We use quantum Monte Carlo method to determine the magnetic and transport properties of coupled square lattice spin and fermionic planes as a model for a metal-insulator interface. Specifically, layers of Ising spins with an intralayer exchange constant $J$ interact with the electronic spins of several adjoining metallic sheets via a coupling $J_H$. When the chemical potential cuts across the band center, that is, at half-filling, the Néel temperature of antiferromagnetic $\left(J>0\right)$ Ising spins is enhanced by the coupling to the metal, while in the ferromagnetic case $\left(J<0\right)$, the metallic degrees of freedom reduce the ordering temperature. In the former case, a gap opens in the fermionic spectrum, driving insulating behavior, and the electron spins also order. This induced antiferromagnetism penetrates more weakly as the distance from the interface increases, and also exhibits a nonmonotonic dependence on $J_H$. For doped lattices, an interesting charge disproportionation occurs where electrons move to the interface layer to maintain half-filling there.

We introduce an integrability-based method enabling the study of semiconductor quantum dot models incorporating both the full hyperfine interaction as well as a mean-field treatment of dipole-dipole interactions in the nuclear spin bath. By performing free-induction decay and spin-echo simulations we characterize the combined effect of both types of interactions on the decoherence of the electron spin, for external fields ranging from low to high values. We show that for spin-echo simulations the hyperfine interaction is the dominant source of decoherence at short times for low fields, and competes with the dipole-dipole interactions at longer times. On the contrary, at high fields the main source of decay is due to the dipole-dipole interactions. In the latter regime an asymmetry in the echo is observed. Furthermore, the nondecaying fraction previously observed for zero-field free-induction decay simulations in quantum dots with only hyperfine interactions, is destroyed for longer times by the mean-field treatment of the dipolar interactions.

A new diluted ferromagnetic semiconductor $(\mathrm{Sr},\mathrm{Na}){(\mathrm{Zn},\mathrm{Mn})}_2{\mathrm{As}}_2$ is reported, in which charge and spin doping are decoupled via Sr/Na and Zn/Mn substitutions, respectively, being distinguished from classic $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$, where charge and spin doping are simultaneously integrated. Different from the recently reported ferromagnetic $(\mathrm{Ba},\mathrm{K}){(\mathrm{Zn},\mathrm{Mn})}_2{\mathrm{As}}_2$, this material crystallizes into the hexagonal ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$ type structure. Ferromagnetism with a Curie temperature up to 20 K has been observed from magnetization. The muon spin relaxation measurements suggest that the exchange interaction between Mn moments of this new system could be different from the earlier diluted magnetic semiconductors (DMS) systems. This system provides an important means for studying ferromagnetism in DMS.

We study the performance of supcode—a family of dynamically correcting pulses designed to cancel simultaneously both Overhauser and charge noise for singlet-triplet spin qubits—adapted to silicon devices with electrostatic control. We consider both natural Si and isotope-enriched Si systems, and in each case we investigate the behavior of individual gates under static noise and perform randomized benchmarking to obtain the average gate error under realistic $1/f$ noise. We find that in most cases supcode pulses offer roughly an order of magnitude reduction in gate error, and especially in the case of isotope-enriched Si, supcode yields gate operations of very high fidelity. We also develop a version of supcode that cancels the charge noise only, “$\delta J$-supcode,” which is particularly beneficial for isotope-enriched Si devices where charge noise dominates Overhauser noise, offering a level of error reduction comparable to the original supcode while yielding gate times that are $30\%–50\%$ shorter. Our results show that the supcode noise-compensating pulses provide a fast, simple, and effective approach to error suppression, bringing gate errors well below the quantum error correction threshold in principle.

One of the great successes of modern condensed matter physics is the discovery of topological insulators (TIs). A thorough investigation of their properties could bring such materials from fundamental research to potential applications. Here, we report on theoretical investigations of the complex band structure (CBS) of two-dimensional (2D) TIs. We utilize the tight-binding form of the Bernevig, Hughes, and Zhang model as a prototype for a generic 2D TI. Based on this model, we outline the conditions that the CBS must satisfy in order to guarantee the presence of topologically protected edge states. Furthermore, we use the Green's function technique to show how these edge states are localized, highlighting the fact that the decay of the edge-state wave functions into the bulk of a TI is not necessarily monotonic and, in fact, can exhibit an oscillatory behavior that is consistent with the predicted CBS of the bulk TI. These results may have implications for electronic and spin transport across a TI when it is used as a tunnel barrier.

We describe measurements of the pyroelectric coefficient of epitaxial layers of ${\mathrm{Ba}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{TiO}}_3$ (BST) and ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ (PZT) using a modulated laser as the heat source in the frequency range 1 Hz to 10 MHz. The pyroelectric coefficient is also measured as a function of applied static electric field and for films grown on ${\mathrm{SrTiO}}_3$, ${\mathrm{DyScO}}_3$, and ${\mathrm{GdScO}}_3$ substrates. The heat diffusion equation is solved in cylindrical coordinates for the temperature field in a multilayer sample heated by a Gaussian-shaped laser beam. The secondary contribution to the pyroelectric effect caused by piezoelectric effects and in-plane thermal expansion is revealed by the difference between the pyroelectric coefficients at high and low frequencies. The secondary pyroelectric effect has the same dependence on applied field as the pyroelectric coefficient. The magnitude of the secondary effect changes with heating frequency because of changing mechanical conditions, but the pyroelectric coefficient has no other frequency dependence between 1 Hz and 10 MHz. The secondary effect is approximately 15% and 20% of the total pyroelectric response for PZT and BST films, respectively.

We investigate a tunnel contact coupled to a double quantum dot (DQD) and employed as a charge monitor for the latter. We consider both the classical limit and the quantum regime. In the classical case, we derive measurement correlations from conditional probabilities, yielding quantitative statements about the parameter regime in which the detection scheme works well. Moreover, we demonstrate that not only the DQD occupation but also the corresponding current may strongly correlate with the detector current. The quantum-mechanical solution, obtained with a Bloch-Redfield master equation, shows that the backaction of the measurement tends to localize the DQD electrons, and thus significantly reduces the DQD current. Moreover, it provides the effective parameters of the classical treatment. It turns out that already the classical description is adequate for most operating regimes.

We study a system consisting of a particle adsorbed on a carbon nanotube resonator. The particle is allowed to diffuse along the resonator, in order to enable study of, e.g., room-temperature mass sensing devices. The system is initialized in a state where only the fundamental vibration mode is excited, and the ring-down of the system is studied by numerically and analytically solving the stochastic equations of motion. We find two mechanisms of dissipation, induced by the diffusing adsorbate. First, short-time correlations between particle and resonator motions means that the net effect of the former on the latter does not average out, but instead causes nonexponential dissipation of vibrational energy. For vibrational amplitudes that are much larger than the thermal energy this dissipation is linear; for small amplitudes the decay takes the same form as that of a nonlinearly damped oscillator. Second, the particle diffusion mediates a coupling between vibration modes that opens a new dissipation channel by enabling energy transfer from the fundamental mode to the excited modes, which rapidly reach thermal equilibrium.

The thermopower of few-electron quantum dots is crucially influenced by on-dot electron-electron interactions, particularly in the presence of Kondo correlations. In this paper, we present a comprehensive picture which elucidates the underlying relations between the thermopower and the spectral density function of two-level quantum dots. The effects of various electronic states, including the Kondo states originating from both spin and orbital degrees of freedom, are clearly unraveled. Such a physical picture is affirmed by accurate numerical data obtained with a hierarchical equations of motion approach. Our findings and understandings provide an effective and viable way to control the thermoelectric properties of strongly correlated quantum dot systems.

Quantum Hall measurements are performed in a dilute high-quality GaAs two-dimensional hole system with carrier densities of (4.4–$5.3)\times {10}^{10}{\text{cm}}^{-2}$. With the $r_s$ value approximately 20, the interaction effect is already strong even in the absence of magnetic field. In the integer quantum Hall regime up to 2 T, a series of reentrant insulating phases (RIPs) are observed between fillings 1, 2, 3, and 4, with peak locations periodic in $1/B$. Moreover, the RIP peaks are also characterized by a striking inductance in response to ac excitations, which is likely due to the onset of a Wigner crystal state. The multiple RIP peaks indicate a more complex phase diagram than previously perceived.

We study charge transport through single molecule magnet (SMM) junctions in the cotunneling regime as a tool for investigating the properties of the excited-state manifolds of neutral ${\mathrm{Mn}}_{12}$ SMs. This study is motivated by a recent transport experiment [S. Kahle et al., Nano Lett. 12, 518 (2012)] that probed the details of the magnetic and electronic structure of ${\mathrm{Mn}}_{12}$ SMMs beyond the ground-state spin manifold. A giant spin Hamiltonian and master equation approach is used to explore theoretically the cotunneling transport through ${\mathrm{Mn}}_{12}$-Ac SMM junctions. We identify SMM transitions that can account for both the strong and weak features of the experimental differential conductance spectra. We find the experimental results to imply that the excited spin-state manifolds of the neutral SMM have either different anisotropy constants or different $g$ factors in comparison with its ground-state manifold. However, the latter scenario accounts best for the experimental data.

We study massless Dirac fermions in a supercritical Coulomb potential with the emphasis on that its low-energy physics is universal and parametrized by a single quantity per supercritical angular momentum channel. This low-energy parameter with the dimension of length is defined only up to multiplicative factors and thus each supercritical channel exhibits the discrete scale invariance. In particular, we show that the induced vacuum polarization has a power-law tail whose coefficient is a sum of log-periodic functions with respect to the distance from the potential center. This coefficient can also be expressed in terms of the energy and width of so-called atomic collapse resonances. Our universal predictions on the vacuum polarization and its relationship to atomic collapse resonances shed light on the longstanding fundamental problem of quantum electrodynamics and can in principle be tested by graphene experiments with charged impurities.

We investigate time-dependent, room-temperature quantum electronic transport in GaAs/AlGaAs double-barrier tunneling structures (DBTSs). The open-boundary Wigner-Boltzmann transport equation is solved by the stochastic ensemble Monte Carlo technique, coupled with Poisson's equation and including electron scattering with phonons and ionized dopants. We observe well-resolved and persistent terahertz-frequency current-density oscillations in uniformly doped, dc-biased DBTSs at room temperature. We show that the origin of these intrinsic current oscillations is not consistent with previously proposed models, which predicted an oscillation frequency given by the average energy difference between the quasibound states localized in the emitter and main quantum wells. Instead, the current oscillations are driven by the long-range Coulomb interactions, with the oscillation frequency determined by the ratio of the charges stored in the emitter and main quantum wells. We discuss the tunability of the frequency by varying the doping density and profile.

The magnetothermopower and the magnetoresistance of single Co-Ni/Cu multilayered nanowires with various thicknesses of the Cu spacer are investigated. Both kinds of measurement are performed as a function of temperature (50–325 K) and under applied magnetic fields perpendicular to the nanowire axis, with magnitudes up to −15% at room temperature. A linear relation between thermopower $S$ and electrical conductivity σ of the nanowires is found, with the magnetic field as an implicit variable. Combining the linear behavior of the $S$ vs σ relation and the Mott formula, the energy derivative of the resistivity is determined. In order to extract the true nanowire materials parameters from the measured thermopower, a simple model based on the Mott formula is employed to distinguish the individual thermopower contributions of the sample. By assuming that the nondiffusive thermopower contributions of the nanowire can be neglected, it is found that the magnetic-field-induced changes of thermopower and resistivity are equivalent. The emphasis in the present paper is on the comparison of the magnetoresistance and magnetothermopower results and it is found that the same correlation is valid between the two sets of data for all samples, irrespective of the relative importance of the giant magnetoresistance or anisotropic magnetoresistance contributions in the various individual nanowires.

Transverse dielectric response of hexagonal cadmium selenide (CdSe) nanowires was investigated using first-principles quantum mechanical calculations. Scaling behavior of polarizability was found to closely follow a simple dielectric cylinder model even for small nanowires with a diameter of a few nanometers. The spatial dependence of the dielectric response in the nanowires was analyzed in terms of maximally localized Wannier functions in order to elucidate the model behavior. Localized $d$ electrons at cadmium atoms were found responsible for the simple analytic scaling of the polarizability, and the dielectric response in the center of nanowire was found converged to that of bulk already for 3 nm diameter nanowires.

The dynamical phase transition mechanism of $\left({\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}\right){\mathrm{TiO}}_3$ (NBT) was studied using inelastic neutron scattering. Softening was observed of multiple phonon modes in the phase transition sequence of NBT. As usual, the softening of the zone center transverse optical modes ${\Delta }_5$ and ${\Sigma }_3$ was observed in the (200) and (220) zones, showing the Ti vibration instabilities in ${\mathrm{TiO}}_6$ octahedra for both cubic-tetragonal (C-T) and tetragonal-rhombohedral (T-R) phase transitions. In these two phase transitions, however, ${\mathrm{Ti}}^{4+}$ has different preferential displacement directions. Surprisingly, the longitudinal optic mode also softens significantly toward the zone center in the range of the transition temperature, indicating the ${\mathrm{Na}}^{+}/{\mathrm{Bi}}^{3+}$ vibration instability against ${\mathrm{TiO}}_6$ octahedra during the T-R phase transition. Strong inelastic diffuse scattering shows up near $M$(1.5, 0.5, 0) and $R$(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the $M_3$ and $R_{25}$ optic modes for the corresponding transitions. This reveals the different rotation instabilities of ${\mathrm{TiO}}_6$ in the corresponding transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures but did not show close correlations to the formation of polar nanoregions. Additional instabilities could be the origin of the complexity of phase transitions and crystallographic structures in NBT.

We present a theory of magnetotransport through an electronic orbital, where the electron spin interacts with a (sufficiently) large external spin via an exchange interaction. Using a semiclassical approximation, we derive a set of equations of motions for the electron density matrix and the mean value of the external spin that turns out to be highly nonlinear. The dissipation via the electronic leads is implemented in terms of a quantum master equation that is combined with the nonlinear terms of the spin-spin interaction. With an anisotropic exchange coupling a variety of dynamics is generated, such as self-sustained oscillations with parametric resonances or even chaotic behavior. Within our theory we can integrate a Maxwell-demon-like closed-loop feedback scheme that is capable of transporting particles against an applied bias voltage and that can be used to implement a spin filter to generate spin-dependent oscillating currents of opposite directions.

Weyl fermions, which are fermions with definite chiralities, can give rise to anomalous breaking of the symmetry of the physical system that they are a part of. In their $(3+1)$-dimensional realizations in condensed matter systems, i.e., the so-called Weyl semimetals, this anomaly gives rise to topological electromagnetic response of magnetic fluctuations, which takes the form of nonlocal interaction between magnetic fluctuations and electromagnetic fields. We study the physical consequences of this nonlocal interaction, including electric field assisted magnetization dynamics, an extra gapless magnon dispersion, and polariton behaviors that feature “sibling” bands in small magnetic fields.

Using scanning susceptibility microscopy, we shed light on the dynamics of individual superconducting vortices and examine the hypotheses of the phenomenological models traditionally used to explain the macroscopic ac electromagnetic properties of superconductors. The measurements, carried out on a ${\text{2H-NbSe}}_2$ single crystal at relatively high temperature ($T=6.8$ K), show a linear amplitude dependence of the global ac susceptibility for excitation amplitudes between 0.3 and 2.6 Oe. We observe that the low amplitude response, typically attributed to the oscillation of vortices in a potential well defined by a single, relaxing, Labusch constant, actually corresponds to strongly nonuniform vortex shaking. This is particularly pronounced in the field-cooled disordered phase, which undergoes a dynamic reorganization above 0.8 Oe as evidenced by the healing of lattice defects and a more uniform oscillation of vortices. These observations are corroborated by molecular dynamics simulations when choosing the microscopic input parameters from the experiments. The theoretical simulations allow us to reconstruct the vortex trajectories, providing deeper insight into the thermally induced hopping dynamics and the vortex lattice reordering.