We have grown single crystals of ${\mathrm{Na}}_2\mathrm{BaNi}{\left({\mathrm{PO}}_4\right)}_2$, a spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat, and thermal conductivity measurements at ultralow temperatures. The main results are as follows: (i) At zero magnetic field, ${\mathrm{Na}}_2\mathrm{BaNi}{\left({\mathrm{PO}}_4\right)}_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted ${120}^{\circ }$ spin structure, which is in a plane including the crystallographic $c$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $c$ axis. (ii) With increasing field along the $c$ axis, a 1/3 magnetization plateau is observed, which means that the canted ${120}^{\circ }$ spin structure is transformed into an up-up-down (UUD) spin structure. With even higher fields, the UUD phase further evolves into possible V and ${\mathrm{V}}^{\prime }$ phases. (iii) With increasing field along the $a$ axis, the canted ${120}^{\circ }$ spin structure is possibly transformed into an umbrella phase and a V phase. Therefore ${\mathrm{Na}}_2\mathrm{BaNi}{\left({\mathrm{PO}}_4\right)}_2$ is a rare example of a spin-1 ETLAF with single-crystalline form that exhibits easy-axis spin anisotropy and a series of quantum spin state transitions.

Topological quantum computation based on Majorana bound states may enable new paths to fault-tolerant quantum computing. Several recent experiments have suggested that the vortex cores of topological superconductors, such as iron-based superconductors, may host Majorana bound states at zero energy. However, quantum computation with these zero-energy vortex bound states requires precise and fast manipulation of individual vortices, which is difficult to do in a scalable manner. To address this issue, we propose a control scheme based on local heating via, for example, scanning optical microscopy to braid vortex-bound Majorana zero modes in a two-dimensional topological superconductor. First, we derive the conditions required for transporting a single vortex between two defects in the superconducting material by trapping it with a hot spot generated by local optical heating. Equipped with critical conditions for the vortex motion, we then establish the ideal material properties for vortex braiding and describe how transition errors resulting from finite speed and/or temperature can be minimized. Our work paves the way toward optical or microscopic control of zero-energy vortex bound states in two-dimensional topological superconductors.

First-principles calculations using the hybrid density functional are carried out to study the defect levels and photoluminescence of bismuth doped in perovskites $R{\mathrm{AlO}}_3$ ($R=\mathrm{Y}$, Gd, La) and $\mathrm{La}B{\mathrm{O}}_3$ ($B=\mathrm{Al}$, Ga, In). Bismuth dopants are confirmed to be predominantly ${\mathrm{Bi}}^{3+}$ occupying the $R$ site, i.e., ${\mathrm{Bi}}_R^0$ ($R=\mathrm{Y}$, Gd, La). The variation of the electron trap depth ${\mathrm{Bi}}_R\left(0/-1\right)$ is shown mainly due to the shift of the conduction band, while the hole trap level ${\mathrm{Bi}}_R\left(+1/0\right)$ shows the correlation with the shortest $R\text{–}\mathrm{O}$ bond length in hosts. Based on the defect level diagram from the first-principles calculation, the transition types of excitation and emission are predicted. Among the systems considered, the lowest excited level that produces the photoluminescence is the ${}^{3}P_{0,1}$ levels of the $6s^{1}6p^1$ configuration of ${\mathrm{Bi}}^{3+}$, except for ${\mathrm{Bi}}^{3+}$-doped ${\mathrm{LaAlO}}_3$, which is the valence band to ${\mathrm{Bi}}^{3+}$ charge transfer state. The photoluminescence involving ${}^{3}P_{0,1}$ states shows similar Stokes shift in the series, and the excitation and emission energies exhibit almost linear correlation with the band gaps of hosts of a slope $\sim 0.22$. Furthermore, our calculations show that there is no tendency of forming excessive ${\mathrm{Bi}}^{3+}$ pairs in all the five $RB{\mathrm{O}}_3$:${\mathrm{Bi}}^{3+}$, while the ${\mathrm{Gd}}^{3+}\text{−}{\mathrm{Gd}}^{3+}$ coupling in ${\mathrm{GdAlO}}_3$ provides a “bridge” for energy migration from an excited isolated ${\mathrm{Bi}}^{3+}$ ion to the ${\mathrm{Bi}}^{3+}$ pairs that are naturally present due to random distribution, leading to the 495-nm emission being observed uniquely in bismuth-doped ${\mathrm{GdAlO}}_3$ among the series of systems considered. The results lay the basis for manipulating the trap levels and excitation and emission wavelengths in ${\mathrm{Bi}}^{3+}$-doped perovskite oxides.

The Weyl semimetal $\mathrm{Mo}{\mathrm{Te}}_2$ offers a rare opportunity to study the interplay between Weyl physics and superconductivity. Recent studies have found that Se substitution can boost the superconductivity up to 1.5 K, but suppresses the $T_d$ structure phase that is essential for the emergence of the Weyl state. A microscopic understanding of the possible coexistence of enhanced superconductivity and the $T_d$ phase has not been established so far. Here, we use scanning tunneling microscopy to study an optimally doped superconductor $\mathrm{Mo}{\mathrm{Te}}_{1.85}{\mathrm{Se}}_{0.15}$ with bulk $T_c\sim 1.5\mathrm{K}$. By means of quasiparticle interference imaging, we identify the existence of a low-temperature $T_d$ phase with broken inversion symmetry where superconductivity globally coexists. Furthermore, we find that the superconducting coherence length, extracted from both the upper critical field and the decay of density of states near a vortex, is much larger than the characteristic length scale of the existing chemical disorder. Our findings of robust superconductivity arising from a Weyl semimetal normal phase in $\mathrm{Mo}{\mathrm{Te}}_{1.85}{\mathrm{Se}}_{0.15}$ make it a promising candidate for realizing topological superconductivity.

Using pure spin current transport measurements in lateral spin-valve devices, we study the spin relaxation in an $n$-type nondegenerate Ge layer, which is moderately doped Ge (P: $\sim {10}^{18}$ ${\mathrm{cm}}^{-3}$). The obtained spin diffusion length $\left({\lambda }_{\mathrm{Ge}}\right)$ of the nondegenerate Ge is two to three times greater than that of heavily doped degenerate Ge (P: $\sim {10}^{19}$ ${\mathrm{cm}}^{-3}$) in the temperature range from 8 to 100 K. We find that the electron spin lifetime ($\tau$) for the nondegenerate Ge is monotonically increased with decreasing temperature $\left(T\right)$. The increase in $\tau$ at temperatures less than 50 K can be interpreted in terms of the donor-driven spin relaxation mechanism including the 1/$\sqrt{T}$ behavior in multivalley semiconductors, proposed by Song et al. [Y. Song, O. Chalaev, and H. Dery, Phys. Rev. Lett. 113, 167201 (2014)]. We note that it is important for the $\tau$ of the moderately doped nondegenerate Ge to partly consider the $T$-independent component of spin relaxation in addition to the 1/$\sqrt{T}$ component.

An effective field theory of composite Dirac fermions was proposed by Son [Phys. Rev. X 5, 031027 (2015)] as a theory of the half-filled Landau level with explicit particle-hole symmetry. We compute the electromagnetic response of this Son-Dirac theory on the level of the random phase approximation (RPA), where we pay particular attention to the effect of an additional composite-fermion dipole term that is needed to restore Galilean invariance. We find that once this dipole correction is taken into account, spurious interband transitions and collective modes that are present in the response of the unmodified theory either cancel or are strongly suppressed. We demonstrate that this gives rise to a consistent theory of the half-filled Landau level valid at all frequencies, at least to leading order in the momentum. In addition, the dipole contribution modifies the Fermi-liquid response at small frequency and momentum, which is a prediction of the Son-Dirac theory within the RPA that distinguishes it from a separate description of the half-filled Landau level by Halperin, Lee, and Read within the RPA.

We theoretically investigate the one-color injection currents and shift currents in zigzag graphene nanoribbons by applying a static electric field across the ribbon, which breaks the inversion symmetry to generate nonzero second-order optical responses by dipole interaction. These two types of currents can be separately excited by specific light polarization, circularly polarized lights for injection currents and linearly polarized lights for shift currents. Based on a tight binding model formed by carbon $2p_z$ orbitals, we numerically calculate the spectra of injection coefficients and shift conductivities, as well as their dependence on the static field strength and ribbon width. The spectra show many peaks associated with the optical transition between different subbands, and the positions and amplitudes of these peaks can be effectively controlled by the static electric field. By constructing a simple two band model, the static electric fields are found to modify the edge states in a nonperturbative way, and their associated optical transitions dominate the current generation at low photon energies. For typical parameters, such as a static field ${10}^6$ V/m and light intensity 0.1 GW/${\mathrm{cm}}^2$, the magnitude of the injection and shift currents for a ribbon with width of 5 nm can be as large as the order of 1 $\mu \mathrm{A}$. Our results provide a physical basis for realizing passive optoelectronic devices based on graphene nanoribbons.

We study the cross-plane thermal transport in ${\text{MoS}}_2$ using density functional theory by adequately accounting for the effect of widespread van der Waals (vdW) forces. We show that past studies report a large variation in the reported values of cross-plane thermal conductivity due to no or limited benchmarking of vdW functionals in computations and variations in sample sizes in experiments. In particular, we show that the validation of the considered vdW functionals based on the correct reproduction of harmonic properties (structural parameters, heat capacity) only is insufficient for the correct description of the thermal transport physics. Further, after having validated the suitability of different functionals, we find that the phonons contributing to cross-plane conductivity have an order of magnitude larger mean free path than that for the basal-plane thermal transport in ${\text{MoS}}_2$ at room temperature. The cross-plane transport is quasiballistic with a more than 80% contribution coming from phonons having mean free paths larger than 500 nm. Furthermore, we demonstrate that four-phonon scattering lowers the cross-plane thermal conductivity by more than 35% at room temperature and is required for a correct description of cross-plane thermal transport physics in layered materials.

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative.

We study the Hubbard model with non-Hermitian asymmetric hopping terms. The conjugate hopping terms are introduced for two spin components so that the negative sign is canceled out. This ensures that the quantum Monte Carlo simulation is free from the negative sign problem. We analyze the antiferromagnetic order and its suppression by the non-Hermiticity.

Understanding and controlling the transport properties of interacting fermions is a key forefront in quantum physics across a variety of experimental platforms. Motivated by recent experiments in one-dimensional (1D) electron channels written on the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface, we analyze how the presence of different forms of spin-orbit coupling (SOC) can enhance electron pairing in 1D waveguides. We first show how the intrinsic Rashba SOC felt by electrons at interfaces such as ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ can be reduced when they are confined in one dimension. Then, we discuss how SOC can be engineered, and show using a mean-field Hartree-Fock-Bogoliubov model that SOC can generate and enhance spin-singlet and -triplet electron pairing. Our results are consistent with two recent sets of experiments [Briggeman et al., Nat. Phys. 17, 782787 (2021); Sci. Adv. 6, eaba6337 (2020)] that are believed to engineer the forms of SOC investigated in this work, which suggests that metal-oxide heterostructures constitute attractive platforms to control the collective spin of electron bound states. However, our findings could also be applied to other experimental platforms involving spinful fermions with attractive interactions, such as cold atoms.

The coherence-to-decoherence transition is studied in a nanowire modeled as a one-dimensional tight-binding lattice in the presence of an external field and in linear interaction with a boson heat bath, characterized by Ohmic dissipation. The focus of attention is the probability propagator which quantifies the likelihood of a quantum particle, such as an electron to end up at an arbitrary site at a time $t$, given that it was at the origin initially—and from it—the particle-current and the mean-squared displacement. If the bath is absent, the probability operator exhibits quantum coherence as can be captured by say, Bloch oscillation and Wannier-Stark (dynamic) localization. The coupling with the bath—which can be weak or strong—leads to decoherence that will be quantified in the text. The Ohmic model of the spectral function of bath excitations contains a cutoff frequency, which if greater than the temperature (in energy units) defines the “low-temperature” regime, whereas the opposite limit implies “high temperatures.” Results will be presented in both these regimes separately.

We present here a microscopic theory of electronic complexes in charged $\mathrm{In}{\mathrm{As}}_x{\mathrm{P}}_{1-x}$ quantum dots in InP nanowires with a hexagonal cross section and determine the potential use of an array of such quantum dots as a synthetic spin chain for the possible construction of a topological qubit. The single-particle energies and wave functions are obtained by diagonalizing a microscopic atomistic tight-binding Hamiltonian of multiple quantum dots in the basis of $s{p}^3{d}^5{s}^{*}$ local atomic orbitals for a given random distribution of arsenic (As) vs phosphorus (P) atoms. The conduction band electronic states are found grouped into $s, p$, and $d$ quantum dot shells. For a double dot, the electronic shells can be understood in terms of interdot tunneling despite the random distribution of As atoms in each quantum dot. The single- and double-dot structures were charged with a finite number of electrons. The many-body Hamiltonian including Coulomb electron-electron interactions was constructed using single atomistic particle states and then diagonalized in the space of many-electron configurations. For a single dot filled with $N_e=1–7$ electrons, the ground state of a half-filled $p$-shell configuration with $N_e=4$ was found with total electronic spin $S=1$. The low-energy spectrum obtained using exact diagonalization of a Hamiltonian of a charged double dot filled with $N_e=8$ electrons, i.e., half-filled $p$ shells in each dot, was successfully fitted to the Hubbard-Kanamori and antiferromagnetic Heisenberg spin-1 Hamiltonians. The atomistic simulation confirmed the potential of InAsP/InP quantum dots in a nanowire for the design of synthetic spin chains.

We experimentally observed an ultrahigh blocking temperature, above 600 K, in an exchange-biased Fe/FeO bilayer after field annealing at 773 K. This is far above the 198 K Néel temperature of bulk FeO. By comparing with an Fe/FeO/Fe trilayer and studying the Fe thickness dependence, we find that the magnetic proximity effect, due to coupling between the Fe layer and the ultrathin amorphous FeO layer, is responsible for the high blocking temperature. The realization of high-temperature exchange bias using an ultrathin antiferromagnet paves the way to further miniaturize magnetic devices for high-information-density applications.

The non-Hermitian skin effect exhibits a collapse of extended bulk modes into an extensive number of localized boundary states in the open boundary condition. Here, we demonstrate a disorder-driven phase transition of a trivial non-Hermitian system to the higher-order non-Hermitian skin effect phase. In contrast to clean systems, the disorder-induced boundary modes form an arc in the complex energy plane, which is a manifestation of the disorder-driven energy-dependent phase transition. At the phase transition, localized corner modes and bulk modes characterized by trivial Hamiltonians coexist within a single band, while they remain separated in the complex energy plane. This behavior is analogous to the mobility edge phenomena in disordered Hermitian systems. Using effective medium theory and numerical diagonalizations, we provide a systematic characterization of the disorder-driven phase transition.

Oxide heterostructures provide unique opportunities to modify the properties of quantum materials through a targeted manipulation of spin, charge, and orbital states. Here, we use resonant x-ray reflectometry to probe the electronic structure of thin slabs of ${\mathrm{YVO}}_3$ embedded in a superlattice with $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$. We extend the previously established methods of reflectometry analysis to a general form applicable to $t_{2g}$ electron systems and extract quantitative depth-dependent x-ray linear dichroism profiles. Our data reveal an artificial, layered orbital polarization, where the average occupation of $xz$ and $yz$ orbitals in the interface planes next to $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ is inverted compared to the central part of the ${\mathrm{YVO}}_3$ slab. This phase is stable down to 30 K and the bulklike orbital ordering transitions are absent. We identify the key mechanism for the electronic reconstruction to be a combination of epitaxial strain and spatial confinement by the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ layers, in good agreement with predictions from ab initio theory.

We consider random singlet phases of spin-$\frac{1}{2}$, random, antiferromagnetic spin chains, in which the universal leading-order divergence $\frac{ln2}{3}ln\ell$ of the average entanglement entropy of a block of $\ell$ spins, as well as the closely related leading term $\frac{2}{3}{l}^{-2}$ in the distribution of singlet lengths are well known by the strong-disorder renormalization group (SDRG) method. Here, we address the question of how large the subleading terms of the above quantities are. By an analytical calculation performed along a special SDRG trajectory of the random XX chain, we identify a series of integer powers of $1/l$ in the singlet-length distribution with the subleading term $\frac{4}{3}{l}^{-3}$. Our numerical SDRG analysis shows that, for the XX fixed point, the subleading term is generally $O\left(l^{-3}\right)$ with a nonuniversal coefficient and also reveals terms with half-integer powers: $l^{-7/2}$ and $l^{-5/2}$ for the XX and XXX fixed points, respectively. We also present how the singlet lengths originating in the SDRG approach can be interpreted and calculated in the XX chain from the one-particle states of the equivalent free-fermion model. These results imply that the subleading term next to the logarithmic one in the entanglement entropy is $O\left({\ell }^{-1}\right)$ for the XX fixed point and $O\left({\ell }^{-1/2}\right)$ for the XXX fixed point with nonuniversal coefficients. For the XX model, where a comparison with exact diagonalization is possible, the order of the subleading term is confirmed but we find that the SDRG fails to provide the correct nonuniversal coefficient.

We study defect generation in a quantum XY-spin chain arising due to the linear drive of the many-body Hamiltonian in the presence of a time-dependent fast Gaussian noise. The main objective of this work is to quantify analytically the effects of noise on the defect density production. In the absence of noise, it is well known that in the slow sweep regime, the defect density follows the Kibble-Zurek (KZ) scaling behavior with respect to the sweep speed. We consider time-dependent fast Gaussian noise in the anisotropy of the spin-coupling term $[{\gamma }_0=(J_1-J_2)/(J_1+J_2)]$ and show via analytical calculations that the defect density exhibits anti-Kibble-Zurek (AKZ) scaling behavior in the slow sweep regime. In the limit of large chain length and long time, we calculate the entropy and magnetization density of the final decohered state and show that their scaling behavior is consistent with the AKZ picture in the slow sweep regime. We have also numerically calculated the sublattice spin correlators for finite separation by evaluating the Toeplitz determinants and find results consistent with the KZ picture in the absence of noise, while in the presence of noise and slow sweep speeds the correlators exhibit the AKZ behavior. Furthermore, by considering the large $n$-separation asymptotes of the Toeplitz determinants, we further quantify the effect of the noise on the spin-spin correlators in the final decohered state. We show that while the correlation length of the sublattice correlator scales according to the AKZ behavior, we obtain different scaling for the magnetization correlators.

A many-body interacting system of classical kicked rotor serves as a prototypical model for studying Floquet heating dynamics. Having established the fact that this system exhibits a long-lived prethermal phase with a quasiconserved average Hamiltonian before entering into the chaotic heating regime, we use spatiotemporal fluctuation correlation of kinetic energy as a two-point observable to probe the above dynamic phases. We remarkably find the diffusive transport of fluctuation in the prethermal regime suggesting an underlying hydrodynamic picture in a generalized Gibbs ensemble with a definite temperature that depends on the driving parameter and the initial conditions. On the other hand, the heating regime is characterized by a diffusive growth of kinetic energy where the correlation is sharply localized around the fluctuation center for all time. Consequently, we attribute nondiffusive and nonlocalized structure of correlation to the crossover regime, connecting the prethermal phase to the heating phase, where the kinetic energy displays a complicated growth structure. We understand these numerical findings using the notion of relative phase matching where the prethermal phase (heating regime) refers to an effectively coupled (isolated) nature of the rotors. We exploit the statistical uncorrelated nature of the angles of the rotors in the heating regime to find the analytical form of the correlator that mimics our numerical results in a convincing way.

We use the nonequilibrium dynamical mean-field theory formalism to compute the equilibrium and nonequilibrium resonant inelastic x-ray scattering (RIXS) signal of a strongly interacting fermionic lattice model with a coupling of dispersionless phonons to the total charge on a given site. In the atomic limit, this model produces phonon subbands in the spectral function, but not in the RIXS signal. Electron hopping processes however result in phonon-related modifications of the charge excitation peak. We discuss the equilibrium RIXS spectra and the characteristic features of nonequilibrium states induced by photo-doping and by the application of a static electric field. The latter produces features related to Wannier-Stark states, which are dressed with phonon sidebands. Thanks to the effect of field-induced localization, the phonon features can be clearly resolved even in systems with weak electron-phonon coupling.