Charge noise is the main hurdle preventing high-fidelity operation, in particular that of two-qubit gates, of semiconductor-quantum-dot-based spin qubits. While certain sweet spots where charge noise is substantially suppressed have been demonstrated in several types of spin qubits, the existence of one for coupled singlet-triplet qubits is unclear. We theoretically demonstrate, using full configuration-interaction calculations, that a range of nearly sweet spots appears in the coupled singlet-triplet qubit system when a strong enough magnetic field is applied externally. We further demonstrate that ramping to and from the judiciously chosen nearly sweet spot using sequences based on the shortcut to adiabaticity offers maximal gate fidelities under charge noise and phonon-induced decoherence. These results should facilitate realization of high-fidelity two-qubit gates in singlet-triplet qubit systems.

Monolayer ${\mathrm{NbSe}}_2$ is a metallic two-dimensional (2D) transition-metal dichalcogenide material. Owing to the lattice structure and the strong atomic spin-orbit coupling (SOC) field, monolayer ${\mathrm{NbSe}}_2$ possesses Ising-type SOC which acts as an effective Zeeman field, leading to the unconventional topological spin properties. In this paper, we numerically calculate spin-dependent optical conductivity of monolayer ${\mathrm{NbSe}}_2$ using Kubo formula based on an effective tight-binding model which includes $d_{z^2}, d_{x^2-y^2}$, and $d_{xy}$ orbitals of Nb atoms. Numerical calculation indicates that the up- and down-spin have opposite sign of Hall current, so the pure spin Hall current can be generated in monolayer ${\mathrm{NbSe}}_2$ under light irradiation, owing to the topological nature of monolayer ${\mathrm{NbSe}}_2$, i.e., finite spin Berry curvature. The spin Hall angle is also evaluated. The optically induced spin Hall current can be enhanced by the electron doping and persists even at room temperature. Our results will serve to design opt-spintronics devices such as spin current harvesting by light irradiation on the basis of 2D materials.

We consider a heterostructure of semiconductor layers sandwiched between two superconductors, forming a two-dimensional Josephson junction. Applying a Zeeman field perpendicular to the junction can render a topological superconducting phase with the chiral Majorana edge mode. We show that the phase difference between two superconductors can efficiently reduce the magnetic field required to achieve the chiral topological superconductivity, providing an experimentally feasible setup to realize chiral Majorana edge modes. We also construct a lattice Hamiltonian of the setup to demonstrate the chiral Majorana edge mode and the Majorana bound state localized in vortices.

Using time-domain terahertz spectroscopy in pulsed magnetic fields up to 31 T, we measure the terahertz optical conductivity in an optimally doped thin film of the high-temperature superconducting cuprate ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_4$. We observe systematic changes in the circularly polarized complex optical conductivity that are consistent with cyclotron absorption of $p$-type charge carriers characterized by a cyclotron mass of $4.9m_{\mathrm{e}}\pm 0.8m_{\mathrm{e}}$ and a scattering rate that increases with magnetic field. These results open the door to studies aimed at characterizing the degree to which electron-electron interactions influence carrier masses in cuprate superconductors.

We study the dynamics of periodically kicked many-body systems away from the high-frequency regime, and discuss a family of Floquet systems where the notion of prethermalization can be naturally extended to intermediate and low driving frequencies. We investigate numerically the dynamics of both integrable and nonintegrable systems, and report on the formation of a long-lived prethermal plateau, akin to the high-frequency limit, where the system thermalizes with respect to an effective Hamiltonian captured by the inverse-frequency expansion (IFE). Unlike the high-frequency regime, we find that the relevant heating times are model dependent: we analyze the stability of the prethermal plateau to small perturbations in the drive period and show that, in a spin chain whose IFE is intractable, the plateau duration is insensitive to the perturbation strength, in contrast to a chain where the IFE admits the resummation of an entire subseries. Infinitesimal perturbations are enough to restore the ergodic properties of the system, and decrease residual finite-size effects. Although the regime where the Floquet system leaves the prethermal plateau and starts heating up to infinite temperature is not captured by the IFE, we provide evidence that the evolved subsystem is described well by a thermal state with respect to the IFE Hamiltonian, with a gradually changing temperature, in accord with the eigenstate thermalization hypothesis.

${\mathrm{Ba}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$, the quasi-one-dimensional spin-1 ($S$ = 1) compound, has rather intricate magnetic properties. The compound consists of structural Ru trimers, which together form the zig-zag chains where the Ru has two inequivalent crystallographic sites. While at high temperature both the inequivalent Ru ions stay in the $4+$ state with effective $S=$ 1 spin state, upon lowering the temperature, the magnetic moment of the central Ru atom is completely quenched accompanied by a change in the octahedral environment. This effectively gives rise to a bond-alternating chain and provides an opportunity to study the excitation of such a spin network in a real material. We have used microscopic tools such as neutron scattering and muon spin relaxation along with the density functional theory based calculations to address the spin state of the two inequivalent Ru ions in this material. From our neutron powder diffraction, on lowering of temperature, we find a large tetragonal distortion of the central ${\mathrm{RuO}}_6$ octahedra of the trimer. The splitting of the $t_{2g}$ level of the central Ru due to this distortion is found to be significant leading to the quenching of the moment underscoring the Hund's exchange. The nonmagnetic central Ru promotes a strong antiferromagnetic superexchange between the other two Ru ions in the trimer, which gives rise to a dimeric state. The presence of spin dimers is reflected by the manifestation of a gap in the spin excitation spectra. The spin-dimer formation in ${\mathrm{Ba}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ is at par with the effective model proposed by Affleck and Haldane for the $S$ = 1 bond alternating chains in the light of valence bond solid formalism. Eventually, at a lower temperature, a long-range ordered antiferromagnetic state emerges from the gapped dimer state due to the significant interdimer interactions.

Van der Waals (vdW) ferromagnets are an important class of materials for spintronics applications. The recent discovery of atomically vdW magnets ${\mathrm{CrI}}_3$ and ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ has triggered a renaissance in the area of two-dimensional (2D) magnetism. Herein we systematically studied 2H-${\mathrm{Mn}}_{0.28}{\mathrm{TaS}}_2$ single crystal, a 2D vdW ferromagnet with $T_c\sim 82.3\mathrm{K}$, and a large in-plane magnetic anisotropy. Mn $K$-edge x-ray absorption spectroscopy was measured to provide information on its electronic state and local atomic environment. The detailed magnetic isotherms measured in the vicinity of $T_c$ indicates that the spin coupling inside 2H-${\mathrm{Mn}}_{0.28}{\mathrm{TaS}}_2$ is of a 3D Heisenberg type coupled with the attractive long-range interaction between spins that decay as $J\left(r\right)\approx r^{-4.85}$. Both resistivity $\rho \left(T\right)$ and thermopower $S\left(T\right)$ exhibit anomalies near $T_c$, confirming that the hole-type transport carriers strongly interact with local moments. An unusual angle-dependent magnetoresistance is further observed, suggesting a possible field-induced novel magnetic structure.

The interplay between topological electronic structure and superconductivity has attracted tremendous research interests recently as they could induce topological superconductivity (TSCs) which may be used to realize topological qubits for quantum computation. Among various TSC candidates, superconducting $\mathrm{Ba}{\mathrm{Sn}}_3$ ($T_{\mathrm{c}}\sim 4.4\mathrm{K}$) has been predicted to be a topological Dirac semimetal (TDS) hosting two pairs of Dirac points along the Γ-$A$ direction. Here, by combining the use of angle-resolved photoemission spectroscopy and ab initio calculations, we identified the predicted topological Dirac fermions and confirmed the TDS nature of the compound. In addition, we observed surface states connecting the Dirac points. Our observations demonstrate $\mathrm{Ba}{\mathrm{Sn}}_3$ as a superconductor with nontrivial topological electronic structures.

We show that the valley Chern number of the low-energy band in twisted double bilayer graphene can be tuned through two successive topological transitions, where the direct band gap closes, by changing the electric field perpendicular to the plane of the graphene layers. The two transitions with Chern number changes of $-3$ and $+1$ can be explained by the formation of three satellite Dirac points around the central Dirac cone in the moiré Brillouin zone due to the presence of trigonal warping. The satellite cones have opposite chirality to the central Dirac cone. Considering the overlap of the bands in energy, which lead to metallic states, we construct the experimentally observable phase diagram of the system in terms of the indirect band gap and the anomalous valley Hall conductivity. We show that while most of the intermediate phase becomes metallic, there is a narrow parameter regime where the transition through three insulating phases with different quantized valley Hall conductivity can be seen. We systematically study the effects of variations in the model parameters on the phase diagram of the system to reveal the importance of particle-hole asymmetry and trigonal warping in constructing the phase diagram. We also study the effect of changes in interlayer tunneling on this phase diagram.

Tuning interfacial thermal conductance has been a key task for the thermal management of nanoelectronic devices. Here, we study how the interfacial thermal conductance is greatly influenced by modulating the mass distribution of the interlayer of one-dimensional atomic chain. By nonequilibrium Green's function and machine learning algorithm, the maximum/minimum value of thermal conductance and its corresponding mass distribution are calculated. Interestingly, the mass distribution corresponding to the maximum thermal conductance is not a simple function, such as the linear and exponential distribution predicted in previous works, it is similar to a sinusoidal curve around a linear distribution for larger thickness interlayer. Further, the mechanism of the abnormal results is explained by analyzing the phonon transmission spectra and density of states. Additionally, the mass distributions are applied to Si/Ge and $\mathrm{Si}\text{/}{\mathrm{Si}}^{\mathrm{isotope}}$ three-dimensional systems. The work provides deep insight into optimizing and designing interfacial thermal conductance by modulating mass distribution of interlayer atoms.

The nature of the fractional quantum Hall effect at $\nu =1/2$, observed in wide quantum wells almost three decades ago, is still under debate. Previous studies have investigated it using the variational Monte Carlo method, which assumes that the transverse wave function and the gap between the symmetric and antisymmetric subbands obtained in a local density approximation at zero magnetic field remain valid even at high perpendicular magnetic fields; this method also ignores the effect of Landau level mixing. We develop in this work a three-dimensional fixed-phase diffusion Monte Carlo method, which gives, in a single framework, the total energies of various candidate states in a finite width quantum well, including Landau level mixing, directly in a large magnetic field. This method can be applied to one-component states and also to two-component states in the limit where the symmetric and antisymmetric bands are nearly degenerate. Our three-dimensional fixed-phase diffusion Monte Carlo calculations find that the one-component composite-fermion Fermi sea and the one-component Pfaffian states are very close in energy for a range of quantum-well widths and densities, suggesting that the observed 1/2 fractional quantum Hall state in wide quantum wells is likely to be the one-component Pfaffian state. We hope that this will motivate further experimental studies of this state.

In this work we investigate the electromagnetic local density of states (LDOS) near a twisted bilayer graphene (TBG) deposited over a general isotropic substrate. The band structure of the TBG is calculated within a tight-binding framework, and then used to determine the TBG's conductivity. The latter presents a nontrivial dependence upon the angle of twist, which shows up in the LDOS, allowing for a moiré pattern-dependent quantum emission. For some specific twist angles we show that it is possible to either enhance or decrease the LDOS by an order of magnitude at selected frequencies when compared to the monolayer. This impressive variation is explained in terms of the presence/absence of well defined surface plasmon polaritons (SPPs). Altogether our findings demonstrate that TBG is an alternative, versatile material platform for controlling spontaneous emission and enhancing light-matter interactions, and pave the way for further studies and applications of quantum emission in the emerging class of two-dimensional moiré materials.

We study backscattering of electrons and conductance suppression in a helical edge channel in two-dimensional topological insulators with broken axial spin symmetry in the presence of nonmagnetic point defects that create bound states. In this system the tunneling coupling of the edge and bound states results in the formation of composite helical edge states in which all four partners of both Kramers pairs of the conventional helical edge states and bound states are mixed. The backscattering is considered as a result of inelastic two-particle scattering of electrons, which are in these composite states. Within this approach we find that sufficiently strong backscattering occurs even if the defect creates only one energy level. The effect is caused by electron transitions between the composite states with energy near the bound state level. We study the deviation from the quantized conductance due to scattering by a single defect as a function of temperature and Fermi level. The results are generalized to the case of scattering by many different defects with energy levels distributed over the band gap. In this case the conductance deviation turns out to be quite strong and comparable with experiment even at a sufficiently low density of defects. Interestingly, under certain conditions, the temperature dependence of the conductance deviation becomes very weak over a wide temperature range.

We explore the topological aspect of dynamics in a microelectromechanical system (MEMS), which is a combination of an electric-circuit system and a mass-spring system. The simplest example is a sequential chain of capacitors and springs. It is shown that such a chain exhibits novel topological dynamics with respect to its oscillation modes. On one hand, when it undergoes free oscillation, the system is governed by the Su-Schrieffer-Heeger model, and the topological charge is given by a winding number. Topological and trivial phases are differentiated by measuring the dynamics of the outermost plate. On the other hand, when it undergoes periodical motion in time, the system is governed by an inversion-symmetric-trimer model, and the topological phases are characterized by an inversion-symmetry indicator. There emerge topological edge states, which are well signaled by measuring electromechanical-impedance resonance. Our results will open an attractive field of topological MEMS.

We theoretically investigate high harmonic generation (HHG) from silicon thin films with thicknesses from a few atomic layers to a few hundreds of nanometers, to determine the most efficient thickness for producing intense HHG in the reflected and transmitted pulses. For this purpose, we employ a few theoretical and computational methods. The most sophisticated method is the ab initio time-dependent density functional theory coupled with the Maxwell equations in a common spatial resolution. This enables us to explore such effects as the surface electronic structure and light propagation, as well as electronic motion in the energy band in a unified manner. We also utilize a multiscale method that is applicable to thicker films. Two-dimensional approximation is introduced to obtain an intuitive understanding of the thickness dependence of HHG. From these ab initio calculations, we find that the HHG signals are the strongest in films with thicknesses of 2–15 nm, which is determined by the bulk conductivity of silicon. We also find that the HHG signals in the reflected and transmitted pulses are identical in such thin films. In films whose thicknesses are comparable to the wavelength in the medium, the intensity of HHG signals in the reflected (transmitted) pulse is found to correlate with the magnitude of the electric field at the front (back) surface of the thin film.

A numerical solution to the time evolution equation of the Wigner distribution function (WDF) with an accuracy necessary to simulate the passage of a wave packet past a barrier is developed, where quantum effects require high accuracy and fine discretization. A wave packet incident on a barrier, a portion of which tunnels through, demonstrates behavior that can define various characteristic transmission and reflection delay (TARD) times useful in the simulation of electron emission. A model for the TARD times is proposed that relies only on the asymptotic maxima of the position $\rho (x,t)$ and wave number $\rho (k,t)$ densities given by the WDF and applied to a ballistic trajectory model for the (faster) transmitted and (slower) reflected parts. The dependence of the TARD times on barrier width, symmetry, and abruptness is analyzed. For symmetrical barriers with characteristics similar to field emission barrier heights and widths, TARD times are on the order of a fraction of a femtosecond. The TARD times for when tunneling predominates are contrasted to tunneling times in the literature. Use of the TARD times in simulations of field emission in nanogap devices or to model ultrashort pulses generated under rapidly changing conditions for electron sources are proposed.

The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes, and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. The symmetry analysis of the ARPES spectra demonstrates that the $d_{xz}$ band gains quasiparticle spectral weight compared to the $d_{yz}$ band for negative antisymmetric strain $\mathrm{\Delta }{\epsilon }_{yy}$ suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the $d_{xz}$ and $d_{yz}$ orbital within a Hund's metal picture. We also discuss the influence of orbital mixing.

TaAs and NbAs are two of the earliest identified Weyl semimetals that possess many intriguing optical properties, such as chirality-dependent optical excitations and giant second harmonic generation (SHG). Linear and nonlinear optics have been employed as tools to probe the Weyl physics in these crystals. Here, we extend these studies to address two important aspects: determining the complete anisotropic dielectric response and exploring if and how they can reveal essential Weyl physics. We determine the complete anisotropic dielectric functions of TaAs and NbAs by combining spectroscopic ellipsometry and density functional theory (DFT). Parameterized Lorentz oscillators are reported from 1.2–6 eV (experiment) and 0–6 eV (DFT), and good agreement is shown between them. Both linear and nonlinear optical properties have been previously reported to reveal Weyl physics. We suggest that strong optical resonances from trivial bands are the likely origin of the large optical SHG previously reported at these energies. Furthermore, by comparing the contribution of a small $k$-space centered around the Weyl cones with the total linear dielectric function, we find that these contributions are highly anisotropic and are $<25\%$ of the total dielectric function $\mathrm{below}0.5\mathrm{eV}$; $\mathrm{above}1\mathrm{eV}$, these contributions are negligible. Thus, the study of Weyl physics using optical techniques requires very low energies, and even there, a careful assessment is required in distinguishing the much smaller contributions of the Weyl bands from the dominant contributions of the trivial bands and Drude response to the total dielectric function.

While it is commonly accepted that the activation energy of the thermally activated polaron hopping transport in disordered organic semiconductors can be decoupled into a disorder and a polaron contribution, their relative weight is still controversial. This feature is quantified in terms of the so-called $C$ factor in the expression for the effective polaron mobility: ${\mu }_e\propto \exp \left[-E_a/k_{B}T-C{\left(\sigma /k_{B}T\right)}^2\right]$, where $E_a$ and $\sigma$ are the polaron activation energy and the energy width of a Gaussian density of states (DOS), respectively. A key issue is whether the universal scaling relation (implying a constant $C$ factor) regarding the polaron formation energy is really obeyed, as recently claimed in the literature [Seki and Wojcik, J. Chem. Phys. 145, 034106 (2016)]. In the present work, we reinvestigate this issue on the basis of the Marcus transition rate model using extensive kinetic Monte Carlo simulations as a benchmark tool. We compare the polaron-transport simulation data with results of analytical calculations by the effective medium approximation and multiple trapping and release approaches. The key result of this study is that the $C$ factor for Marcus polaron hopping depends on first the degree of carrier localization, i.e., the coupling between the sites, further whether quasiequilibrium has indeed been reached, and finally the $\sigma /E_a$ ratio. This implies that there is no universal scaling with respect to the relative contribution of polaron and disorder effect. Finally, we demonstrate that virtually the same values of the disorder parameter $\sigma$ are determined from available experimental data using the $C$ factors obtained here irrespective of whether the data are interpreted in terms of Marcus or Miller-Abrahams rates. This implies that molecular reorganization contributes only weakly to charge transport, and it justifies the use of the zero-order Miller-Abrahams rate model for evaluating the DOS width from temperature-dependent charge transport measurements regardless of whether or not polaron effects are accounted for.