We propose a model of spin-polarized current state for electrons in bilayer graphene. The model resolves the puzzles as revealed by experiments that (a) the energy gap $E_{\mathrm{gap}}$ of the insulating ground state at the charge neutrality point (CNP) can be closed by a perpendicular electric field of either polarity, (b) $E_{\mathrm{gap}}$ increases significantly with increasing the magnetic field $B$, (c) the particle-hole spectrum is asymmetric in the presence of $B$, (d) there is a peak structure in the electric conductivity at small $B$ at the CNP, and (e) there are quantum Hall states stemming from lifting of degeneracy in the lowest Landau level. The model predicts that the ground state of the system close to the CNP is a ferrimagnet at finite $B$ and the Hall current is spin polarized.

Hybrid-exchange density functional theory calculations are carried out to determine the effects of A-site doping on the electronic and magnetic properties of the manganite series ${\mathrm{La}}_{1\text{−}x}$${\mathrm{Ca}}_x$${\mathrm{MnO}}_3$. This study focuses on the ground state of an ordered Ca occupancy in a periodic structure. It is shown that the hybrid-exchange functional, Becke three-parameter Lee-Yang-Parr (B3LYP), provides an accurate and consistent description of the electronic structure for ${\mathrm{LaMnO}}_3$, ${\mathrm{CaMnO}}_3$, and ${\mathrm{La}}_{0.75}$${\mathrm{Ca}}_{0.25}$${\mathrm{MnO}}_3$. We have quantified the relevant structural, magnetic, and electronic energy contributions to the stability of the doped compound. An insight into the exchange coupling mechanism for the low hole density region of the phase diagram, where a polaron (anti-Jahn-Teller) forms, is also provided. This study completes a microscopic description of the lightly doped insulator with an antiferromagnetic-to-ferromagnetic and metal-to-insulator transition.

We investigate the valley depolarization due to the electron-hole exchange interaction in monolayer ${\text{MoS}}_2$. Both the long- and short-range parts of the intra- and intervalley electron-hole exchange interactions are calculated. We find that both the long- and short-range exchange interactions can cause the inter- and intravalley bright exciton transitions. With the intravalley bright exciton transition channel nearly forbidden due to the large splitting of the valence bands, the intervalley channel due to the exchange interaction can cause the valley depolarization efficiently by the Maialle-Silva-Sham mechanism [Phys. Rev. B 47, 15776 (1993)]. With only the long-range exchange interaction, the calculations show good agreement with the recent valley polarization experiments, including the time-resolved valley polarization measurement, the pump-probe experiment, and the steady-state photoluminescence (PL) polarization measurement. We further show that for the A exciton with large (small) center-of-mass momentum, the long-range exchange interaction can cause the fast (slow) intervalley exciton transition.

The growth process of germanium inverted quantum hut (IQH) structures, which are embedded in a silicon lattice, has been studied using anomalous x-ray scattering techniques. These self-assembled IQH structures exhibit strong photoluminescence (PL) although the number density of the huts is rather small. We show here that these IQH structures form by the intermixing of germanium with previously deposited silicon producing an intriguing composition variation that keeps the out-of-plane lattice parameter of the alloy almost constant. We have identified a zero strain cubic structure, which extends towards the tip of these IQHs to accommodate large-scale interdiffusion of germanium in silicon lattice. A substantial increase in intensity of the PL peak at around 0.8 eV as the temperature is lowered from 70 to 10 K and a corresponding activation energy of 49 meV indicates that photon induced carriers are predominantly captured at the tip of the embedded quantum hut structures.

We report large amplitude quantum oscillations and negative differential conductance in the bias voltage-dependent photocurrent of p-i-n GaAs diodes with an AlAs barrier in the intrinsic ($i$) region. The oscillations appear only when the devices are illuminated with above-band gap radiation. They are strongly suppressed by a weak (∼2 T) in-plane magnetic field. Their period, amplitude, and magnetic field dependence are explained in terms of the quantized motion of confined photoexcited electrons and holes in the triangular potential wells formed by the AlAs barrier and the strong electric field in the intrinsic region. With increasing electric field, the energy levels of the electrons (holes) successively reach the top of their confining potentials, thus leading to a larger overlap of their wave functions with the free carriers in the $p$- (and $n$-) doped electrodes and to the observed oscillatory modulation of the recombination rate and photocurrent as a function of the applied voltage. The effect on the photocurrent oscillations amplitude of placing a layer of InAs quantum dots in the AlAs barrier layer is also examined.

The large atomic-size mismatch between In and Ga and the large lattice-mismatch strain between InAs and GaAs make the InAs/GaAs (001) growth interface a complex alloy system, the understanding of which can enhance control of device synthesis and nanostructure self-assembly. We present a detailed first-principles analysis of the full progression of surface reconstructions observed on the InAs/GaAs(001) wetting layer during early stages of In deposition. We use systematic techniques to identify the most likely surface reconstruction prototypes of the InAs wetting layer on GaAs(001) using density functional theory (DFT) and then develop several cluster expansion Hamiltonians in order to thoroughly explore surface alloy disorder due to species substitution of In, Ga, and As at the surface. We use these results to construct a first principles 0-K surface phase diagram of the InAs wetting layer on GaAs(001) and test the sensitivity of our predicted phase diagram to DFT approximations and convergence errors. We find two alloy configurations of the $(4\times 3)$ structural prototype that are likely ground-state surface reconstructions, and simulated scanning tunneling micrographs (STM) of these reconstructions indicate that they can explain prominent features of experimentally obtained STM of the InAs/GaAs $(4\times 3)$ surface.

We report the layer-number dependence of the averaged interlayer thermal resistances ($R_{\mathrm{int}}$) of the suspended and supported few-layer graphene (FLG), simulated by equilibrium molecular dynamics (EMD). The existence of a silicon dioxide substrate significantly decreases the $R_{\mathrm{int}}$ of FLG at low layer number. We use the model of long-wavelength dynamics of a nanolayer adsorbed on a deformable crystal [Kosevich and Syrkin, Phys. Lett. A 135, 298 (1989)] to explain the appearance of the substrate-induced gaps in the FLG dispersion curves and phonon radiation into the deformable substrate from these gap modes. The enhanced thermal conductance in the cross-plane direction is ascribed to the phonon radiation from FLG into the deformable substrate, which partially transfers the flow of phonon energy in FLG from the in-plane to the cross-plane direction and to the substrate. To confirm this, we calculate the cross-plane thermal resistance of three-layer graphene supported by an effective SiO${}_2$ substrate in which atomic masses are increased by a factor of 1000. This makes the substrate almost immovable and suppresses phonon radiation from the supported FLG by complete phonon reflection at the interface. The cross-plane thermal resistance of three-layer graphene supported on such a substrate is found to be the same as its suspended counterpart.

When atomically thin two-dimensional (2D) materials are layered, they often form incommensurate noncrystalline structures that exhibit long-period moiré patterns when examined by scanning probes. In this paper, we present an approach that uses information obtained from ab initio calculations performed on short-period crystalline structures to derive effective Hamiltonians that are able to efficiently describe the influence of the moiré pattern superlattices on electronic properties. We apply our approach to the cases of graphene on graphene (G/G) and graphene on hexagonal boron nitride (G/BN), deriving explicit effective Hamiltonians that have the periodicity of the moiré pattern and can be used to calculate electronic properties of interest for arbitrary twist angles and lattice constants.

Lithium titanate (LTO) is a promising candidate as an anode material in future generations of lithium ion batteries due to its high intrinsic safety and stability. In this work, we investigate the diffusion barriers for lithium ions in two different crystal structures of LTO using the density functional theory. Our calculations show that the activation barriers vary between 0.30–0.48 eV for the spinel phase ${\mathrm{Li}}_4{\mathrm{Ti}}_5{\mathrm{O}}_{12}$ and between 0.20–0.51 eV in the lithiated rocksalt phase ${\mathrm{Li}}_7{\mathrm{Ti}}_5{\mathrm{O}}_{12}$. The origins of the rather broad ranges of activation energies are related to different chemical environments of the diffusion channels due to mixed occupancies of some sites in LTO. Our results reveal that the determination of lithium diffusion constants in LTO can not be carried out by using a single activation barrier. Instead, the local environment of the diffusion paths must be considered to correctly capture the variety of activation barriers. Moreover, we find the sites which have mixed occupation in LTO to trap lithium vacancies in the spinel phase. This effect is not observed in the rocksalt phase. This behavior explains the low lithium diffusivity found in experiments for lithium concentrations in the vicinity of the spinel phase.

We examine the effect of small amounts of magnetic substituents in the $A$ sites of the frustrated spinels ${\mathrm{MgCr}}_2{\mathrm{O}}_4$ and ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$. Specifically, we look for the effects of spin and lattice disorder on structural changes accompanying magnetic ordering in these compounds. Substitution of Co${}^{2+}$ on the nonmagnetic Zn${}^{2+}$ site in ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ where 0 $<$ $x$ $\le$ 0.2 completely suppresses the spin-Jahn-Teller distortion of ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ although these systems remain frustrated, and magnetic ordering occurs at very low temperatures of $T$ $<$ 20 K. On the other hand, the substitution of Jahn-Teller active Cu${}^{2+}$ for Mg${}^{2+}$ and Zn${}^{2+}$ in ${\mathrm{Mg}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ and ${\mathrm{Zn}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ where 0 $<$ $x$ $\le$ 0.2 induce Jahn-Teller ordering at temperatures well above the Néel temperatures of these solid solutions, and yet spin interactions remain frustrated with long-range magnetic ordering occurring below 20 K without any further lattice distortion. The Jahn-Teller distorted solid solutions ${\mathrm{Mg}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ and ${\mathrm{Zn}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ adopt the orthorhombic $Fddd$ structure of ferrimagnetic ${\mathrm{CuCr}}_2{\mathrm{O}}_4$. Total neutron scattering studies of ${\mathrm{Zn}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ suggest that there are local $A{\mathrm{O}}_4$ distortions in these Cu${}^{2+}$-containing solid solutions at room temperature and that these distortions become cooperative when average structure distortions occur. Magnetism evolves from compensated antiferromagnetism in ${\mathrm{MgCr}}_2{\mathrm{O}}_4$ and ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ to uncompensated antiferromagnetism with substitution of magnetic cations on the nonmagnetic cation sites of these frustrated compounds. The sharp heat capacity anomalies associated with the first-order spin-Jahn-Teller transitions of ${\mathrm{MgCr}}_2{\mathrm{O}}_4$ and ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ become broad in ${\mathrm{Mg}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$, ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$, and ${\mathrm{Zn}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ when $x$ $>$ 0. We present a temperature-composition phase diagram summarizing the structural ground states and magnetic properties of the studied spinel solid solutions.

We exploit a natural projected entangled-pair state (PEPS) representation for the resonating Affleck-Kennedy-Lieb-Tasaki loop (RAL) state. By taking advantage of PEPS-based analytical and numerical methods, we characterize the RAL states on various two-dimensional lattices. On square and honeycomb lattices, these states are critical since the dimer-dimer correlations decay as a power law. On the kagome lattice, the RAL state has exponentially decaying correlation functions, supporting the scenario of a gapped spin liquid. We provide further evidence that the RAL state on the kagome lattice is a ${\mathbb{Z}}_2$ spin liquid, by identifying the four topological sectors and computing the topological entropy. Furthermore, we construct a one-parameter family of PEPS states interpolating between the RAL state and a short-range resonating valence bond state and find a critical point, consistent with the fact that the two states belong to two different phases. We also perform a variational study of the spin-1 kagome Heisenberg model using this one-parameter PEPS.

We report dual spin pumping in magnetic bilayers made of a ferromagnetic insulator yttrium iron garnet (YIG) and a ferromagnetic metal permalloy (Py). At the YIG ferromagnetic resonance (FMR), we detect a charge voltage in Py caused by YIG spin pumping. At the Py FMR, we measure the charge voltage generated by Py spin rectification. A striking simultaneous enhancement of both voltages is found at the equal resonance condition of both FMRs, which we attribute to dynamic coupling of the dual spin pumping. Our results demonstrate that Py enables electrical detection of both dc and ac spin currents in the spin pumping from YIG, which reveals an alternative path for developing insulator spintronics.

Using a Ginzburg-Landau-Devonshire model that includes the coupling of polarization to strain, we calculate the fluctuation spectra of ferroelectric domain walls. The influence of the strain coupling differs between 180${}^{\circ }$ and 90${}^{\circ }$ walls due to the different strain profiles of the two configurations. The finite speed of acoustic phonons, $v_s$, retards the response of the strain to polarization fluctuations, and the results depend on $v_s$. For $v_s\to \infty$, the strain mediates an instantaneous electrostrictive interaction, which is long-range in the 90${}^{\circ }$ wall case. For finite $v_s$, acoustic phonons damp the wall excitations, producing a continuum in the spectral function. As $v_s\to 0$, a gapped mode emerges, which corresponds to the polarization oscillating in a fixed strain potential.

A theoretical study is presented on the odd-frequency spin-singlet pairing that arises in nonuniform even-frequency superconductors as a consequence of broken translation symmetry. The effect of the odd-frequency pairing on the superfluid density and the spin susceptibility is analyzed by using the quasiclassical theory of superconductivity. It is shown that (1) the superfluid density is reduced by the formation of the odd-frequency singlet pairs and (2) the odd-frequency pairing increases the spin susceptibility even though its spin symmetry is singlet. The two unusual phenomena are related to each other through a generalized Yosida formula by taking into account both the even- and odd-frequency pairing effects.

Using the Keldysh-Usadel formalism, we theoretically study the 0-$\pi$ transition profiles and current-phase relations of magnetic $SFSFS$ and $SFSFFS$ Josephson nanojunctions in the diffusive regime. By allowing the magnetizations of the ferromagnetic layers to take arbitrary orientations, the strength and direction of the charge supercurrent flowing through the ferromagnetic regions can be controlled via the magnetization rotation in one of the ferromagnetic layers. Depending on the junction parameters, we find opposite current flow in the ferromagnetic layers, revealing that, remarkably, such configurations possess well-controlled 0 and $\pi$ states simultaneously, creating a three-terminal 0-$\pi$ spin switch. We demonstrate that the spin-controlled 0-$\pi$ profiles trace back to the proximity induced odd-frequency superconducting correlations generated by the ferromagnetic layers. It is also shown that the spin-switching effect can be more pronounced in $SFSFFS$ structures. The current-phase relations reveal the important role of the middle $S$ electrode, where the spin-controlled supercurrent depends crucially on its thickness and phase differences with the outer $S$ terminals.

A self-consistent scheme for determining the optimal fraction of exact exchange for full-range hybrid functionals is presented and applied to the calculation of band gaps and dielectric constants of solids. The exchange-correlation functional is defined in a similar manner to the PBE0 functional, but the mixing parameter is set equal to the inverse macroscopic dielectric function and it is determined self-consistently by computing the optimal dielectric screening. We found excellent agreement with experiments for the properties of a broad class of systems, with band gaps ranging between 0.7 and 21.7 eV and dielectric constants within 1.23 and 15.9. We propose that the eigenvalues and eigenfunctions obtained with the present self-consistent hybrid scheme may be excellent inputs for $G_0{W}_0$ calculations.

Magnetophotoluminescence and magnetotransport were studied in a GaAs/AlGaAs triple quantum well. Oscillations of the photoluminescence intensity observed in tilted magnetic fields were found to correspond to the interlayer tunneling quantum Hall gap collapses detected in magnetoresistance measurements and predicted to occur in tilted magnetic fields. The obtained experimental data were shown to agree well with the theory developed for double quantum wells. This implies that the observed quantum Hall gap collapses are mostly caused by the tunneling between a pair of quantum wells. Our results reveal spectroscopic evidence of the quantum Hall gap collapses. Indications of interlayer correlation effects influencing a character of the inter-Landau-level gaps were found.

We obtain the exact Kohn-Sham potentials $V_{\mathrm{KS}}$ of time-dependent density-functional theory for one-dimensional Hubbard chains, driven by a dc external field, using the time-dependent electron density and current density obtained from exact many-body time evolution. The exact $V_{\mathrm{xc}}$ is compared to the adiabatically exact $V_{\mathrm{xc}}^{\mathrm{ad}}$ and the “instantaneous ground state” $V_{\mathrm{xc}}^{\mathrm{igs}}$. The latter is shown to work effectively in some cases when the former fails. Approximations for the exchange-correlation potential $V_{\mathrm{xc}}$ and its gradient, based on the local density and on the local current density, are also considered and both physical quantities are observed to be far outside the reach of any possible local approximation. Insight into the respective roles of ground-state and excited-state correlation in the time-dependent system, as reflected in the potentials, is provided by the pair correlation function.

We predict that C, N, and O dopants in TlBr can display large, localized magnetic moments. Density functional theory based electronic structure calculations show that the moments arise from partial filling of the crystal-field-split localized $p$ states of the dopant atoms. A simple model is introduced to explain the magnitude of the moments.

We study decoherence of an electron spin qubit in a quantum dot due to charge noise. We find that at the lowest order in spin-orbit interaction, the pure dephasing channel is suppressed for both $1/f$ charge noise and Johnson noise, so that charge noise leads to a pure relaxation channel of decoherence. Because of the weaker magnetic field dependence, the spin relaxation rate due to charge noise could dominate over phonon noise at low magnetic fields in a gate-defined GaAs or Si quantum dot or an InAs self-assembled quantum dot. Furthermore, in a large InAs self-assembled quantum dot, spin relaxation due to phonon noise could be suppressed in high magnetic fields due to the suppression of the coupling matrix element, so that the spin relaxation due to charge noise could dominate in both low and high magnetic fields. Numerically, in a 1 T magnetic field, the spin relaxation time due to typical charge noise is about 100 s in Si, $0.1$ s in GaAs for a gate-defined quantum dot with a 1 meV confinement, and 10 $\mu$s (or 1 s) in InAs self-assembled quantum dot with a 4 meV (or 30 meV) confinement.