We study the coupled spin and orbital dynamics of electrons in one- and two-dimensional quantum dots driven by few cycle pulses of polarization-structured terahertz vector fields. Emphasis is put on the use of radially and azimuthally polarized, cylindrical vector beams for spin-flip processes caused by transitions between electronic states characterized by spin-position entanglement and corresponding injection of spin currents. We demonstrated how different topologies of the beams result in different selection rules and corresponding spin and position dynamics. These results point to new possibilities for spatiotemporal control of the coupled spin and orbital electron dynamics via polarization shaping of the driving electromagnetic pulse.

We investigate thermoelectric transport through interfaces with inelastic scatterings by developing a quantum theory, which has been extensively validated by existing results. We find that under temperature bias, a three-terminal conductor-insulator-conductor interface can function as an electricity generator caused by phonon-mediated electron scatterings with heat-charge current separation. Unlike conventional thermoelectrics, which is a bulk property caused by the Seebeck effect, this thermoelectric behavior is a property of an interface driven by electron-phonon scatterings.

Surface electronic structure and its one-dimensionality above and below the Fermi level ($E_{\mathrm{F}}$) are surveyed on the Bi/GaSb(110)-($2\times 1$) surface hosting quasi-one-dimensional (Q1D) Bi chains, using conventional (one-photon) and two-photon angle-resolved photoelectron spectroscopy (ARPES) and theoretical calculations. ARPES results reveal that the Q1D electronic states are within the projected bulk band gap. Circular dichroism of two-photon ARPES and density-functional-theory calculation indicate clear spin and orbital polarization of the surface states consistent with the giant sizes of Rashba-type spin-orbit interaction (SOI), derived from the strong contribution of heavy Bi atoms. The surface conduction band above $E_{\mathrm{F}}$ forms a nearly straight constant-energy contour, suggesting its suitability for application in further studies of one-dimensional electronic systems with strong SOI. A tight-binding model calculation based on the obtained surface electronic structure successfully reproduces the surface band dispersions and predicts possible one- to two-dimensional crossover in the temperature range of 60–100 K.

We report the full sequence of excitonic states in monolayer and few-layer black phosphorus (BP). High-quality BP samples are acquired by treating the silicon oxide substrate with a self-assembled monolayer hydrophobic molecule, which facilitates the access to their ground and higher-lying states of intrinsic excitons through the reflection and photoluminescence excitation spectra. Combined with first-principles and model calculations, we further determine that the exciton binding energy shrinks by ∼6-fold from 940 meV in monolayer to 160 meV in bilayer BP, yet it only decreases slightly to 108 meV in trilayer. This could be attributed to the strong interlayer coupling and lattice shrinking in multilayer BP samples. Our results shed light on the unique excitonic optical properties of two-dimensional BP.

Two-dimensional multivalley electronic systems in which the dispersion of individual pockets has low symmetry give rise to quantum Hall ferroelectric and nematic states in the presence of strong quantizing magnetic fields. We investigate the local signatures of these states arising near impurities that can be probed via scanning tunneling microscopy (STM) spectroscopy. For quantum Hall ferroelectrics, we demonstrate a direct relation between the dipole moment measured at impurity bound states and the ideal bulk dipole moment obtained from the modern theory of polarization. We also study the many-body problem with a single impurity via exact diagonalization and find that near strong impurities a nontrivial excitonic state can form with specific features that can be easily identified via STM spectroscopy.

We report the observation of tens of minor lines of the combinational spin-flip Raman scattering in a CdTe:Mn quantum well by means of the high-resolution optical spectroscopy. Classification of this manifold leads to four characteristic values of energy, that correspond to four different types of pair clusters of Mn ions: the nearest, second, third, etc., neighbors. All the four energies show up in a single experiment with a very high precision, providing experimental grounds for a deeper understanding of the $d-d$ exchange interactions in a diluted magnetic semiconductor and demonstrating the capacity of the employed method. The major (nearest-neighbor) exchange constant $J_1=6.15\pm 0.05$ K was found to consent with its previously reported value. Other detected characteristic energies are as follows ($\pm 0.05$ K): $J_{\left(2\right)}=1.80$ K, $J_{\left(3\right)}=1.39$ K, $J_{\left(4\right)}=0.81$ K.

The electric-field tunable Rashba spin-orbit coupling at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface shows potential applications in spintronic devices. However, different gate dependence of the coupling strength has been reported in experiments. On the theoretical side, it has been predicted that the largest Rashba effect appears at the crossing point of the $d_{\text{xy}}$ and $d_{\text{xz},\text{yz}}$ bands. In this work, we study the tunability of the Rashba effect in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ by means of backgating. The Lifshitz transition was crossed multiple times by tuning the gate voltage so that the Fermi energy is tuned to approach or depart from the band crossing. By analyzing the weak antilocalization behavior in the magnetoresistance, we find that the maximum spin-orbit coupling effect occurs when the Fermi energy is near the Lifshitz point. Moreover, we find strong evidence for a single spin winding at the Fermi surface.

We perform a systematic ab initio study of the work function and its uniform strain dependence for graphene and silicene for both tensile and compressive strains. The Poisson ratios associated with armchair and zigzag strains are also computed. Based on these results, we obtain the deformation potential, crucial for straintronics, as a function of the applied strain. Further, we propose a particular experimental setup with a special strain configuration that generates only the electric field, while the pseudomagnetic field is absent. Then, applying a real magnetic field, one should be able to realize experimentally the spectacular phenomenon of the collapse of Landau levels in graphene or related two-dimensional materials.

It is widely accepted that topological superconductors can only have an effective interpretation in terms of curved geometry rather than gauge fields due to their charge neutrality. This approach is commonly employed in order to investigate their properties, such as the behavior of their energy currents. Nevertheless, it is not known how accurately curved geometry can describe actual microscopic models. Here, we demonstrate that the low-energy properties of the Kitaev honeycomb lattice model—a topological superconductor that supports localized Majorana zero modes at its vortex excitations—are faithfully described in terms of Riemann-Cartan geometry. In particular, we show analytically that the continuum limit of the model is given in terms of the Majorana version of the Dirac Hamiltonian coupled to both curvature and torsion. We numerically establish the accuracy of the geometric description for a wide variety of couplings of the microscopic model. Our work opens up the opportunity to accurately predict dynamical properties of the Kitaev model from its effective geometric description.

Databases compiled using ab initio and symmetry-based calculations now contain tens of thousands of topological insulators and topological semimetals. This makes the application of modern machine learning methods to topological materials possible. Using gradient boosted trees, we show how to construct a machine learning model which can predict the topology of a given existent material with an accuracy of 90%. Such predictions are orders of magnitude faster than actual ab initio calculations. We use machine learning models to probe how different material properties affect topological features. Notably, we observe that topology is mostly determined by the “coarse-grained” chemical composition and crystal symmetry and depends little on the particular positions of atoms in the crystal lattice. We identify the sources of our model's errors and we discuss approaches to overcome them.

Impurities and defects in Kondo insulators can have an unusual impact on dynamics that blends with effects of intrinsic electron correlations. Such crystal imperfections are difficult to avoid, and their consequences are incompletely understood. Here we study magnetic impurities in Kondo insulators via perturbation theory of the s-d Kondo impurity model adapted to small-band-gap insulators. The calculated magnetization and specific heat agree with recent thermodynamic measurements in samarium hexaboride (${\mathrm{SmB}}_6$). This qualitative agreement supports the physical picture of multichannel Kondo screening of local moments by electrons and holes involving both intrinsic and impurity bands. Specific heat is thermally activated in zero field by Kondo screening through subgap impurity bands and exhibits a characteristic upturn as the temperature is decreased. In contrast, magnetization obtains a dominant quantum correction from partial screening by virtual particle-hole pairs in intrinsic bands. We point out that magnetic impurities could impact de Haas–van Alphen quantum oscillations in ${\mathrm{SmB}}_6$, through the effects of Landau quantization in intrinsic bands on the Kondo screening of impurity moments.

First-principles, real-time-cumulant, and Bethe-Salpeter-equation calculations fully capture the detailed satellite structure that occurs in response to the sudden creation of the core hole in both photoemission and x-ray absorption spectra of the transition-metal compounds $\mathrm{SrTi}{\mathrm{O}}_3$ and rutile $\mathrm{Ti}{\mathrm{O}}_2$. Analysis of the excited-state, real-space charge-density fluctuations betrays the physical nature of these many electron excitations that are shown to reflect the materials’ solid-state electronic structure and chemical bonding. This first-principles development of the cumulant-based core hole spectral function is generally applicable to other systems and should become a standard tool for all similar spectroscopic analysis going beyond the quasiparticle physics of the photoelectric effect.

The recent experimental observations of decaying energy oscillations with increasing magnetic field in semiconductor-superconductor Majorana nanowires is in contrast with the theoretical expectations based on the presence of Majorana zero modes localized at the ends of the system. These observations have been recently theoretically justified by considering a position-dependent spin-orbit coupling that can emerge due to a tunnel gate. Here, we show that the window in parameter space where this phenomenology occurs is vanishingly small when compared to the parameter region where Majorana oscillations should increase in amplitude with the applied magnetic field. Further, including a position-dependent effective potential that is also induced due to a tunnel gate practically removes this small window. Using extensive numerical calculations, we show that, as expected, increasing amplitude oscillations of the hybridization energy represent a generic property of topological Majorana zero modes, while decreasing amplitude oscillations are a generic property of low-energy trivial Andreev bound states, recently called partially separated Andreev bound states. By averaging over several realistic parameter configurations, we identify robust features of the hybridization energy that can be observed in a typical differential conductance experiment without fine-tuning the control parameters.

We propose and show that antichiral edge states can be realized in a gyromagnetic photonic crystal (GPC) with a honeycomb lattice consisting of two interpenetrating triangular sublattices $A$ and $B$. When sublattices $A$ and $B$ are immersed in opposite external magnetic fields respectively, the band structure of the GPC tilts and antichiral edge states emerge. These special edge states propagate in the same direction at the two opposite parallel zigzag boundaries of the GPC, which are completely distinguished from the well-studied topological edge states in chiral photonic systems where the edge states transport in opposite directions at the opposite two parallel zigzag boundaries. We show that these unique antichiral edge states originate from the overall coupling effects of the counterclockwise energy flux vortexes of sublattice $A$ and clockwise energy flux vortexes of sublattice $B$, so that two copropagating one-way transport channels are created on the boundaries. We further demonstrate that these antichiral edge states are also strongly robust against backscattering from the obstacles at the zigzag edges. Our findings clearly indicate that deeply digging into the GPC systems can help to find rich novel and significant topological physics. Antichiral edge states are of significance not only in basic physics, but also in offering useful insights and routines to design novel electromagnetic and optical functional devices, such as the compact multichannel one-way waveguide.

${\mathrm{LaMnO}}_3$ is considered as a prototypical Jahn-Teller perovskite compound, exhibiting a metal-to-insulator transition at $T_{JT}=750$ K related to the joint appearance of an electronic orbital-ordering and a large lattice Jahn-Teller distortion. From first principles, we revisit the behavior of ${\mathrm{LaMnO}}_3$ and show that it is not only prone to orbital ordering but also to charge ordering. Both charge and orbital orderings appear to be enabled by rotations of the oxygen octahedra and the subtle competition between them is monitored by a large tetragonal compressive strain, which is itself a Jahn-Teller active distortion. Equally, the competition of ferromagnetic and antiferromagnetic orders is dependent on the same tetragonal strain. Our results further indicate that the metal-to-insulator transition can be thought as a Peierls transition that is enabled by spin symmetry breaking. Therefore, dynamical spin fluctuations in the paramagnetic state stabilize the insulating phase by the instantaneous symmetry breaking they produce and which is properly captured from static density functional theory calculations. As a basis to our discussion, we introduce canonical notations for lattice distortions in perovskites that distort the oxygen octahedra and are connected to charge and orbital orderings.

$\mathrm{LiOs}{\mathrm{O}}_3$ synthesized under high pressure in recent years is a rare metal since it undergoes a nonpolar to polar phase transition at $T_s=140\mathrm{K}$. Forming a polar axis through a phase transition in a metal seems against common sense. It is also not clear whether the transition to a polar phase in the oxide fits the mechanism predicted by Anderson and Blount in 1965. As monitored by an anomaly of resistivity in $\mathrm{LiOs}{\mathrm{O}}_3$ at $T_s$ reported recently, $T_s$ increases under pressure. The structural study under high pressure could give us a useful clue for understanding how dipoles form in this metallic oxide. Here, we report the identification of a polar phase of $\mathrm{LiOs}{\mathrm{O}}_3$ at room temperature under high pressure by using in situ probes of Raman and synchrotron x-ray diffraction. In the Raman study, the pressure-induced modes and their responses to polarized light, the linewidth change, the peak profile change, and mode softening have been directly compared with the corresponding changes of $\mathrm{LiOs}{\mathrm{O}}_3$ on cooling through $T_s$ at ambient pressure. Whereas a complete set of Raman modes from the $R3c$ phase can be found at $P\ge 15.5\mathrm{GPa}$, a Raman mode of the $R3c$ phase appears in the $R\overline{3}c$ phase at 4.11 GPa. A significant drop in the linewidth occurs at 12.6 GPa that coincides with the critical pressure for the phase transition to the polar phase detected by x-ray diffraction. Fitting the peak profile of a Raman mode to the Fano formula also indicates a clear change of electron-phonon coupling at 16 GPa. In contrast to a sharp structural transition to the polar phase at $T_s$ under ambient pressure, our results reveal all the structural ingredients to facilitate the polar phase over a broad range of pressure. A bond valence sum analysis has been introduced to reveal the local structural instability under pressure. The transition to the polar phase in metallic $\mathrm{LiOs}{\mathrm{O}}_3$ is solely caused by optimizing the local structure in order to make the bond valence sum close to the formal valence of the Li ion.

Inspired by the discoveries of electronic topological phases and topological insulators, topologically protected boundary states in classical wave-based systems have attracted considerable interest in the last decade. Most recently, acoustic higher-order topological insulators and Kekulé-distorted sonic lattices have been proposed to support topological corner states and zero-dimensional bound states. Here, we demonstrate a domain wall induced topological corner state that is bound at the crossing point among finite acoustic graphenelike crystals. The approach is based on designing multipolar pseudospin resonances, which give rise to topologically trivial and nontrivial transitions across the domain walls that flank this unusual corner excitation toward the crossing point. By deliberately adding a substantial amount of defects into the cavities of the sonic lattice, we find that the pseudospin induced topological corner state remains entirely unaffected and pinned spectrally to the complete audible band gap. Our findings may thus have the potential to broaden the possibilities for sound confinement and focusing.

We report on the efficient spin-orbit torque (SOT) switching in a single ferromagnetic layer induced by a new type of inversion asymmetry, the composition gradient. The SOT of 6- to 60-nm epitaxial FePt thin films with a $L{1}_0$ phase is investigated. The magnetization of the FePt single layer can be reversibly switched by applying electrical current with a moderate current density. Different from previously reported SOTs which either decreases with or does not change with the film thickness, the SOT in FePt increases with the film thickness. We found the SOT in FePt can be attributed to the composition gradient along the film normal direction. A linear correlation between the SOT and the composition gradient is observed. This Rapid Communication introduces a platform to engineer large SOTs for lower-power spintronics.