Linear and nonlinear transport properties through a quantum point contact based on oxides two-dimensional electron gas is examined using the tight-binding method and the $\mathbf{k}·\mathbf{p}$ approach. The ballistic transport is analyzed in contacts realized at the (001) interface between band insulators ${\text{LaAlO}}_3$ and ${\text{SrTiO}}_3$ by using the Landauer-Büttiker method for many sub-bands derived from three Ti $3d$ orbitals ($d_{yz}$, $d_{zx}$, and $d_{xy}$) in the presence of an out-of-plane magnetic field. We focus especially on the role played by the atomic spin-orbit coupling and the inversion-symmetry-breaking term. Three different transport regimes stem out: The first, at low energies, involving the first $d_{xy}$-like sub-bands, where the conductance quantization is clearly observed; a second one, at intermediate energies, entailing further $d_{xy}$-like sub-bands, where the sub-band splitting induced by the magnetic field is quenched; the third one, where the mixing between light $d_{xy}$-like, heavy $d_{yz}$-like, and $d_{zx}$-like sub-bands is so strong that the conductance plateaus turn out to be very narrow. Very good agreement is found with recent experiments exploring the transport properties at low energies.

The electronic nematic phase emerging with spontaneous rotation symmetry breaking is a central issue of modern condensed-matter physics. In particular, various nematic phases in iron-based superconductors and high-$T_{\mathrm{c}}$ cuprate superconductors are extensively studied recently. Electric quadrupole moments (EQMs) are one of the order parameters characterizing these nematic phases in a unified way, and elucidating EQMs is a key to understanding these nematic phases. However, the quantum-mechanical formulation of the EQMs in crystals is a nontrivial issue because the position operators are nonperiodic and unbound. Recently, the EQMs have been formulated by local thermodynamics, and such thermodynamic EQMs may be used to characterize the fourfold rotation symmetry breaking in materials. In this paper, we calculate the thermodynamic EQMs in iron-based superconductors LaFeAsO and FeSe as well as a cuprate superconductor ${\mathrm{La}}_2{\mathrm{CuO}}_4$ by a first-principles calculation. We show that owing to the orbital degeneracy, the EQMs in iron-based superconductors are mainly determined by the geometric properties of wave functions. This result is in sharp contrast to the cuprate superconductor, in which the EQMs are dominated by distortion of the Fermi surface.

Germanium is crystalized in the cubic diamond structure, but its high energy hexagonal Ge (lonsdaleite) phase has many novel properties such as direct band gap. Using first-principles calculations, we show that the hexagonal lonsdaleite phase of Ge can be stabilized by introducing carriers, either electrons or holes, because Ge in the cubic and hexagonal phases form a type-I band alignment with both electrons and holes localized at the hexagonal site. This result is distinct from that in zinc-blende compounds such as ZnSe, because due to the lack of inversion symmetry, the crystal-field splitting, zone folding, and symmetry-controlled level repulsion between valence and conduction band states lead to a type-II band alignment between its cubic and hexagonal phases, so the hexagonal (wurtzite) phase of ZnSe can only be stabilized, in principle, by holes. This distinction reveals that, due to the symmetry differences, the well-investigated understanding of band structure differences between zinc-blende and wurtzite phases should not be simply extended to that of diamond and lonsdaleite phases despite the remarkable structure resemblance between the two cases.

Pauli spin blockade in double quantum dots has matured into a prime technique for precise measurements of nanoscale system parameters. In this work we demonstrate that systems with site-dependent $g$ tensors and spin-polarized leads allow for a complete characterization of the $g$ tensors in the dots by magnetotransport experiments alone. Additionally, we show that special polarization configurations can enhance the often elusive magnetotransport signal, rendering the proposed technique robust against noise in the system and inducing a giant tunnel magnetoresistance effect. Finally, we incorporate the effects of the spin-orbit interaction and show that in this case the leakage current contains information about the $g$ tensors and the degree of spin polarization in the leads.

The surfaces of perovskite oxides affect their functional properties, and while a bulk-truncated $(1\times 1)$ termination is generally assumed, its existence and stability is controversial. Here, such a surface is created by cleaving the prototypical ${\mathrm{SrTiO}}_3$(001) in ultrahigh vacuum, and its response to thermal annealing is observed. Atomically resolved noncontact atomic force microscopy (nc-AFM) shows that intrinsic point defects on the as-cleaved surface migrate at temperatures above $200{}^{\circ }\mathrm{C}$. At $400{}^{\circ }\mathrm{C}\text{–}500{}^{\circ }\mathrm{C}$, a disordered surface layer forms, albeit still with a $(1\times 1)$ pattern in low-energy electron diffraction (LEED). Purely ${\mathrm{TiO}}_2$-terminated surfaces, prepared by wet-chemical treatment, are also disordered despite their $(1\times 1)$ periodicity in LEED.

We investigate the vibrational properties of $\mathrm{V}{\mathrm{O}}_2$, particularly the low-temperature M1 phase by first-principles calculations using the density functional theory as well as Raman spectroscopy. In the theoretical aspect, the phonon calculation requires structure optimization and force calculations on a large supercell to obtain an accurate result, and strong correlation effects in $\mathrm{V}{\mathrm{O}}_2$ make this task very challenging. We perform the structural optimization using SCAN meta-GGA functional and obtain the optimized crystal structures with the correct energy hierarchy as well as correct electronic properties for the rutile and ${\mathrm{M}}_1$ phases of $\mathrm{V}{\mathrm{O}}_2$ including both the nonmagnetic (NM) and antiferromagnetic (AFM) spin ordering. We show both the NM-${\mathrm{M}}_1$ and AFM-${\mathrm{M}}_1$ are insulators, whereas NM-R and AFM-R are metals. However, FM-R and FM-${\mathrm{M}}_1$ are found be same as energetically favorable and becomes half-metal. Based on the harmonic approximation around the optimized structures obtained from the NM calculations at zero temperature, we reproduce the phonon softening in the rutile phase as well as the phonon stiffening in the ${\mathrm{M}}_1$ phase. Our phonon calculations reveal that both the NM-${\mathrm{M}}_1$ phase and AFM-${\mathrm{M}}_1$ are dynamically stable, whereas FM-${\mathrm{M}}_1$ is found to be dynamically unstable. In addition, we perform Raman experiment on $\mathrm{V}{\mathrm{O}}_2$ thin films which stimulate enormous interests in the field of thin-film engineering for transition metal oxides. We demonstrate from the comparison between theoretical calculations and Raman measurements that the subtle change of the V-V dimer due to the strain effect can be reliably detected by the Raman spectroscopy, which could be a new framework to determine the subtle change of crystal structure for transition metal oxide thin films.

Rydberg atoms in optical tweezer arrays provide a playground for nonequilibrium quantum many-body physics. The PXP model describes the dynamics of such systems in the strongly interacting Rydberg blockade regime and notably exhibits weakly nonergodic dynamics due to quantum many-body scars. Here, we study the PXP model in a strong staggered external field, which has been proposed to manifest quasiparticle confinement in light of a mapping to a lattice gauge theory. We characterize this confining regime using both numerical exact diagonalization and perturbation theory around the strong-field limit. In addition to the expected emergent symmetry generated by the staggered field, we find a second emergent symmetry that is special to the PXP model. The interplay between these emergent symmetries and the Rydberg blockade constraint dramatically slows down the system's dynamics beyond naive expectations. We devise a nested Schrieffer-Wolff perturbation theory to properly account for the new emergent symmetry, and we show that this treatment is essential to understand the numerically observed relaxation time scales. We also discuss connections to Hilbert space fragmentation, and we trace the origin of the new emergent symmetry to a “nearly-$\text{SU}\left(2\right)$” algebra discovered in the context of many-body scarring.

The free energy and the entropy of magnetic skyrmions with respect to the collinear state are calculated for a $\left({\mathrm{Pt}}_{0.95}{\mathrm{Ir}}_{0.05}\right)/\mathrm{Fe}$ bilayer on Pd(111) via atomistic spin model simulations. The simulations are carried out starting from very low temperatures where the skyrmion number is conserved up to the range where skyrmions are constantly created and destroyed by thermal fluctuations, highlighting their quasiparticle nature. The higher entropy of the skyrmions at low temperature leads to a reduced free energy, such that the skyrmions become energetically preferred over the collinear state due to entropic stabilization as predicted by linear spin-wave theory. Going beyond the linear spin-wave approximation, a sign change is shown to occur in the free energy as well as the entropy at elevated temperature.

The hierarchy of the coupling strengths in a physical system often engenders an effective model at low energies where the decoupled high-energy modes are integrated out. Here, using neutron scattering, we show that the spin excitations in the breathing pyrochlore lattice compound $\mathrm{Cu}\mathrm{In}{\mathrm{Cr}}_4{\mathrm{S}}_8$ are hierarchical and can be approximated by an effective model of correlated tetrahedra at low energies. At higher energies, intratetrahedron excitations together with strong magnon-phonon couplings are observed, which suggests the possible role of the lattice degree of freedom in stabilizing the spin tetrahedra. Our work illustrates the spin dynamics in $\mathrm{Cu}\mathrm{In}{\mathrm{Cr}}_4{\mathrm{S}}_8$ and demonstrates a general effective-cluster approach to understand the dynamics on the breathing-type lattices.

Recent studies show the nonrelativistic antiferromagnetic ordering could generate momentum-dependent spin splitting analogous to the Rashba effect but free from the requirement of relativistic spin-orbit coupling. Whereas the classification of such compounds can be illustrated by different spin-splitting prototypes (SSTs) from symmetry analysis and density-functional-theory calculations, the huge variation in chemical bonding and structures of these diverse compounds possibly clouds the issue of how much of the variation in spin splitting can be traced back to the symmetry-defined characteristics, rather to the underlining chemical and structural diversity. The alternative model Hamiltonian approaches do not confront the issues of chemical and structural complexity but often consider only the magnetic sublattice, dealing with the all-important effects of the nonmagnetic ligands via renormalizing the interactions between the magnetic sites. To this end, we constructed a DFT model Hamiltonian that allows us to study SSTs at constant chemistry while retaining the realistic atomic-scale structure including ligands. This is accomplished by using a single, universal magnetic skeletal lattice (${\mathrm{Ni}}^{2+}$ ions in rocksalt NiO) and designing small displacements of the nonmagnetic (oxygen) sublattice which produce, by design, the different SST magnetic symmetries. We show that (i) even similar crystal structures having very similar band structures can lead to contrasting behavior of spin splitting vs momentum, and (ii) even subtle deformations of the nonmagnetic ligand sublattice could cause a giant spin splitting in AFM-induced SST. This is a paradigm shift relative to the convention of modeling magnets without considering the nonmagnetic ligand that mediates indirect magnetic interaction (e.g., superexchange).

We report a comprehensive inelastic neutron scattering study of the hybrid molecule-based multiferroic compound $({\mathrm{ND}}_4{)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$ in the zero-field incommensurate cycloidal phase and the high-field quasicollinear phase. The spontaneous electric polarization changes its direction concurrently with the field-induced magnetic transition, from mostly aligned with the crystallographic $a$ axis to the $c$ axis. To account for such a change in polarization direction, the underlying multiferroic mechanism was proposed to switch from the spin-current model induced via the inverse Dzyaloshinskii-Moriya interaction to the $p\text{−}d$ hybridization model. We perform a detailed analysis of the inelastic neutron data of $({\mathrm{ND}}_4{)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$ using linear spin-wave theory to quantify magnetic interaction strengths and investigate the possible impact of different multiferroic mechanisms on the magnetic couplings. Our result reveals that the spin dynamics of both multiferroic phases can be well described by a Heisenberg Hamiltonian with easy-plane anisotropy. We do not find notable differences between the optimal model parameters of the two phases. The hierarchy of exchange couplings and the balance among frustrated interactions remain the same between two phases, suggesting that magnetic interactions in $({\mathrm{ND}}_4{)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$ are much more robust than the electric polarization in response to delicate reorganizations of the electronic degrees of freedom in an applied magnetic field.

$\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$ has been proposed to have magnetic topological states as a potential Weyl semimetal. We synthesized single crystals of $\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$ and systematically investigated their structural and physical properties. $\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$ has an isostructural septuple-layered structure that is similar to the magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ but possesses transpositional Mn and Sb atoms between the sublayers. Magnetic and specific-heat measurements revealed a ferrimagnetic phase transition with a Curie temperature $T_{\mathrm{C}}$ of ∼31 K in $\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$, which originates from the interaction of the interexchanged ${\mathrm{Mn}}^{2+}$ ions. As the temperature decreases below $T_{\mathrm{C}}$, negative longitudinal magnetoresistance and anomalous Hall effect are observed, implying a non-negligible connection between the magnetism and expected transport carriers that may be driven by topological bands. Our results indicate that ferrimagnetic $\mathrm{Mn}{\mathrm{Sb}}_2{\mathrm{Te}}_4$ provides insights for further studies on magnetic topological materials.

Considering a noncentrosymmetric pinning texture composed of a square array of triangular holes, the magnetic flux penetration and expulsion are investigated experimentally and theoretically. A direct visualization of the magnetic landscape obtained using a magneto-optical technique on a Nb film is complemented by a multiscale numerical modeling. This combined approach allows the magnetic flux dynamics to be identified from the single flux quantum limit up to the macroscopic electromagnetic response. Within the theoretical framework provided by time-dependent Ginzburg-Landau simulations, an estimation of the in-plane current anisotropy is obtained and its dependence with the radius of the curvature of hole vertices is addressed. These simulations show that current crowding plays an important role in channeling the flux motion, favoring hole-to-hole flux hopping rather than promoting interstitial flux displacement in between the holes. The resulting anisotropy of the critical current density gives rise to a distinct pattern of discontinuity lines for increasing and decreasing applied magnetic fields, in sharp contrast to the invariable patterns reported for centrosymmetric pinning potentials. This observation is partially accounted for by the rectification effect, as demonstrated by finite-element modeling. At low temperatures, where magnetic field penetration is dominated by thermomagnetic instabilities, highly directional magnetic flux avalanches with a fingerlike shape are observed to propagate along the easy axis of the pinning potential. This morphology is reproduced by numerical simulations. Our findings demonstrate that anisotropic pinning landscapes and, in particular, ratchet potentials produce subtle modifications to the critical state field profile that are reflected in the distribution of discontinuity lines.

We study ballistic transport in integrable quantum lattice models, i.e., in the spin XXZ and Hubbard chains, close to the noninteracting limit. It is by now well established that the stiffnesses of spin and charge currents reveal, at high temperatures, a discontinuous reduction (jump) when the interaction is introduced. We show that the jumps are related to the large degeneracy of the parent noninteracting models and, more generally, can appear in other integrable models with macroscopic degeneracies. These degeneracies are properly captured by the degenerate perturbation calculations which may be performed for large systems. We find that the discontinuities and the quasilocality of the conserved current in this limit can be traced back to the nonlocal character of an effective interaction. From the latter observation we identify a class of observables which show discontinuities.

Using a hydrodynamic approach, we show that charge diffusion in two dimensional Coulomb interacting liquids with broken momentum conservation is intrinsically anomalous. The charge relaxation is governed by an overdamped, superdiffusive plasmon mode. We demonstrate that the diffusing particles follow Lévy flight trajectories, and study the hydrodynamic collective modes under the influence of magnetic fields. The latter are shown to slow down the superdiffusive process. The results are argued to be relevant to electron liquids in solids, as well as plasmas.

An exciting development in the field of correlated systems is the possibility of realizing two-dimensional (2D) phases of quantum matter. For a system of bosons, an example of strong correlations manifesting themselves in a 2D environment is provided by helium adsorbed on graphene. We construct the effective Bose-Hubbard model for this system which involves hard-core bosons $(U\approx \infty )$, repulsive nearest-neighbor $(V>0)$ and small attractive $(V^{\prime }<0)$ next-nearest-neighbor interactions. The mapping onto the Bose-Hubbard model is accomplished by a variety of many-body techniques which take into account the strong He-He correlations on the scale of the graphene lattice spacing. Unlike the case of dilute ultracold atoms where interactions are effectively pointlike, the detailed microscopic form of the short-range electrostatic and long-range dispersion interactions in the helium-graphene system is crucial for the emergent Bose-Hubbard description. The result places the ground state of the first layer of ${}^4\mathrm{He}$ adsorbed on graphene deep in the commensurate solid phase with $1/3$ of the sites on the dual triangular lattice occupied. Because the parameters of the effective Bose-Hubbard model are very sensitive to the exact lattice structure, this opens up an avenue to tune quantum phase transitions in this solid-state system.

Active control of heat flow is of both fundamental and applied interest in thermal management and energy conversion. Here, we present a fluctuational electrodynamic study of thermal radiation between twisted bilayer graphene (TBLG), motivated by its unusual and highly tunable plasmonic properties. We show that near-field heat flow can vary by more than 10-fold over only a few degrees of twist, and observe a larger variation with increasing chemical potential and decreasing temperature. Further, we identify special angles leading to heat flow extrema, which are dictated by the Drude weight in the intraband optical conductivity of TBLG, and are roughly linear with the chemical potential. As the twist angle decreases, we observe multiband thermal transport due to the increasing role of interband transitions, in analogy to monolayer graphene in a magnetic field. We also briefly discuss the effect of a small angular deviation and a substrate, which are experimentally relevant. Our findings are understood via the surface plasmons in TBLG, and highlight its potential for manipulating radiative heat flow.

Photogenerated fully spin-polarized current has attracted considerable attention in spintronics owing to its high efficiency for information processing but low energy consumption. However, the short spin carrier lifetime of the primary materials significantly impedes the practical application of spin-polarized current and presents a major challenge. Here, on the basis of density functional theory and nonadiabatic molecular dynamics simulations, we design two prototypes of two-dimensional (2D) ferromagnetic/nonmagnetic semiconductor heterostructures ($\mathrm{Cr}{\mathrm{I}}_3/\mathrm{Mo}{\mathrm{Se}}_2$ and $\mathrm{Cr}{\mathrm{Br}}_3/\mathrm{Mo}{\mathrm{S}}_2$) with demonstrated ultralong photogenerated spin carrier lifetime. It is shown that the photogenerated spin holes can be effectively injected from the ferromagnetic layer to the nonmagnetic layer, and fully spin-polarized electrons will be confined in the ferromagnetic layer due to their type-II spin-polarized channel and large spin-flip gap. More surprisingly, the photogenerated spin carrier lifetime of these heterostructures is up to the time scale of nanoseconds. Such an ultralong spin carrier lifetime is mainly originated from small nonadiabatic coupling induced by weak spin electron-phonon interaction and slow nuclear velocity. Our results provide insights into the design of 2D semiconductor spintronic devices with an ultralong fully spin-polarized current lifetime.