We report the pairwise chemical interaction and the direct $d-d$ interactions of substitutional $3d$ transition-metal impurities in various insulator hosts for various charged conditions by first-principles calculations. Our calculations show that for nearest-neighbor impurity pairs, the direct $d-d$ interactions play an important role in the binding of the impurities and the degree of these interactions increases as the defect charge state is varied from positive to negative. This allows for direct control of the transition-metal impurity clustering and possible secondary phase formation by $n$- or $p$-type doping.

We report on a magnetotransport study in a high-mobility 2D hole gas hosted in a pure Ge/SiGe quantum well subject to dc electric fields and high-frequency microwave radiation. We find that under applied dc bias the differential resistivity exhibits a pronounced maximum at a magnetic field which increases linearly with the applied current. We associate this maximum with the fundamental peak of Hall field-induced resistance oscillations (HIRO) which are known to occur in 2D electron gases in GaAs/AlGaAs systems. After taking into account the Dingle factor correction, we find that the position of the HIRO peak is well described by the hole effective mass $m^{☆}\approx 0.09m_0$, obtained from microwave photoresistance in the same sample.

We study the toplogical checkerboard lattice model around the $\nu =\frac{1}{3}$ fractional quantum Hall phase using numerical exact diagonalization without Landau level projections. We add local perturbations, modified hoppings, and on-site potentials, and observe phase transitions from the fractional quantum Hall phase to metallic and insulating phases when the strength and number of impurities is increased. In addition to evaluating the energy spectrum, we identify the phase diagrams by computing the topological Chern number of the many-body ground state manifold, and we show how the ground states lose their correlations due to the impurities by evaluating the spectrum of the one-body reduced density matrix. Our results show that the phase transition from the fractional quantum Hall phase to the metallic phase occurs for both impurity hoppings and potentials. Strong impurity hoppings cause a further transition into the insulating state, regardless of the sign of the hopping, when their density is high enough. In contrast, the same happens only for attractive potentials. Furthermore, the mixing to the higher band in a two-band model, generally denoted as Landau level mixing, is measured concluding that the lowest Landau level projection works well even with remarkably strong interactions and in the presence of impurities.

The dielectric function of orthorhombic ${{\mathrm{Cu}}_2\mathrm{CdGeS}}_4$ and ${{\mathrm{Cu}}_2\mathrm{CdSiS}}_4$ single crystals (space group $Pmn{2}_1$) is measured experimentally in the spectral range of 0.7–5.2 eV, and the results are interpreted via comparison with ab initio calculations. Polarized Raman-scattering measurements of both semiconductors are reported. All observed vibrational modes are assigned to specific lattice eigenmodes of the wurtz-stannite structure. Good agreement between theoretical and experimental mode frequencies is established. The differences in the experimental Raman spectra of the two compounds are well correlated with the effect of exchanging Si and Ge predicted by the theory. The strongest lines in the Raman-scattering spectra of both semiconductors are identified as being due to the “breathing” motion of ${\mathrm{GeS}}_4$ and ${\mathrm{SiS}}_4$ octahedra.

We analyze the backscattering current induced by a time-dependent constriction as a tool to probe fractional topological insulators. We demonstrate an enhancement of the total current for a fractional topological insulator induced by the dominant tunneling excitation, contrary to the decrease present in the integer case for not too strong interactions. This feature allows us to unambiguously identify fractional quasiparticles. Furthermore, the dominant tunneling processes, which may involve one or two quasiparticles depending on the interactions, can be clearly determined.

Motivated by the absence of both spin freezing and a cooperative Jahn-Teller effect at the lowest measured temperatures, we study the ground state of ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$. We solve a general spin-orbital model on both the honeycomb and the decorated honeycomb lattice, revealing rich phase diagrams. The spin-orbital model on the honeycomb lattice contains an SU(4) point, where previous studies have shown the existence of a spin-orbital liquid with algebraically decaying correlations. For realistic parameters on the decorated honeycomb lattice, we find a phase that consists of clusters of nearest-neighbor spin singlets, which can be understood in terms of dimer coverings of an emergent square lattice. While the experimental situation is complicated by structural disorder, we show qualitative agreement between our theory and a range of experiments.

We investigate the influence of spin currents during ultrafast laser-induced demagnetization of magnetic bilayer structures by a new all-optical method to measure material- and/or depth-resolved magnetization dynamics. By describing the magneto-optical response of the bilayers in the complex Kerr plane, it is shown that the material-specific magnetization dynamics of the individual layers can be measured by a marginal adjustment to any conventional time-resolved magneto-optical Kerr effect setup. We use this technique to trace superdiffusive spin currents in magnetic Ni/Fe bilayers, providing new insight on its importance to ultrafast laser-induced demagnetization.

It is shown that under certain conditions spin transfer results in conservative magnetization dynamics. This dynamics occurs along closed precessional-type trajectories, and it admits a special integral of motion which is reduced to the usual magnetic energy when the spin current is reduced to zero. The existence of this conservative dynamics is due to the symmetry properties of the magnetization dynamics equation with respect to simultaneous inversions of magnetization and time. When an external dc magnetic field is applied parallel to the spin polarization, the conservative magnetization dynamics is transformed into relaxations. It is demonstrated that there exists such a state function (Lyapunov function) that monotonically either increases or decreases during these relaxations, depending on the directions of the injected current and applied dc magnetic field. These results hold in the absence of intrinsic (thermal) damping. When the intrinsic damping is included in the description, mutual compensation between field-induced and damping-induced nonconservative effects may occur, which may eventually lead to the appearance of limit cycles, that is, of magnetization self-oscillations.

We explore the local quantum coherence and the local quantum uncertainty, based on Wigner-Yanase skew information, in the ground state of the anisotropic spin-1/2 XY chain in a transverse magnetic field. We show that the skew information, as a figure of merit, supplies the necessary information to reveal the occurrence of the second-order phase transition and the completely factorized ground state in the XY model. Additionally, in the same context, we also discuss the usefulness of a simple experimentally friendly lower bound of local quantum coherence. Furthermore, we demonstrate how the connection between the appearance of nonanalyticities in the local quantum uncertainty of the ground state and the quantum phase transitions does not hold in general, by providing explicit examples of the situation. Lastly, we discuss the ability of the local quantum coherence to accurately estimate the critical point of the phase transition, and we investigate the robustness of the factorization phenomenon at low temperatures.

The magnetic moment $M$ of superconducting samples with a nonzero demagnetizing factor is one of the most important but not always completely understood sample characteristics. Here we calculate the magnetic moment of a slab of type-I superconductor in an arbitrarily oriented magnetic field, applying a recently developed model of the intermediate state (IS) [V. Kozhevnikov et al., Phys. Rev. B 89, 100503(R) (2014)]. We introduce and employ a novel form of the thermodynamic potential, which allows us to derive a general formula for the total free energy of the slab. The magnetic moment, calculated from the total free energy, is consistent with experimental data reported in this paper as well as with data available in the literature. Also, the calculated moment solves a long-standing puzzle of experimental $M\left(H\right)$ curves $(H$ is the applied field) and provides insight in the field driven evolution of the magnetic flux density in the domain structure of the IS. This paper completes our presentation of a theoretical model which consistently addresses all properties of the IS, thus solving the problem of the IS in a slab that was put forward by Landau almost eight decades ago.

We study the magnetic topological transition of the isofrequency curves in the wave vector space from a closed ellipsoid to an open hyperboloid in a metamaterial based on artificial transmission lines. In the radio frequency band we directly measure the emission pattern of a point source placed in the center of the lattice and demonstrate the elliptical wave fronts below the topological transition frequency and hyperbolic wave fronts above the topological transition frequency.

We study fluctuation properties of a one-dimensional anisotropic Heisenberg model out of equilibrium, focusing in particular on the gapped regime. Within the open-system setting we study large-deviation properties of the spin current. Numerically evaluating the first four current cumulants in a nonequilibrium stationary state at high energies, we find that the first two cumulants scale with the system size in a diffusive way, while the third and the fourth cumulants do not. This means that the model is not an ordinary diffusive spin conductor.

We study the behavior of the recently proposed “strange correlator” [Y.-Z. You, Z. Bi, A. Rasmussen, K. Slagle, and C. Xu, Phys. Rev. Lett. 112, 247202 (2014)] in spin-1 Heisenberg antiferromagnetic chains with uniaxial single-ion anisotropy. Using projective quantum Monte Carlo, we are able to directly access the strange correlator in a variety of phases, as well as to examine its critical behavior at a quantum phase transition between trivial and nontrivial symmetry protected topological phases. After finding the expected long-range behavior in these two symmetry conserving phases, we go on to verify the topological nature of two-leg and three-leg spin-1 Heisenberg antiferromagnetic ladders. This demonstrates the power of the strange correlator in distinguishing between trivial and nontrivial symmetry protected topological phases.

From our general index theorem, which characterizes faithfully the topological intrinsic boundary-bulk correspondence of topological superconductors and insulators, we reveal rigorously that four topologically distinct types of Majorana zero modes can emerge at the ends of superconducting wires of various symmetry classes. More intriguingly, we establish three exotic one-dimensional models that have different types of topological charge of Majorana zero modes and disclose exactly the corresponding topological properties, whose distinct topological essences may be tested experimentally. Moreover, we also address their application in universal quantum manipulation, which is promising for realizing universal topological quantum computation.

Using ab initio total energy calculations we have studied the relation between the step atomic configuration and its properties (step energy, donor/acceptor behavior, and step interaction) on a ${\beta }_2(2\times 4)$ reconstructed GaAs (001) surface. The results have been tested against the widely used elastic dipole model for the step energy and step interaction considered valid for stress-free surfaces. We have found that acceptor-behaving steps have an attractive interaction and donor-behaving steps have a repulsive interaction in contrast with the elastic dipole model which predicts always a repulsive interaction between like-oriented steps. To account for the attractive interaction we consider the electrostatic dipole interaction having the $L^{-2}$ scaling with the step distance $L$ and therefore compatible with the standard elastic model. Using a model charge distribution with localized point charges at the step based on the electron counting model we show that the electrostatic step interaction can indeed be generally attractive and of the same order of magnitude of the negative elastic dipole interaction. Our results show however that the usually employed dipole model is unable to account for the repulsive/attractive step interaction between donorlike/acceptorlike steps. Therefore, the ab initio results suggest an important electronic contribution to the step interaction, at least at the short step distances accessible to the first-principles study. Our results explain qualitatively many experimental observations and provide an explanation to the step bunching phenomenon on GaAs(001) induced by doping or by critical growth conditions as due to the stabilization of attractively interacting step structures. These ideas would lead to the development of a bottom-up surface step engineering.

We calculate the electronic structure and magnetic properties of hydrogenated graphite surfaces using van der Waals density functional theory (DFT) and model Hamiltonians. We find, as previously reported, that the interaction between hydrogen atoms on graphene favors adsorption on different sublattices along with an antiferromagnetic coupling of the induced magnetic moments. On the contrary, when hydrogenation takes place on the surface of graphene multilayers or graphite (Bernal stacking), the interaction between hydrogen atoms competes with the different adsorption energies of the two sublattices. This competition may result in all hydrogen atoms adsorbed on the same sublattice and, thereby, in a ferromagnetic state for low concentrations. Based on the exchange couplings obtained from the DFT calculations, we have also evaluated the Curie temperature by mapping this system onto an Ising-like model with randomly located spins. Remarkably, the long-range nature of the magnetic coupling in these systems makes the Curie temperature size dependent and larger than room temperature for typical concentrations and sizes.

Neutron spectroscopy is used to investigate the magnetic fluctuations in ${\mathrm{Fe}}_{1+x}\mathrm{Te}$—a parent compound of chalcogenide superconductors. Incommensurate “stripelike” excitations soften with increased interstitial iron concentration. The energy crossover from incommensurate to stripy fluctuations defines an apparent hourglass dispersion. Application of sum rules of neutron scattering find that the integrated intensity is inconsistent with an $S=1{\mathrm{Fe}}^{2+}$ ground state and significantly less than $S=2$ predicted from weak crystal field arguments pointing towards the ${\mathrm{Fe}}^{2+}$ being in a superposition of orbital states. The results suggest that a highly anisotropic order competes with superconductivity in chalcogenide systems.

The temperature dependence of the in-plane optical conductivity has been determined for ${\mathrm{Fe}}_{1.03}\mathrm{Te}$ above and below the magnetic and structural transition at $T_N\simeq 68$ K. The electron and hole pockets are treated as two separate electronic subsystems: a strong, broad Drude response that is largely temperature independent, and a much weaker, narrow Drude response with a strong temperature dependence. Spectral weight is transferred from high to low frequency below $T_N$, resulting in the dramatic increase of both the low-frequency conductivity and the related plasma frequency. The change in the plasma frequency is due to an increase in the carrier concentration resulting from the closing of the pseudogap on the electron pocket, as well as the likely decrease of the effective mass in the antiferromagnetic state.