MBE-grown Sn- and Mg-doped epitaxial ${\mathrm{In}}_2{\mathrm{O}}_3$ thin-film samples with varying doping concentrations have been measured using positron Doppler spectroscopy and compared to a bulk crystal reference. Samples were subjected to oxygen or vacuum annealing and the effect on vacancy type defects was studied. Results indicate that after oxygen annealing the samples are dominated by cation vacancies, the concentration of which changes with the amount of doping. In highly Sn-doped ${\mathrm{In}}_2{\mathrm{O}}_3$, however, these vacancies are not the main compensating acceptor. Vacuum annealing increases the size of vacancies in all samples, possibly by clustering them with oxygen vacancies.

Based on the first-principles calculations, we reveal that TlN, a simple binary compound with wurtzite structure, is a three-dimensional (3D) topological insulator (TI) with effectively negative spin-orbit coupling ${\lambda }_{\text{eff}}<0$, which makes it distinguished from other TIs by showing opposite spin-momentum locking effect in its surface states. The sign of ${\lambda }_{\text{eff}}$ depends on the hybridization between N-$2p$ and Tl-$5d$ states, and can be tuned from negative to positive by lattice strain or chemical substitution, which drives the system into a Dirac semimetal with 3D Dirac cones in its bulk states. Such topological phase transition can be realized by electronic mechanism without breaking any crystal symmetry.

We introduce and model spin-Rabi oscillations based on exciton-polaritons in semiconductor microcavities. The phase and polarization of oscillations can be controlled by resonant coherent pulses and the propagation of oscillating domains gives rise to phase-dependent interference patterns in real space. We show that interbranch polariton-polariton scattering controls the propagation of oscillating domains, which can be used to realize logic gates based on an analog variable phase.

We demonstrate that the quasi-one-dimensional (1D) current transport, experimentally observed in graphene as measured by a collinear four-point probe in two electrode configurations A and B, can be interpreted using the sensitivity functions of the two electrode configurations (configurations A and B represents different pairs of electrodes chosen for current sources and potential measurements). The measured sheet resistance in a four-point probe measurement is averaged over an area determined by the sensitivity function. For a two-dimensional conductor, the sensitivity functions for electrode configurations A and B are different. But when the current is forced to flow through a percolation network, e.g., graphene with high density of extended defects, the two sensitivity functions become identical. This is equivalent to a four-point measurement on a line resistor, hence quasi-1D transport. The sensitivity analysis presents a formal definition of quasi-1D current transport, which was recently observed experimentally in chemical-vapor-deposition graphene. Our numerical model for calculating sensitivity is verified by comparing the model to analytical calculations based on conformal mapping of a single extended defect.

Using first-principles calculations to obtain the crystal and electronic structures, we show that the $1\times 1$ phase of silicene is energetically more favorable than the $\sqrt{3}\times \sqrt{3}$ silicene superstructure on a semiconducting Bi/Si(111)-$\sqrt{3}\times \sqrt{3}$ substrate. The band gap of the system is found to be influenced strongly through the participation of Bi orbitals, which possess a larger spin-orbit coupling strength compared to Si. In particular, the nontrivial (topological) band gap of a few meV in freestanding $1\times 1$ silicene enlarges to 124 meV and becomes trivial in the presence of the substrate. We further show how an out-of-the-plane external electric field can be used to tune the band gap and restore the nontrivial topological phase.

We consider strongly interacting one-dimensional electron liquids where elementary excitations carry either spin or charge. At small temperatures a spinon created at the bottom of its band scatters off low-energy spin and charge excitations and follows the diffusive motion of a Brownian particle in momentum space. We calculate the mobility characterizing these processes and show that the resulting diffusion coefficient of the spinon is parametrically enhanced at low temperatures compared to that of a mobile impurity in a spinless Luttinger liquid. We briefly discuss that this hints at the relevance of spin in the process of equilibration of strongly interacting one-dimensional electrons, and comment on implications for transport in clean single-channel quantum wires.

Using chiral decomposition, we are able to find analytically the zero modes and the conditions for such modes to exist in the Kitaev ladder model and superconducting nanowires with Dresselhaus spin-orbit coupling. As a result, we are able to calculate the number of zero modes in these systems for arbitrary given parameters in the semi-infinite limit. Moreover, we find that when suitable resonance condition is satisfied exact zero modes exist even in finite systems. Because of this, the energy of the lowest positive energy shows oscillation with respect to the chemical potential.

The dynamics of a magnetic vortex in a circular disk with an off-centered point contact is theoretically studied based on Thiele's equation and two-vortices model. It is shown that the steady-state rotational motion of a vortex core is induced by an electric current, and the threshold current of this rotational motion increases as the distance between the point contact and the center of the disk increases. The increase rate of the threshold current depends on the curling of the vortex because the Oersted field acts as an attractive or repulsive force between the vortex core and the point contact, depending on the curling of the vortex.

We address the unsolved question on the structure of relaxor ferroelectrics at the atomic level by characterizing lead-free piezoceramic solid solutions $(100-x)\left({\mathrm{Bi}}_{1/2}{\mathrm{Na}}_{1/2}\right){\mathrm{TiO}}_3\text{−}x{\mathrm{BaTiO}}_3 \left(\mathrm{BNT}\text{−}x\mathrm{BT}\right)$ (for $x=1,4,6$, and 15). Based on the relative intensity between spectral components in quadrupolar perturbed ${}^{23}\mathrm{Na}$ nuclear magnetic resonance, we present direct evidence of the coexistence of cubic and polar local symmetries in these relaxor ferroelectrics. In addition, we demonstrate how the cubic phase vanishes whenever a ferroelectric state is induced, either by field cooling or changing the dopant amount, supporting the relation between this cubic phase and the relaxor state.

We report neutron scattering measurements of the structural correlations associated with the apparent relaxor transition in ${\mathrm{K}}_{1-x}{\mathrm{Li}}_x{\mathrm{TaO}}_3$ for $x=0.02$ [KLT(0.02)]. This compound displays a broad and frequency-dependent peak in the dielectric permittivity, which is the accepted hallmark of all relaxors. However, no evidence of elastic diffuse scattering or any soft-mode anomaly is observed in KLT(0.02) [J. Wen et al., Phys. Rev. B 78, 144202 (2008)], a situation that diverges from that in other relaxors such as ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$. We resolve this dichotomy by showing that the structural correlations associated with the transition in KLT(0.02) are purely dynamic at all temperatures, having a time scale on the order of $\sim$ THz. These fluctuations are overdamped, nonpropagating, and spatially uncorrelated. Identical measurements made on pure ${\mathrm{KTaO}}_3$ show that they are absent (within experimental error) in the undoped parent material. They exhibit a temperature dependence that correlates well with the dielectric response, which suggests that they are associated with local ferroelectric regions induced by the ${\mathrm{Li}}^{+}$ doping. The ferroelectric transition that is induced by the introduction of ${\mathrm{Li}}^{+}$ cations is therefore characterized by quasistatic fluctuations, which represents a stark contrast to the soft-harmonic-mode-driven transition observed in conventional perovskite ferroelectrics such as ${\mathrm{PbTiO}}_3$. The dynamic, glasslike structural correlations in KLT(0.02) are much faster than those measured in random-field-based lead-based relaxors, which exhibit a frequency scale of order $\sim$ GHz and are comparatively better correlated spatially. Our results support the view that static random fields give rise to the relaxor phenomena, and that the glasslike dynamics observed here characterize a nascent response.

Spin-transfer torque (STT) in tunnel junctions with ferromagnetic leads is one of the essential underlying phenomena of modern spintronics. Here, we present a theoretical study of STT in ferrimagnet- (FI-) based tunnel junctions where two FI metal electrodes are separated by a thin nonmagnetic insulating barrier. We show that electronic structure parameters, such as bandwidths and exchange splittings of the FI leads strongly influence STT. In particular, the STT spatial distribution within the leads shows a striking spin-modulated wavelike behavior resulting from the interplay between the exchange splittings of the two FI sublattices. Additionally, we identify the fundamental parameter for quantifying STT characteristic lengths in FI metals, which will also be accessible to experiments, for instance, by ferromagnetic resonance and spin pumping measurements.

The energy-dependent scattering of fermions from a localized orbital at an energy-dependent rate $\Gamma \left(\varepsilon \right)\propto {\left|\varepsilon \right|}^r$ gives rise to quantum critical points (QCPs) in the pseudogap single-impurity Anderson model separating a local moment phase with an unscreened spin moment from a strong-coupling phase which slightly deviates from the screened phase of standard Kondo problem. Using the time-dependent numerical renormalization group (TD-NRG) approach we show that local dynamic properties always equilibrate towards a steady-state value even for quenches across the QCP but with systematic deviations from the thermal equilibrium depending on the distance to the critical coupling. Local nonequilibrium properties are presented for interaction quenches and hybridization quenches. We augment our numerical data by an analytical calculation that becomes exact at short times and find excellent agreement between the numerics and the analytical theory. For interaction quenches within the screened phase we find a universal function for the time-dependent local double occupancy. We trace back the discrepancy between our results and the data obtained by a time-dependent Gutzwiller variational approach to restrictions of the wave-function ansatz in the Gutzwiller theory: while the NRG ground states properly account for the formation of an extended spin moment which decouples from the system in the unscreened phase, the Gutzwiller ansatz only allows the formation of the spin moment on the local impurity orbital.

Gutzwiller projection is a way to construct many-body wave functions that could carry topological order or symmetry-protected topological (SPT) order. However, an important issue is to determine whether or not a given Gutzwiller-projected wave function (GWF) carries a nontrivial SPT order, and which SPT order is carried by the wave function. In this paper, we numerically study the SPT order in a spin $S=1$ GWF on the kagome lattice. Using the standard Monte Carlo method, we directly confirm that the GWF has (1) gapped bulk with short-range correlations, (2) a trivial topological order via a nondegenerate ground state, and zero topological entanglement entropy, (3) a nontrivial $U\left(1\right)\times U\left(1\right)$ SPT order via the Hall conductances of the protecting $U\left(1\right)\times U\left(1\right)$ symmetry, and (4) a symmetry-protected gapless boundary. This represents numerical evidence of continuous symmetry-protected topological order in two-dimensional bosonic lattice systems.

We have studied the quantum oscillations in the conductivity of metallic, $p$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_{2.1}{\mathrm{Te}}_{0.9}$. The dependence of the oscillations on the angle of the magnetic field with the surface as well as the Berry phase determined from the Landau level fan plot indicate that the observed oscillations arise from surface carriers with a characteristic Dirac dispersion. Several quantities characterizing the surface conduction are calculated employing the Lifshitz-Kosevich theory. The low value of the Fermi energy with respect to the Dirac point is consistent with the metallic character of the bulk hole carriers. We conclude that, due to the peculiar shape of the valence band, the Shubnikov–de Haas oscillations of the bulk carriers are shifted to higher magnetic fields, which allows for the detection of the quantum oscillations from the topological surface states at a lower field.

The quantum dimer model on the square lattice is a $\text{U}\left(1\right)$ gauge theory that addresses aspects of the physics of high-$T_c$ superconductors. Using a quantum Monte Carlo method, we show that the theory exists in a confining columnar valence bond solid phase. The interfaces separating distinct columnar phases display plaquette order, which, however, is not realized as a bulk phase. Static “electric” charges are confined by flux tubes that consist of multiple strands, each carrying a fractionalized flux $\frac{1}{4}$. A soft pseudo-Goldstone mode (which becomes exactly massless at the Rokhsar-Kivelson point) extends deep into the columnar phase, with potential implications for high-$T_c$ physics.

We use first-principles calculations to understand the behavior of the Seebeck coefficient $\left(S\right)$ in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ as a function of isotropic pressure. We perform calculations up to 5 GPa using density functional theory and with thermoelectric properties extracted using Boltzmann transport equations. We find that with the increase in pressure the system becomes more metallic, in agreement with previous calculations on ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. For $p$-type doping the overall behavior is a decrease in $S$ with an increase in pressure. At small values of hole doping $\left(p=1.8\times {10}^{18}{\mathrm{cm}}^{-3}\right)$, we obtain an anomalous variation of $S$ under 2 GPa, which is an indication of the electronic topological transition. For $n$-type doping, $S$ slightly increases with pressure.

We present an atomistic-continuum model to simulate ultrashort-pulse laser melting processes in semiconductor solids on the example of silicon. The kinetics of transient nonequilibrium phase transition mechanisms is addressed with a molecular dynamics method at atomic level, whereas the laser light absorption, strong generated electron-phonon nonequilibrium, fast diffusion of and heat conduction due to photoexcited free carriers are accounted for in the continuum. We give a detailed description of the model, which is then applied to study the mechanism of short laser pulse melting of freestanding Si films. The effect of laser-induced pressure and temperature of the lattice on the melting kinetics is investigated. Two competing melting mechanisms, heterogeneous and homogeneous, were identified. Apart from the classical heterogeneous melting mechanism, the nucleation of the liquid phase homogeneously inside the material significantly contributes to the melting process. The simulations showed, that due to the open diamond structure of the crystal, the laser-generated internal compressive stresses reduce the crystal stability against the homogeneous melting. Consequently, the latter can take a massive character within several picoseconds upon the laser heating. Due to the negative volume of melting of modeled Si material, −7.5%, the material contracts upon the phase transition, relaxes the compressive stresses, and the subsequent melting proceeds heterogeneously until the excess of thermal energy is consumed. The threshold fluence value, at which homogeneous nucleation of liquid starts contributing to the classical heterogeneous propagation of the solid-liquid interface, is found from the series of simulations at different laser input fluences. For the example of Si, the laser melting kinetics of semiconductors was found to be noticeably different from that of metals with a fcc crystal structure.