Recently, an exciting discovery of orientation-dependent superconductivity was made in two-dimensional electron gas (2DEG) at the interfaces of ${\mathrm{LaAlO}}_3/{\mathrm{KTaO}}_3$ (LAO/KTO) and EuO/${\mathrm{KTaO}}_3$ (EuO/KTO). The superconducting transition temperature can reach a $T_c$ of up to $\sim 2.2$ K, which is significantly higher than its $3d$ counterpart ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (LAO/STO) with a $T_c$ of $\sim 0.2$ K. However, the underlying origin remains to be understood. To uncover the nature of electrons in KTO-based interfaces, we employ x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray spectroscopy (RIXS) to study LAO/KTO and EuO/KTO with different orientations. We reveal the absence of $dd$ orbital excitations in all the measured samples. Our RIXS results are well reproduced by calculations that considered itinerant $5d$ electrons hybridized with O $2p$ electrons. This suggests that there is a lack of localized Ta $5d^1$ electrons in KTO interface superconductors, which is consistent with the absence of magnetic hysteresis observed in magnetoresistance (MR) measurements. These findings offer new insights into our understanding of superconductivity in Ta $5d$ interface superconductors and their potential applications.

Orbital-free density functional theory constitutes a computationally highly effective tool for modeling electronic structures of systems ranging from room-temperature materials to warm dense matter. Its accuracy critically depends on the employed kinetic energy (KE) density functional, which has to be supplied as an external input. In this work we consider several nonlocal and Laplacian-level KE functionals and use an external harmonic perturbation to compute the static density response at $T=0$ K in the linear and beyond-linear response regimes. We test for the satisfaction of exact conditions in the limit of uniform densities and for how approximate KE functionals reproduce the density response of realistic materials (e.g., Al and Si) against the Kohn-Sham DFT reference, which employs the exact KE. The results illustrate that several functionals violate exact conditions in the uniform electron gas (UEG) limit. We find a strong correlation between the accuracy of the KE functionals in the UEG limit and in the strongly inhomogeneous case. This empirically demonstrates the importance of imposing the limit of UEG response for uniform densities and validates the use of the Lindhard function in the formulation of kernels for nonlocal functionals. This conclusion is substantiated by additional calculations for bulk aluminum (Al) with a face-centered cubic (fcc) lattice and silicon (Si) with an fcc lattice, body-centered cubic (bcc) lattice, and semiconducting crystal diamond state. The analysis of fcc Al, and fcc as well as bcc Si data follows closely the conclusions drawn for the UEG, allowing us to extend our conclusions to realistic systems that are subject to density inhomogeneities induced by ions.

Time-dependent density functional theory (TDDFT) has established itself as a computationally efficient and effective alternative to the many-body perturbation theory for calculating the optical properties of solids. Within the framework of linear response formalism, TDDFT exhibits good agreement between optical absorption spectra and experimental data, as well as the possibility of directly determining exciton binding energies. However, the family of exchange-correlation kernels, known as long-range-corrected kernels, which accurately capture excitonic features, face challenges in simultaneously generating visually appealing spectra and precise exciton binding energies. Recently, progress has been made in addressing this discrepancy through a hybrid-TDDFT approach. Our study reveals that the key lies in the numerical treatment of the long-range Coulomb singular term. We conduct a meticulous investigation of the impact of the term in both pure-TDDFT and hybrid approaches using a Wigner-Seitz truncated kernel. Notably, we show that computing this term presents formidable technical difficulties in both approaches, pointing to the need for an improved description of the electron-hole interaction.

We consider the effect of the uniaxial strain applied to a graphene monolayer with a realized quantum valley Hall state, for which we use a version of the Haldane model. In its specific point, the latter model has two spectral valleys: the gapless one and the gapped one. Using analytical and numerical arguments, we show that this state is unstable against mechanical deformations of the lattice, which influences the energy spectrum, the density of states, and the conductivity tensor. In particular, the Hall conductivity in the near-DC regime may surpass largely the known plateau value along with the simultaneous sign change. Above effects pave the way to the applied graphene strain engineering or straintronics. Namely, they can be used in quantum logical gates utilizing a controllable strain manipulation for reversing or on- and off-switching of the Hall current.

We study an analytically solvable and experimentally relevant number-conserving periodically driven $p$-wave superconductor. Such a system is found to support generalized Majorana zero and $\pi$ modes, which, despite being non-Hermitian, are still capable of encoding qubits. Moreover, appropriate winding numbers characterizing the topology of such generalized Majorana modes are defined and explicitly calculated. We further discuss the fate of the obtained generalized Majorana modes in the presence of finite charging energy. Finally, we shed light on the quantum computing prospects of such modes by demonstrating the robustness of their encoded qubits and explicitly braiding a pair of generalized Majorana modes.

We consider multicomponent Abelian Higgs (AH) gauge theories with multiparameter scalar quartic potentials, which are extensions of the $\text{SU}\left(N\right)$-invariant AH theories, with a smaller global symmetry group. In particular, we consider an AH model with a two-parameter scalar potential and $\text{SO}\left(N\right)$ global symmetry. We discuss the renormalization-group flow of the $\text{SO}\left(N\right)$-invariant AH field theory and the phase diagram and critical behavior of a corresponding three-dimensional (3D) noncompact lattice AH model. We argue that the phase diagram of 3D noncompact $\text{SO}\left(N\right)$- and $\text{SU}\left(N\right)$-symmetric lattice AH models are qualitatively similar. In both cases, there are three phases: the high-temperature Coulomb phase, and the low-temperature molecular and Higgs phases that differ for the topological properties of the gauge correlations. However, the main features of the low-temperature ordered phases and, in particular, of the Higgs phase, differ significantly in $\text{SO}\left(N\right)$ and $\text{SU}\left(N\right)$ models. In particular, in $\text{SO}\left(N\right)$ models they depend on the sign of the self-interaction parameter $v$ that controls the symmetry breaking from $\text{SU}\left(N\right)$ to $\text{SO}\left(N\right)$. As a consequence, the universal features of the transitions related with the spontaneous breaking of the global symmetry (those between the high-temperature Coulomb phase and the low-temperature molecular and Higgs phases) also depend on the sign of $v$.

The multicomponent density functional theory is faced with the challenge of capturing various types of inter- and intraparticle exchange-correlation effects beyond those introduced by the conventional electronic exchange-correlation functionals. Herein, we focus on evaluating the electron-proton/muon correlation functionals appearing in molecular/condensed-phase systems where a proton/muon is treated as a quantum particle on equal footing with electrons, beyond the Born-Oppenheimer paradigm. Five recently developed local correlation functionals, i.e., the epc series and $\mathrm{e}\mu \mathrm{c}$-1, are selected and their performances are analyzed by employing a two-particle model that includes an electron and a positively charged particle (PCP) with a variable mass, interacting through Coulombic forces, within a double harmonic trap. Using the Kohn-Sham (KS) inversion procedure, the exact two-component KS characterization of the model is deduced and its properties are compared to those derived from the considered functionals. The analysis demonstrates that these local functionals achieve their original parameterization objectives to reproduce the one-PCP densities and the electron-PCP correlation energies, but all fall short of reproducing the underlying PCP correlation potentials correctly. Moreover, a comprehensive error analysis reveals that the density-driven errors have a non-negligible contribution to the success of the considered functionals. Overall, the study shows the strengths as well as shortcomings of the considered functionals hopefully paving the way for designing more robust functionals in the future.

We present a device architecture of hybrid-edge and dual-gated quantum point contact. We demonstrate improved electrostatic control over the separation, position, and coupling of each broken-symmetry compressible strip in graphene. Via low-temperature magnetotransport measurement, we demonstrate selective manipulation over the evolution, hybridization, and transmission of arbitrarily chosen quantum Hall states in the channel. With gate-tunable tunneling spectroscopy, we characterize the energy gap of each symmetry-broken quantum Hall state with high resolution on the order of ∼0.1 meV.

Motivated by scanning tunneling microscopy experiments on ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{Cs}, \mathrm{Rb}, \mathrm{K})$ that revealed periodic real-space modulation of electronic states at low energies, I show using model calculations that a triple-Q chiral pair density wave (CPDW) is generated in the superconducting state by a charge order of $2a\times 2a$ superlattice periodicity, intertwined with a time-reversal symmetry breaking orbital loop current. In the presence of such a charge order and orbital loop current, the superconducting critical field is enhanced beyond the Chandrasekhar-Clogston limit. The CPDW correlation survives even when the long-range superconducting phase coherence is diminished by a magnetic field or temperature, stabilizing an exotic granular superconducting state above and in the vicinity of the superconducting transition. The presented results suggest that the CPDW can be regarded as the origin of the pseudogap observed near the superconducting transition.

Hydrous minerals hold the key to unlocking the enduring mystery of the water cycle deep inside the Earth. Tremendous efforts have been devoted to identifying geologically viable minerals meeting stringent pressure-temperature-density stability requirements for descent into deep Earth, and such pursuits remain active. Here, we identify two hydrous iron silicates, $\alpha$- and $\beta \text{−}{\mathrm{FeSiO}}_4{\mathrm{H}}_2$, formed by a reaction of Earth-abundant ${\mathrm{FeSiO}}_3$ and ${\mathrm{H}}_2\mathrm{O}$ and stabilized at the pressure-temperature conditions in cold subducting slabs. These phases have a sufficiently high density for a stable descent into the Earth's lower mantle, and then decompose to release water after reaching equilibrium with the mantle geotherm. Moreover, $\mathrm{Mg}\left(\mathrm{Fe}\right){\mathrm{SiO}}_4{\mathrm{H}}_2$ solutions are found to be more stable than the pure substances and can serve as effective carriers to transport substantial amounts of water to lower-mantle regions via the cold subduction zones. These findings establish a viable and robust material basis for the deep-Earth water cycle, with major implications for elucidation of many prominent geological processes.

Friction is generated due to the relative motion between two contacting objects, involving a multitude of atoms in the dissipation of translational kinetic energy. Current models often simplify the scenario of two objects in relative motion by depicting it as an elastic object excited by either a moving rigid body or an external force. Such approaches overlook the many-body nature inherent in friction systems and are insufficient in accounting for certain essential characteristics of friction. Here, we propose a parameter-free phononic dynamics model derived from the atomistic Green's function method, which includes all-atomic interactions in the friction process. Our model demonstrates that the phonon spectrum induced by friction exhibits narrow peaks around the washboard frequency and its harmonics, a phenomenon consistent with experimental observations. Furthermore, our model elucidates the mechanisms by which friction excites phonons and how these nonequilibrium phonons contribute to energy dissipation. Under the framework of phononic dynamics, the spring-stiffness-dependent, velocity-strengthening and velocity-weakening friction can be quantitatively explained without any assumptions.

The quantum anomalous Hall (QAH) effect has attracted significant attention due to its potential applications in low-power-consumption spintronic devices. In this study, we performed density functional theory calculations to investigate the stability, electronic, and topological properties of ${\mathrm{Ti}}_3{\mathrm{O}}_5$ monolayer. Our results demonstrate that ${\mathrm{Ti}}_3{\mathrm{O}}_5$ monolayer is thermally and dynamically stable. In the absence of spin-orbit coupling (SOC), the monolayer exhibits spin-polarized Weyl semimetal behavior and the Weyl points are protected by the vertical mirror symmetry. Specifically, the Weyl points can be preserved when the magnetization direction is parallel to xoy plane. By altering the magnetization direction out of plane, the Weyl points are opened and the system is transformed into the QAH phase $\left(\right|C|=1)$. To validate our findings, we constructed a 12-band tight-binding model based on first-principles calculations, which successfully reproduces the QAH state by incorporating exchange coupling and the SOC term. Furthermore, it is observed that a transition from ferromagnetic QAH insulator to antiferromagnetic semiconductor occurs when biaxial compressive strain is applied. These results open up exciting possibilities for the realization of two-dimensional Weyl semimetals and the QAH effect in a single material, which has significant applications for the fields of spintronics and topological microelectronics.

In this paper, a modified method of anomaly detection using convolutional autoencoders is employed to predict phase transitions in several statistical mechanical models on a square lattice. We show that, when the autoencoder is trained with input data of various phases, the mean-square-error loss function can serve as a measure of disorder, and its standard deviation becomes an excellent indicator of critical points. We find that various types of phase transition points, including first-order, second-order, and topological ones, can be faithfully detected by the peaks in the standard deviation of the loss function. Besides, the values of transition points can be accurately determined under the analysis of finite-size scaling. Our results demonstrate that the present approach has general application in identification and classification of phase transitions even without a priori knowledge of the systems in question.

A magnetic resonance phenomenon is proposed, in which only the orbital degrees of freedom of electrons participate. This previously unexploited magnetic resonance may be called electron orbital resonance (EOR). To this end, a nonmagnetic, singlet ground state is necessary. In addition, the system needs to exhibit the Van Vleck paramagnetism. Conditions for EOR are met by the rare earth perovskite ${\mathrm{LaCoO}}_3$ (LCO). The ${}^{5}D$ state of LCO is shown to exhibit the van Vleck paramagnetism that result in EOR. The frequency versus magnetic field relation for EOR in LCO is shown to be readily accessible with present day instrumentation. We predict that conventional electron spin resonance signals will be observed in the ${}^{5}D$ state of LCO in addition to EOR.

We study the excitation spectrum of the one-dimensional spin-$\frac{1}{2}$ XXZ chain with antiferromagnetic Ising anisotropy across a magnetic quantum phase transition induced by the application of a site-dependent transverse magnetic field. Motivated by the chain antiferromagnet ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, we consider a situation where the transverse magnetic field has a strong uniform component and a weaker staggered part. To determine the nature of the excitations giving rise to the spin dynamical structure factor, we use a combination of analytical approaches and the numerically exact time-dependent matrix product state method. We identify below the quantum phase transition high-energy many-body two-magnon and three-magnon repulsively bound states which are clearly visible due to the staggered component of the magnetic field. At high magnetic fields and low temperature, single magnons dominate the dynamics. Our theory results are in very good agreement with terahertz spectroscopy experimental results.

We develop a theory of the nonlinear optical responses in superconducting systems in the presence of a dc supercurrent. The optical transitions between particle-hole pair bands across the superconducting gap are allowed in clean superconductors as the inversion symmetry breaking by supercurrent. Vertex correction is included in optical conductivity to maintain the U(1) gauge symmetry in the mean-field formalism, which contains the contributions from collective modes. We show two pronounced current-dependent peaks in the second-harmonic generation ${\sigma }^{\left(2\right)}(2\omega ,\omega ,\omega )$ at the gap edge $2\hslash \omega =2\mathrm{\Delta }$ and $\hslash \omega =2\mathrm{\Delta }$ and one in the photocurrent effect ${\sigma }^{\left(2\right)}(0,\omega ,-\omega )$ at $\hslash \omega =2\mathrm{\Delta }$, all of which diverge in the clean limit. We demonstrate this in the models of a single-band superconductor with $s$-wave and $d$-wave pairings, and Dirac fermion systems with $s$-wave pairing. Our theory predicts that the current-induced peak in $\text{Im}\left[{\sigma }^{\left(2\right)}\left(\omega \right)\right]$ is proportional to the square of the supercurrent density in the $s$-wave single-band model, with the same order of magnitude as the recent experimental observation of second-harmonic generation in NbN by Nakamura et al. [Phys. Rev. Lett. 125, 097004 (2020)]. Supercurrent-induced nonlinear optical spectroscopy provides a valuable toolbox to explore novel superconductors.

The electronic structure of chiral crystal ${\mathrm{NbSi}}_2$ is investigated using angle-resolved photoelectron spectroscopy and density-functional theory calculations. In addition to the observation of clear band dispersions over a wide energy range, unexpected spiral-shaped constant energy contours (CECs) are found at specific binding energies. The rotation direction of this spiral signal is reversed in crystals having opposite chirality, indicating that the observed CECs originate from the structural chirality. Our theoretical analysis suggests that a combination of the bulk chiral atomic arrangement and surface states that penetrate sufficiently deep into the bulk is the key to understanding the emergence of spiral CECs.

The optical properties of rare-earth niobates play a crucial role in determining their performance as luminescent materials or host materials. In this study, we performed first-principles calculations based on the hybrid density functional to examine the electronic structures of ${\mathrm{LnNbO}}_4$ (Ln = La–Lu). We found that the usual parameter settings for the functional could not adequately handle the localization and correlation of $4f$ electrons in these full-concentration lanthanide systems, prompting us to adopt a “doping” approach as a more suitable approximation. Our results provide a systematical interpretation of the experimental results on the absorption edge of ${\mathrm{LnNbO}}_4$ (Ln = La–Lu): in addition to the common ${\mathrm{O}}^{2-}-2p$ to ${\mathrm{Nb}}^{5+}-4d$ transition in the ${\left[{\mathrm{NbO}}_4\right]}^{3-}$ group at near $5\text{eV}$, the redshifted absorption edge in ${\mathrm{EuNbO}}_4$ is due to ${\mathrm{O}}^{2-}-2p$ to ${\mathrm{Eu}}^{3+}-4f$ charge transfer absorption, while in ${\mathrm{CeNbO}}_4$, ${\mathrm{PrNbO}}_4$, and ${\mathrm{TbNbO}}_4$, the lower energy absorption edges are attributed to the ionization of ${\mathrm{Ln}}^{3+}-4f$ to conduction bands with a mix of ${\mathrm{Ln}}^{3+}-5d$ compositions. This study also provides a general yet simple approximate approach for studying the electronic structures of full-concentrated insulating lanthanide compounds.

We report a comprehensive study of polarized infrared/terahertz photocurrents in bulk tellurium crystals. We observe different photocurrent contributions and show that, depending on the experimental conditions, they are caused by the trigonal photogalvanic effect, the transverse linear photon drag effect, and the magnetic field induced linear and circular photogalvanic effects. All observed photocurrents have not been reported before and are well explained by the developed phenomenological and microscopic theory. We show that the effects can be unambiguously distinguished by studying the polarization, magnetic field, and radiation frequency dependence of the photocurrent. At frequencies around 30 THz, the photocurrents are shown to be caused by the direct optical transitions between subbands in the valence band. At lower frequencies of 1 to 3 THz, used in our experiment, these transitions become impossible and the detected photocurrents are caused by the indirect optical transitions (Drude-like radiation absorption).

Recently, we have proposed the concept of laser-dressing-effect mediated spin-orbit (SO) control in semiconductor wells, with the laser polarization aligning with the well-growth direction [X. Li et al., Phys. Rev. B 106, 155420 (2022)], facilitating flexible SO control. Here, for covering the full scenario and considering practical applications, we focus on tilted laser fields (TLFs) and build up a generic model incorporating both axial and transverse components of the laser polarization, which are respectively parallel and perpendicular to the well-growth direction, to explore the electrical gate assisted optical multisubband SO control in realistic GaInAs quantum wells with two occupied subbands. We reveal that the two laser components play contrasting roles in affecting SO properties from distinct physical sources, with the axial component directly altering the quantum confinement and the transverse component primarily adjusting the electron density of states. By performing a self-consistent Poisson-Schrödinger calculation (with both TLF and gate bias) in the Hartree approximation, we determine the full scenario of gate assisted TLF multisubband SO control, including intrasubband (Rashba ${\alpha }_{\nu }$ and Dresselhaus ${\beta }_{\nu }$) and intersubband (Rashba $\eta$ and Dresselhaus $\mathrm{\Gamma }$) types, $\nu =1,2$. Notably, we achieve adjusting the intrasubband Rashba ${\alpha }_1$ and ${\alpha }_2$ of the two subbands while meanwhile not only keeping them of opposite signs but also locking them to essentially equal strength, i.e., ${\alpha }_1=-{\alpha }_2$, as a result of intertwined effect of the two laser components. This provides a synchronous symmetric Rashba control of distinct subbands, and facilitates the realization of persistent skyrmion lattice that we recently proposed [J. Fu et al., Phys. Rev. Lett. 117, 226401 (2016)], even with different pitches. Also, depending on the width of wells, we observe that ${\alpha }_1$ can even be pinned at zero as TLF varies while ${\alpha }_2$ can be flexibly adjusted, greatly fascinating for selective multisubband SO control in particular with suppressed spin relaxation for a given subband. As for the intrasubband Dresselhaus ${\beta }_1$ and ${\beta }_2$, which inherit the bulk nature of materials and thus are usually immune to electrical control, we unveil their flexible gate assisted optical tunability. Moreover, regarding the intersubband SO terms, while the Dresselhaus $\mathrm{\Gamma }$ is found weakly dependent on the TLF, the Rashba $\eta$ is optically tunable and is even accompanied by a sign reversal. With the intersubband SO terms, we reveal three distinct scenarios for the spin-resolved multisubband dispersion, featuring the band crossing without spin hybridization and the band anticrossing with and without vertex corrections, respectively. Our work sheds light on experiments incorporating the TLF for a universal control of the Rashba-Dresselhaus SO terms of both intrasubband and intersubband kinds.