{3+}{B}^{3+}{\mathrm{O}}_3$, $A^{2+}{B}^{4+}{\mathrm{O}}_3$, and $A^{1+}{B}^{5+}{\mathrm{O}}_3$ and the $B$-site cations from the $3d$ to the $4d$ and $5d$ row of elements. To fulfill this purpose, we have revisited two families of $3d$ perovskites: $R\mathrm{Cr}{\mathrm{O}}_3$ and $R\mathrm{Fe}{\mathrm{O}}_3$.

Magnetometry and first-principles density functional theory-based calculations were used to investigate the concentration dependence of magnetic properties of amorphous ${\mathrm{Co}}_x{\mathrm{Zr}}_{100-x}$ alloys. A linear increase in saturation magnetization $\left(M_s\right)$ is observed in both the experiments and calculations. Samples with Co content of at least 62 at.% are inferred to be ferromagnetic at 5 K. The experimentally determined ordering temperature is found to scale quadratically with Co concentration indicating a complex interplay between local structural motifs and magnetic parameters such as exchange interaction and anisotropy.

The interplay between competing degrees of freedom can stabilize nontrivial magnetic states in correlated electron materials. Frustration-induced strong quantum fluctuations can evade long-range magnetic ordering, leading to exotic quantum states such as spin liquids in two-dimensional spin lattices such as triangular and kagome structures. However, experimental realization of dynamic and correlated quantum states is rare in three-dimensional (3D) frustrated magnets, wherein quantum fluctuations are less prominent. Here, we report the crystal structure, magnetic susceptibility, electron spin resonance, and specific heat studies accompanied by crystal electric field calculations on the 3D frustrated magnet ${\mathrm{Yb}}_3{\mathrm{Sc}}_2{\mathrm{Ga}}_3{\mathrm{O}}_{12}$. In this material, ${\mathrm{Yb}}^{3+}$ ions form a three-dimensional network of corner-sharing triangles known as a hyperkagome lattice without any detectable antisite disorder. Our thermodynamic results reveal a low-energy state with $J_{\mathrm{eff}}=\frac{1}{2}$ degrees of freedom in the Kramers doublet state. The zero-field-cooled and field-cooled magnetic susceptibilities taken at 0.001 T rule out the presence of spin freezing down to 1.8 K. The Curie-Weiss fit of magnetic susceptibility at low temperature yields a small and negative Curie-Weiss temperature, indicating the presence of a weak antiferromagnetic interaction between $J_{\mathrm{eff}}=\frac{1}{2}$ $\left({\mathrm{Yb}}^{3+}\right)$ moments. The Yb-electron spin resonance displays a broad line of asymmetric shape consistent with the presence of considerable magnetic anisotropy in ${\mathrm{Yb}}_3{\mathrm{Sc}}_2{\mathrm{Ga}}_3{\mathrm{O}}_{12}$. The crystal electric field calculations suggest that the ground state is well separated from the excited states, which are in good agreement with experimental results. The absence of long-range magnetic ordering inferred from specific heat data indicates a dynamic liquidlike ground state at least down to 130 mK. Furthermore, zero-field specific heat shows a broad maximum around 200 mK, suggesting the presence of short-range spin correlations in this three-dimensional frustrated antiferromagnet.

Coupling between spatially separated magnets can be mediated by excitations such as photons and phonons, which is believed to be coherent in a closed system in the absence of traveling waves. We theoretically predict the existence of dynamic exchange coupling in a closed magnetoelastic system mediated by attenuated elastic waves and whose coupling rate is complex in general, leading to an alternating repulsive or attractive spectrum, as a periodic function of the distance between magnets. Furthermore, the ratio between real and imaginary components of coupling can be tuned in situ via applied fields. The presence of dynamic exchange coupling and its competition with coherent coupling are numerically verified according to the generalized Hooke's law in magnetoelastic systems. The predicted mechanism provides a strategy to synchronize precessing magnets as well as other excitations over long distances and paves the way for non-Hermitian engineering of collective modes in hybrid magnonics, phononics, and photonics.

2D transition-metal dioxides and dichalcogenides with $1H$ phase ($1H\text{−}M{X}_2$) have a large number of members and a wide range of material properties, making them promising candidates for numerous applications. The comprehensive, accurate, and rapid evaluation on the mechanical properties of the $1H\text{−}M{X}_2$ family has been an important and challenging issue. Here, a general theoretical model is constructed based on molecular mechanics, which provides an accurate and rapid analytical algorithm for calculating the mechanical properties of the entire family of $1H\text{−}M{X}_2$. The validity of the constructed model is verified by molecular dynamics simulations upon the scale effect on the mechanical behavior of $1H\text{−}{\mathrm{MoS}}_2$. Notably, we report a library composed of the mechanical properties of 34 types of $1H\text{−}M{X}_2$. It is found that the mechanical performances of $1H\text{−}M{X}_2$ depend on the period and group numbers of elements. The obtained results are in good agreement with the existing experimental and numerical results. Furthermore, the roles of molecular structure and force field on the mechanical properties are elucidated, which is beneficial in predicting the mechanical performances of the potential and unreported $1H\text{−}M{X}_2$. The findings offer an important theoretical basis for the reverse design and optimization of $1H\text{−}M{X}_2$ material-based nanodevices through nanostructure-property relationships.

We study topological magnons on an anisotropic square-hexagon-octagon lattice which has been found by a two-dimensional biphenylene network. We propose the concept of type-II Dirac magnonic states where new schemes to achieve topological magnons are unfolded without requiring the Dzyaloshinskii-Moriya interactions (DMIs). In the ferromagnetic states, the topological distinctions at the type-II Dirac points along with one-dimensional closed lines of Dirac magnon nodes are characterized by the Berry phase and the ${\mathbb{Z}}_2$ invariant. We find pair annihilation of the Dirac magnons and use the Wilson loop method to depict the topological protection of the band degeneracy. The Green's function approach is used to calculate chiral edge modes and the magnon density of states. We introduce the DMIs to gap the type-II Dirac magnon points and demonstrate that the Dirac nodal loops are robust against the DMIs within a certain parameter range. The topological phase diagram of magnon bands is given via calculating the Berry curvature and Chern number. We find that the anomalous thermal Hall conductivity gives connection to the magnon edge current. Furthermore, we derive the differential gyromagnetic ratio to exhibit the Einstein–de Haas effect of magnons with topological features.

The boom of semiconductor quantum computing platforms created a demand for computer-aided design and fabrication of quantum devices. Path-integral Monte Carlo (PIMC) can have an important role in this effort because it intrinsically integrates strong quantum correlations that often appear in these multielectron systems. In this paper we present a PIMC algorithm that estimates exchange interactions of three-dimensional electrically defined quantum dots. We apply this model to silicon complementary metal-oxide-semiconductor (CMOS) devices and we benchmark our method against well-tested full configuration interaction simulations. As an application, we study the impact of a single charge trap on two exchanging dots, opening the possibility of using this code to test the tolerance to disorder of CMOS devices. This algorithm provides an accurate description of this system, setting up an initial step to integrate PIMC algorithms into development of semiconductor quantum computers.

Gallium oxide (${\mathrm{Ga}}_2{\mathrm{O}}_3$), as a representative of the four-generation ultrawide band-gap semiconductors, emerges to be a promising candidate for high performance scintillators. However, the full exploitation of its scintillation properties and the understanding of the defect-driven luminescence mechanisms are still lacking especially for $\gamma -\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ nanocrystals (NCs) grown in glass. Here, the easy-to-process glass-making and post-thermal treatment method is used to prepare $\gamma -\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ NC embedded transparent glass-ceramics (GCs). X-ray excited bright visible blue-green emissions are observed from the $\gamma -\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ NC embedded GCs, which are attributed to the donor-acceptor pair (DAP) recombination. Positive luminescence hysteresis is found to occur in the processes of both ultraviolet (UV) light excited photoluminescence and the x-ray excited luminescence (XEL). A method of prolonged UV preirradiation is used to counteract the hysteresis effect, which is desirable from the application point of view. Monte Carlo simulation is employed to reveal the difference between x- and $\gamma -\mathrm{ray}$ photoresponse of the GCs, and the results are compared with those of the ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ crystal. First-principles calculations are carried out to investigate the intrinsic defect properties of $\gamma -\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. Our results provide a theoretical foundation to understand the relevant defects responsible for the observed DAP, and offer potential avenues for tailoring optical performance for various applications. As a proof-of-concept, an x-ray imaging system is built on the studied GC scintillator, exhibiting performance superior to that of the eye-catching ${\mathrm{CsPbBr}}_3$ perovskite NC embedded GCs.