The electronic and magnetic properties of recently discovered important constituent of the earth's lower mantle ${\mathrm{FeO}}_2\mathrm{H}$ was investigated by means of the density functional theory combined with the dynamical mean-field theory. The addition of hydrogen to the parent ${\mathrm{FeO}}_2$ compound, which is an uncorrelated bad metal, destroys the most important ingredient of its electronic structure, namely O-O molecular orbitals. In effect, the physical properties of ${\mathrm{FeO}}_2$ and ${\mathrm{FeO}}_2\mathrm{H}$ turn out to be completely different, ${\mathrm{FeO}}_2\mathrm{H}$ is a correlated metal with a mass renormalization $\left(m^{*}/m\sim 1.7\right)$, and the magnetic moments on Fe ions become localized with the Curie-Weiss type of uniform magnetic susceptibility.

The quantum spin Hall effect (QSHE) has formed the seed for contemporary research on topological quantum states of matter. Since its discovery in HgTe/CdTe quantum wells and InAs/GaSb heterostructures, all such systems have so far been suffering from extremely low operating temperatures, rendering any technological application out of reach. We formulate a theoretical paradigm to accomplish the high temperature QSHE in monolayer-substrate heterostructures. Specifically, we explicate our proposal for hexagonal compounds formed by monolayers of heavy group-V elements (As, Sb, Bi) on a SiC substrate. We show how orbital filtering due to substrate hybridization, a tailored multiorbital density of states at low energies, and large spin-orbit coupling can conspire to yield QSH states with bulk gaps of several hundreds of meV. Combined with the successful realization of Bi/SiC (0001), with a measured bulk gap of $\sim 800$ meV reported previously [F. Reis et al., Science 357, 287 (2017)], our paradigm elevates the QSHE from an intricate quantum phenomenon at low temperatures to a scalable effect amenable to device design and engineering.

This work concerns Ising quasiholes in Moore-Read type lattice wave functions derived from conformal field theory. We commence with constructing Moore-Read type lattice states and then add quasiholes to them. By use of Metropolis Monte Carlo simulations, we analyze the features of the quasiholes, such as their size, shape, charge, and braiding properties. The braiding properties, which turn out to be the same as in the continuum Moore-Read state, demonstrate the topological attributes of the Moore-Read lattice states in a direct way. We also derive parent Hamiltonians for which the states with quasiholes included are ground states. One advantage of these Hamiltonians lies therein that we can now braid the quasiholes just by changing the coupling strengths in the Hamiltonian since the Hamiltonian is a function of the positions of the quasiholes. The methodology exploited in this article can also be used to construct other kinds of lattice fractional quantum Hall models containing quasiholes, for example, investigation of Fibonacci quasiholes in lattice Read-Rezayi states.

A non-Hermitian extension of a Chern insulator and its bulk-boundary correspondence are investigated. It is shown that in addition to the robust chiral edge states that reflect the nontrivial topology of the bulk (nonzero Chern number), anomalous helical edge states localized only at one edge can appear, which are unique to the non-Hermitian Chern insulator.

Two-dimensional materials offer a versatile platform to study high-harmonic generation (HHG), encompassing as limiting cases bulklike and atomiclike harmonic generation [Tancogne-Dejean and Rubio, Sci. Adv. 4, eaao5207 (2018)]. Understanding the high-harmonic response of few-layer semiconducting systems is important and might open up possible technological applications. Using extensive first-principles calculations within a time-dependent density functional theory framework, we show how the in-plane and out-of-plane nonlinear nonperturbative responses of two-dimensional materials evolve from the monolayer to the bulk. We illustrate this phenomenon for the case of multilayer hexagonal BN layered systems. Whereas the in-plane HHG is found not to be strongly altered by the stacking of the layers, we found that the out-of-plane response is strongly affected by the number of layers considered. This is explained by the interplay between the induced electric field, resulting from the electron-electron interaction, and the interlayer delocalization of the wave functions contributing most to the HHG signal. The gliding of a bilayer is also found to affect the high-harmonic emission. Our results will have important ramifications for the experimental study of monolayer and few-layer two-dimensional materials beyond the case of hexagonal BN studied here as the results we found are generic and applicable to all two-dimensional semiconducting multilayer systems.

Recently, a quantum anomalous Hall insulator (QAHI)/superconductor heterostructure has been realized and shows half-quantized conductance plateaus in two-terminal conductance measurements [Q. L. He et al., Science 357, 294 (2017)]. The half-quantized conductance plateaus are considered as a solid evidence of chiral Majorana edge modes. However, there is a strong debate over the origin of the half-quantized conductance plateaus. In this work, we propose a Josephson junction based on the QAHI/superconductor heterostructure to identify the existence of chiral Majorana edge modes. We find that the critical Josephson current dramatically increases to a peak value when a half-quantized conductance plateau ${\sigma }_{12}=e^2/2h$ is showing up for the $N=1$ chiral topological superconductor phase with a single chiral Majorana mode. Furthermore, we show that the critical Josephson current of the $N=1$ chiral topological superconductor exhibits an $h/e$-period oscillation and is robust to disorder, in contrast to the behaviors of conventional two-dimensional electron gas systems. We also estimate experimentally relevant parameters and believe that the supercurrent can be observed in experiments.

Two-dimensional electrons in high-quality nanostructures at low temperatures can form a viscous fluid. We develop a theory of high-frequency magnetotransport in such a fluid. The time dispersion of viscosity should be taken into account at the frequencies about and above the rate of electron-electron collisions. We show that the shear viscosity coefficients as functions of magnetic field and frequency have the resonance at the frequency equal to the doubled cyclotron frequency. We demonstrate that such a resonance manifests itself in the plasmon damping. Apparently, the predicted resonance is also responsible for the peaks and features in photoresistance and photovoltage, recently observed on the ultra-high-mobility GaAs quantum wells. The last fact should be considered as important evidence for forming a viscous electron fluid in such structures.

The negative longitudinal magnetoresistance (NLMR) was observed in type-II Dirac semimetal ${\mathrm{PtSe}}_2$ ultrathin microflakes. The NLMR disappeared when the magnetic field direction deviates from that of the electric field, or when the temperature is above a critical value, even when the Fermi level is away from Dirac point. The physical mechanism behind NLMR may relate to the chiral anomaly of Dirac fermions or the nontopological effect in ${\mathrm{PtSe}}_2$ material.

Electron emission from solids driven by two-color lasers provides great flexibility for the control of electron dynamics in ultrashort spatiotemporal scales due to the interference effect. Here, we construct an analytical model for the highly nonlinear photoelectron emission from a metal surface illuminated by two-color laser fields, by solving the time-dependent Schrödinger equation. The exact solution is valid for arbitrary harmonic orders, laser intensities, phase difference between two lasers, and metal work function and Fermi level. We find two-color lasers can strongly modulate both the electron energy spectra and the emission current up to 99%. Using the same input parameters, our theoretical prediction for the photoemission current modulation depth (93.9%) is almost identical to the experimental result (94%) in [M. Förster et al., Phys. Rev. Lett. 117, 217601 (2016)].

We developed a theoretical framework which extends the method of full counting statistics (FCS) from conventional single-channel Kondo screening schemes to a multichannel Kondo paradigm. The developed idea of FCS has been demonstrated considering an example of a two-stage Kondo (2SK) model. We analyzed the charge-transferred statistics in the strong-coupling regime of a 2SK model using a nonequilibrium Keldysh formulation. A bounded value of the Fano factor, $1\le F\le 5/3$, confirmed the crossover regimes of charge-transferred statistics in the 2SK effect, from Poissonian to super-Poissonian. An innovative way of measuring the transport properties of the 2SK effect, by the independent measurements of charge current and noise, has been proposed.

We inspect the propagation of shear polarized surface waves akin to Love waves through a forest of trees of the same height atop a guiding layer on a soil substrate. An asymptotic analysis shows that the forest behaves like an infinitely anisotropic wedge with effective boundary conditions. We discover that the foliage of trees brings a radical change in the nature of the dispersion relation of these surface waves, which behave like spoof plasmons in the limit of a vanishing guiding layer, and like Love waves in the limit of trees with a vanishing height. When we consider a forest with trees of increasing or decreasing height, this hybrid “spoof Love wave” is either trapped within the trees or converted into a downward propagating bulk (shear) wave. These mechanisms of wave trapping and wave conversion appear to be robust with respect to perturbations of height or position of trees in the metawedge and with respect to three-dimensional effects such as regarding a potential change of elastic wave polarization.

The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to the magnetization of a magnetically ordered metallic sample; in metallic ferrimagnets with chemically distinct sublattices, such as rare-earth transition-metal alloys, it depends on the difference between the sublattice contributions. Here we show that in a highly spin polarized, fully compensated ferrimagnet, where the sublattices are chemically similar, a signal is observed even when the net moment is negligible. We analyze the spectral ellipsometry and MOKE of ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ and show that this behavior is due to a highly spin-polarized conduction band dominated by one of the two manganese sublattices ($4c$) which creates helicity-dependent reflectivity dominated by a broad Drude tail. Our findings open prospects for studying spin dynamics in the infrared.

We use ab initio anisotropic Migdal-Eliashberg formalism to examine the pairing mechanism and the nature of the superconducting gaps in experimentally fabricated borophenes. Our results indicate that the superconducting transition is dominated by a standard phonon-mediated mechanism, and multiple anisotropic superconducting gaps with critical temperatures $T_c$ even approaching 33 K are present in the freestanding form of the fabricated borophenes. These findings provide a different perspective for superconductivity in borophenes.

We chart out the ground state phase diagram and demonstrate the presence of a many-body localized (MBL) phase for an experimentally realizable one-dimensional (1D) constrained dipole boson model in the presence of an Aubry-Andre (AA) potential whose strength ${\lambda }_0$ can be tuned to precipitate an ergodic-MBL transition. We discuss the signature of such a transition in the quantum dynamics of the model by computing its response subsequent to a sudden quench of ${\lambda }_0$. We show that the MBL and the ergodic phases can be clearly distinguished by study of post-quench dynamics and provide an estimate for minimal time up to which experiments need to track the response of the system to confirm the onset of the MBL phase. We suggest experiments which can test our theory.

We report the structural, static, and dynamic properties of ${\mathrm{Cr}}_{0.5}{\mathrm{Fe}}_{0.5}\mathrm{Ga}$ by means of powder x-ray diffraction, dc magnetization, heat capacity, ac susceptibility, magnetic relaxation, and magnetic memory effect measurements. The dc magnetization and ac susceptibility studies reveal a spin-glass transition at around $T_{\mathrm{f}}\simeq 22$ K. An intermediate value of the relative shift in freezing temperature $\delta T_{\mathrm{f}}\simeq 0.017$, obtained from the ac susceptibility data reflects the formation of cluster spin-glass states. The frequency dependence of $T_{\mathrm{f}}$ is also analyzed within the framework of dynamic scaling laws such as power law and Vogel-Fulcher law. The analysis using power law yields a characteristic time constant for a single spin flip $\tau *\simeq 1.1\times {10}^{-10}$ s and critical exponent $z{\nu }^{\prime }=4.2\pm 0.2$. On the other hand, the Vogel-Fulcher law yields the characteristic time constant for a single spin flip ${\tau }_0\simeq 6.6\times {10}^{-9}$ s, Vogel-Fulcher temperature $T_0=21.1\pm 0.1$ K, and an activation energy $E_{\mathrm{a}}/k_{\mathrm{B}}\simeq 16$ K. The value of $\tau *$ and ${\tau }_0$ along with a nonzero value of $T_0$ provide further evidence for the cluster spin-glass behavior. The magnetic field dependent $T_{\mathrm{f}}$ follows the de Almeida-Thouless (AT) line with a non-mean-field type instability, reflecting either a different universality class or strong anisotropy in the spin system. A detailed nonequilibrium dynamics study via relaxation and memory effect experiments demonstrates the evolution of the system through a number of intermediate metastable states and striking memory effects. All the above observations render a cluster spin-glass behavior in ${\mathrm{Cr}}_{0.5}{\mathrm{Fe}}_{0.5}\mathrm{Ga},$ which is triggered by magnetic frustration due to competing antiferromagnetic and ferromagnetic interactions and magnetic site disorder. Moreover, the asymmetric response of magnetic relaxation with respect to the change in temperature, below the freezing temperature can be explained by the hierarchical model.

The temperature dependence of the spin polarized tunnel conductance is investigated with Fe/MgO/Fe tunnel junctions with different structural properties and interfacial chemistry. A global quantitative model is proposed for analyzing the tunnel conductance in both parallel and antiparallel configuration. Three contributions to the temperature dependence can be distinguished. The first one is governed by the Bloch law for the temperature dependence of the magnetization of the electrodes. The second one is unpolarized and follows a power-law associated with a hopping mechanism. Although these two mechanisms are predominant in the decrease of tunnel magnetoresistance with temperature, a third contribution must be considered. This term results from a thermally activated decrease of the effective spin polarization, introduced by an Arrhenius law. It appears in either the parallel or antiparallel conductance, depending on the interface chemical doping, and could then be related to a symmetry dependent diffusion process. Finally, this global analysis appears universal, as it can well fit the temperature dependence of all samples.

A mechanism is advanced suggesting the resolution of the dichotomy of long-lived spin polarization storage versus fast spin reversal at the required time. A system of atoms or molecules is considered interacting through magnetic dipolar forces. The constituents are assumed to possess internal structure allowing for the generation of the alternating-current quadratic Zeeman effect, whose characteristics can be efficiently regulated by quasiresonant dressing. The sample is connected to an electric circuit producing a feedback field acting on spins. By switching on and off the alternating-current quadratic Zeeman effect it is possible to realize spin reversals with a required delay time. The suggested technique of regulated spin reversal can be used in quantum information processing and spintronics.

We discuss the modeling of the electrical manipulation of spin qubits in the linear-response regime where the Rabi frequency is proportional to the magnetic field and to the radio-frequency electric field excitation. We show that the Rabi frequency can be obtained from a generalized $g$-tensor magnetic resonance formula featuring a $g$ matrix and its derivative $g^{\prime }$ with respect to the electric field (or gate voltage) as inputs. These matrices can be easily calculated from the wave functions of the qubit at zero magnetic field. The $g$-matrix formalism therefore provides the complete dependence of the Larmor and Rabi frequencies on the orientation of the magnetic field at very low computational cost. It also provides a compact model for the control of the qubit and a simple framework for the analysis of the effects of symmetries on the anisotropy of the Larmor and Rabi frequencies. The $g$-matrix formalism applies to a wide variety of electron and hole qubits, and we focus on a hole qubit in a silicon-on-insulator nanowire as an illustration. We show that the Rabi frequency of this qubit shows a complex dependence on the orientation of the magnetic field and on the gate voltages that control the symmetry of the hole wave functions. We point out that the qubit may be advantageously switched between two bias points, one where it can be manipulated efficiently, and one where it is largely decoupled from the gate field but presumably longer lived. We also discuss the role of residual strains in such devices in relation to recent experiments.

Quantum computation based on semiconductor electron-spin qubits requires high control of tunnel couplings between the quantum dots and the electron reservoirs. Potential disorder and the increasing complexity of the two-dimensional gate-defined quantum computing devices set high demands on the gate design and the voltage tuning of the tunnel barriers. We present a Green's formalism approach for the calculation of tunnel couplings between a quantum dot and a reservoir. Our method takes into account in full detail the two-dimensional electrostatic potential of the quantum dot, the tunnel barrier, and the reservoir. A wideband limit is employed only far away from the tunnel barrier region where the density of states is sufficiently large. We calculate the tunnel coupling including potential disorder effects, which become increasingly important for large-scale silicon-based spin-qubit devices. Studying the tunnel couplings of a single-electron transistor in Si/SiGe as a showcase, we find that charged defects are the dominant source of disorder leading to variations in the tunnel coupling of four orders of magnitude.