We consider a multiband Hubbard model $H_m$ for Cu and O orbitals in ${\mathrm{Ba}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_{3+\delta }$ similar to the three-band model for two-dimensional cuprates. The hopping parameters are obtained from maximally localized Wannier functions derived from ab initio calculations. Using the cell perturbation method, we derive both a generalized $t\text{−}J$ model $H_{tJ}$ and a one-band Hubbard model $H_H$ to describe the low-energy physics of the system. $H_{tJ}$ has the advantage of having a smaller relevant Hilbert space, facilitating numerical calculations, while additional terms should be included in $H_H$ to accurately describe the multiband physics of $H_m$. Using $H_{tJ}$ and the density matrix renormalization group method, we calculate the wave-vector-resolved photoemission and discuss the relevant features in comparison with recent experiments. In agreement with previous calculations, we find that the addition of an attractive nearest-neighbor interaction of the order of the nearest-neighbor hopping shifts the weight from the $3k_F$ to the holon-folding branch. Kinetic effects also contribute to this process.

The results of systematic experimental studies of quantum oscillations of resistivity (${\rho }_{xx}$), the Hall coefficient ($R_H$), and capacitance ($C$) in GaAs and ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells (QWs) with a simple electron spectrum and HgTe QWs with a complicated nonparabolic spectrum and strong spin-orbit interaction are reported. It is shown that for all the structures under study the oscillations of ${\rho }_{xx}$ in a magnetic field ($B$) are completely determined by the oscillations of the density of states, the oscillation phase of ${\rho }_{xx}\left(B\right)$ over the entire the filling factor ($N$) range is close to zero, which corresponds to the theory, while the oscillation phase of $R_H\left(B\right)$ is close to the theoretical value of $\pi$ only for large $N$ values and increases as $N$ decreases. It is experimentally justified that such a behavior is not associated with approaching the quantum Hall effect regime. A striking result on the ratio of the amplitudes of the resistance and Hall coefficient oscillations is obtained. In GaAs QWs with a simple spectrum characterized by negligibly small Zeeman and spin-orbit splitting, the ratio of amplitudes is close to that predicted theoretically. In HgTe QWs, this ratio is very different and behaves differently in QWs with normal and inverted electron spectra. In HgTe QWs with a normal spectrum, it tends to a theoretical value with an increase in $N$, while for HgTe QWs with an inverted spectrum, it differs significantly from the theoretical one for all available $N$. It is assumed that such a difference in the ratio of amplitudes in GaAs and HgTe QWs is due not to the peculiarities of the energy spectrum of HgTe, but to the peculiarities of electron scattering due to spin-orbit interaction with the potential of the scatterers. This assumption is justified by analysis of experimental results obtained for a heterostructure with an ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{As}$ QW, which spectrum is very close to the GaAs QW spectrum, but characterized by a much stronger spin-orbit splitting value. It has been found that the positions of the resistance and capacitance oscillations, the difference between the phases of the resistance and Hall coefficient oscillations, and its $N$ dependence are close to those observed in GaAs QWs. At the same time the ratio of the amplitude of the resistance oscillations to the Hall coefficient oscillations and its $N$ dependence differ very strongly and they are close to that observed in HgTe quantum wells.

Defect qubits in 4H-SiC are outstanding candidates for numerous applications in the rapidly emerging field of quantum technology. Carbon clusters can act as emission sources that may appear after thermal oxidation of 4H-SiC or during irradiation, which kicks out carbon atoms from their sites. These fluorescent carbon clusters could interfere with the already established vacancy-related qubits that generated with irradiation techniques. In this study, we systematically investigate the electronic structure, formation energy, dissociation energy, vibrational properties, and the full fluorescence spectrum of carbon clusters involving up to four carbon atoms in 4H-SiC by means of density functional theory calculations. All the possible local configurations for these carbon clusters are carefully evaluated. We find the electronic and vibronic properties of the carbon clusters depend strongly on the local configuration of the 4H-SiC lattice. By comparing the calculated and previously observed fluorescence spectra in 4H-SiC, we identify several carbon clusters as stable visible emitters in 4H-SiC. The paired carbon interstitial defects are identified as the source of the 463-nm triplet and the 456.6-nm emitters. The 471.8-nm emitter in 4H-SiC is associated with tricarbon antisite clusters. Our findings provide plausible explanation for the origin of visible emission lines in 4H-SiC and propose the possible configurations of carbon clusters, which are helpful for the quantum information processing application through qubits in 4H-SiC.

The progress in two-dimensional materials has led to rapid experimental developments in quantum plasmonics, where light is manipulated using plasmons. Although numerical methods can be used to quantitatively describe plasmons in spatially inhomogeneous systems, they are limited to relatively small setups. Here, we present a semi-analytical method to describe plasmons in two-dimensional inhomogeneous media within the framework of the random phase approximation (RPA). Our approach is based on the semiclassical approximation, which is formally applicable when the length scale of the inhomogeneity is much larger than the plasmon wavelength. We obtain an effective classical Hamiltonian for quantum plasmons by first separating the in-plane and out-of-plane degrees of freedom and subsequently employing the semiclassical Ansatz for the electrostatic plasmon potential. We illustrate this general theory by considering scattering of plasmons by radially symmetric inhomogeneities. We derive a semiclassical expression for the differential scattering cross section and compute its numerical values for a specific model of the inhomogeneity.

Classical spin liquids have been a very active subject of research in the last few years. A very rich variety of cases have been shown to exist, including short-range and algebraic spin liquids displaying dipolar correlations at zero temperature. In this article, we investigate different families of classical spins liquids by combining analytical techniques and Monte Carlo simulations. Our study relies on the Luttinger-Tisza approximation (LTA), which is associated with the constraint vector function in momentum space, whose general properties allow for a classification of different spin liquids. We show that the general properties of the LTA provide a framework for identifying and accurately characterizing the different types of spin liquids in different geometries. We apply our approach to three different families of spin liquids defined on the checkerboard and kagome lattices, which exhibit a remarkable range of situations, including various cases of algebraic and short-range spin liquids. Additionally, we analyze the effective Gauss law emerging from different kinds of spin liquids and identify states that exhibit additional degeneracy lines. The presence of spin-liquid phases and pinch-point singularities are confirmed by Monte Carlo simulations validating our approach. Our study opens up avenues of research in the study of spin liquids, exploring algebraic spin liquids with higher-rank gauge fields and as critical points dividing different types of classical spin liquids.

We present a finite-temperature second-order perturbation method incorporating spin-orbit coupling to investigate the temperature-dependent site-resolved contributions to the magnetocrystalline anisotropy energy (MAE), specifically $K_1\left(T\right)$, in FePt, MnAl, and FeNi alloys. Our developed method successfully reproduces the results obtained using the force theorem from our previous work. By employing this method, we identify the key sites responsible for the distinctive behaviors of MAE in these alloys, shedding light on the inadequacy of the spin model in capturing the temperature dependence of MAE in itinerant magnets. Moreover, we explore the lattice expansion effect on the temperature dependence of on-site contributions to $K_1\left(T\right)$ in FeNi. Our results not only provide insights into the limitations of the spin model in explaining the temperature dependence of MAE in itinerant ferromagnets but also highlight the need for further investigations. These findings contribute to a deeper understanding of the complex nature of MAE in itinerant magnetic systems.

Many systems that host exact quantum many-body scars (towers of energy-equidistant low entanglement eigenstates) are governed by a Hamiltonian that splits into a Zeeman term and a sum of local terms that annihilate the scar subspace. We show that this unifying structure also applies to models, such as the Affleck-Kennedy-Lieb-Tasaki (AKLT) model or the PXP model of Rydberg-blockaded atoms, which were previously believed to evade this characterization. To fit these models into the local annihilator framework, we need to fractionalize their degrees of freedom and enlarge the associated Hilbert space. The embedding of the original system in a larger space elucidates the structure of their scar states, which are characterized as the common kernel of local annihilators. This simplifies their construction and reveals close analogies with lattice gauge theories.

${\mathrm{LaH}}_{10}$, as a member of hydrogen-rich superconductors, has a superconducting critical temperature of 250 K at high pressures, which exhibits the possibility of solving the long-term goal of room-temperature superconductivity. Considering the extreme pressure and low mass of hydrogen, the nuclear quantum effects in ${\mathrm{LaH}}_{10}$ should be significant and have an impact on its various physical properties. Here, we adopt the method that combines deep potential and quantum thermal bath, which was verified to be able to account for quantum effects in high-accuracy large-scale molecular dynamics simulations. Our method can actually reproduce pressure-temperature phase diagrams of ${\mathrm{LaH}}_{10}$ consistent with experimental and theoretical results. After incorporating quantum effects, the quantum fluctuation driven diffusion of protons is found even in the absence of thermal fluctuation near 0 K. The high mobility of protons is found to be compared to liquid, yet the structure of ${\mathrm{LaH}}_{10}$ is still rigid. These results would greatly enrich our vision to study quantum behavior of hydrogen-rich superconductors.

Quantum phase transitions have long been studied in their relation to quantum fluctuations. These fluctuations can be quantified as the degree of spin squeezing in spin models, where one of the two noncommutative observables breaks the standard quantum limit of measurement by minimizing its uncertainty. However, the understanding of their role in dynamical quantum phase transitions (DQPTs) is still incomplete. In this work, we combine the Loschmidt amplitude, which detects DQPTs, and the spin-squeezing parameter (SSP), the quantification of spin squeezing, to study the spin dynamics in a quenched interacting spin model around DQPT. We show that the extremum, mostly maximum, of SSP occurs near DQPTs when the system is quenched between different phases. These phenomena further unveil the spin correlations during DQPTs, for which the highest contribution aligns with the preferred direction of spin interactions in the postquenched phase. We also demonstrate that the time evolution of SSP differs for various quench scenarios. These findings provide us with physical insights into the dynamics of quantum fluctuations around DQPTs and their relation to the equilibrium phase diagrams.

Layered ternary actinide chalcogenides contain unique structural and magnetic properties that remain underexplored. ${\mathrm{KU}}_2{\mathrm{Te}}_6$ is a new member of the $A\text{−}R\text{−}Q$ ($A=$ alkali; $R=$ actinide; $Q=$ chalcogenide) materials family crystallizing in the $Cmcm$ space group as part of the ${\mathrm{CsTh}}_2{\mathrm{Te}}_6$ structure type. Evidence of a structural anomaly appears near $T_s=48$ K as a weak feature in specific heat and electrical resistivity. Magnetism in this material derives from local non-Kramers ${\mathrm{U}}^{4+}$ ions as suggested by charge balance and crystalline electric-field susceptibility analysis. These ions form one-dimensional $\mathrm{U}\text{−}\mathrm{U}$ chains along the $a$ axis, but do not exhibit signs of magnetic order to $T=0.36$ K. The absence of magnetic order is consistent with the crystalline electric-field splitting of the ${\mathrm{U}}^{4+}$ $J=4$ manifold into a series of nondegenerate singlets.

We theoretically study paramagnetic spin transport based on atomistic simulations of spin dynamics in a one-dimensional model in the presence of a magnetic field along the transport direction. As the magnetic field increases, the spin diffusion length with an antiferromagnetic coupling between neighboring spins presents an interesting nonmonotonic behavior, which is attributed to the competition between the field-driven ordering in the low-field region and the reduction in the paramagnon lifetime in the high-field region. The local increase in magnon temperature in the ordered configuration, although of the same order as or even larger than the background temperature, is found to play a minor role in the spin diffusion length, suggesting that the temperature dependence of the paramagnon transport is mainly dominated by the magnon-phonon scattering.

We study the slopes of the upper critical field $S=\partial H_{{}_{c2}}/\partial T$ at the superconducting transition temperature $T_c$ in anisotropic superconductors with transport (nonmagnetic) scattering employing the Ginzburg-Landau theory, developed for this case by Pokrovsky and Pokrovsky [Phys. Rev. B 54, 13275 (1996)]. We find unexpected behavior of the slopes for a $d$-wave superconductor and, in a more general case, of materials with line nodes in the order parameter. Specifically, the presence of line nodes causes $S$ to decrease with increasing nonmagnetic scattering parameter $P=\hslash /2\pi T_{c0}\tau$ ($T_{c0}$ is for the clean limit, $\tau$ is the scattering time), unlike the nodeless case where the slope increases. In a pure $d\text{-wave}$ case, the slope changes from decreasing to increasing when the scattering parameter approaches $P\approx 0.91P_{\mathrm{crit}}$, where $P_{\mathrm{crit}}\approx 0.28$, at which $T_c\to 0$, which implies the existence of a “gapless” state in $d\text{-wave}$ superconductors with transport scattering in the interval, $0.91P_{\mathrm{crit}}<P<P_{\mathrm{crit}}$. Furthermore, we consider the mixed $(s+d)\text{-wave}$ order parameter with four nodes on a cylindrical Fermi surface when the $d$ part is dominant, or no nodes at all when the $s\text{-wave}$ phase dominates. We find that the presence of nodes causes the slope $S\left(P\right)$ to decrease initially with increasing $P$, whereas in the nodeless state, $S\left(P\right)$ monotonically increases. Therefore relatively straightforward measurements of the disorder dependence of the slope of $H_{c2}$ at $T_c$ can help distinguish between nodal and nodeless order parameters, which is particularly useful for quickly assessing newly discovered superconductors.

We study electric-field-induced second harmonic generation (SHG) in the Kitaev model. This frustrated magnet hosts a quantum spin liquid, featuring fractionalization in terms of mobile Majorana fermion and static ${\mathbb{Z}}_2$ flux-vison elementary excitations. We show that finite-temperature SHG allows to probe characteristic features of both fractional quasiparticle types. In the homogeneous flux state at low temperatures, the SHG susceptibility displays an oscillatory spectrum, which is set by only the fermionic excitations and is subject to temperature-induced Fermi blocking, generic to all higher harmonic generation (HHG). In the intermediate to high-temperature range, intrinsic randomness, which emerges from thermally excited visons leads to drastic changes of the SHG susceptibility, resulting from resonance decoupling over a wide range of energies. At the flux proliferation crossover, we suggest an interpolation between these two temperature regimes. Our results satisfy previously established symmetries for electric-field-induced SHG in Kitaev magnets.

We present a first-principles computational study of the ${\mathrm{NbS}}_2/{\mathrm{WSe}}_2$ junction between two transition metal dichalcogenide monolayers as a prototypical metal/semiconductor two-dimensional (2D) lateral heterostructure (LH) to investigate the effects of electrostatic perturbations on electron transport in 2D LH systems. In order to simulate electrostatic (charged or dipolar) defects in the substrate, we introduce ionic systems (LiF lines) properly positioned in two different configurations and study cases, corresponding to modeling two different phenomena: (i) an electrostatic defect in the middle of the semiconducting part of the heterostructure (qualitatively analogous to a gate voltage opposing transmission), and (ii) an electrostatic perturbation realigning and flattening the electrostatic potential along the asymmetric LH junction. In the former case, we determine a substantial decrease of transmission even for small values of the perturbation (providing information that can be used to achieve a quantitative correlation between substrate-induced defectivity and device performance degradation in experiment), whereas in the latter we predict that electron transport can be significantly enhanced by properly tuning external electrostatic perturbations at the interface.

The potential energy surface (PES) of the interlayer interaction of infinite twisted bilayer graphene is calculated for a set of commensurate moiré patterns using the registry-dependent Kolmogorov-Crespi empirical potential. The calculated PESs have the same shape for all considered moiré patterns, and the unit cell size of the PESs is inversely related to the unit cell size of the moiré pattern. The amplitude of PES corrugations is found to decrease exponentially upon increasing the size of the moiré pattern unit cell. An analytical expression for such a PES including the first Fourier harmonics compatible with the symmetries of both layers is derived. It is shown that the calculated PESs can be approximated by the derived expression with an accuracy within 1%. This means that different physical properties associated with relative in-plane motion of graphene layers are interrelated and can be expressed analytically as functions of the amplitude of PES corrugations. In this way, we obtain the shear mode frequency, the shear modulus, the shear strength, and the barrier for relative rotation of the commensurate twisted layers to a fully incommensurate state for the considered moiré patterns. This barrier may possibly lead to robust macroscopic superlubricity for a twisted graphene bilayer with a commensurate moiré pattern. The conclusions drawn should be valid for diverse two-dimensional systems of twisted commensurate layers.

Narrow-band metasurface perfect absorbers with a high-quality $\left(Q\right)$ factor play a vital role in governing the performance of optoelectronic devices. However, it is quite challenging to realize polarization-independent high-$Q$ perfect absorption at a near-infrared wavelength by only a single-layered structure without using the back reflectors. Here, we theoretically propose a monolayer high-$Q$ near-perfect metasurface absorber with unit cells comprised of four germanium (Ge) split rings. Based on quasibound states in the continuum, the Mie modes with low radiative decay rates are induced by the orientation of each split ring to match the low nonradiative decay rates and achieve high-$Q$ absorption performance. The high reflection background mode excited by four split rings can transform the symmetrically coupled Mie modes into asymmetrically coupled ones and decrease the radiative decay rate of transmission, enabling the monolayer structure to break through the absorption limit of 50% to near 100%. We have theoretically demonstrated that the maximum absorption of the presented structure is as high as 93% at 1530 nm of the $Q$ factor up to 470. The structure is polarization independent due to its $C_4$ symmetry. Our proposed monolayer all-dielectric metasurface without any back reflectors achieves near-perfect absorption at a very narrow bandwidth. This work may provide a paradigm for designing polarization-independent high-$Q$ monolayer perfect absorbers, which is potentially applicable in ultrafast optoelectronic devices and integrated photon systems.