The temperature dependence of magnetic anisotropy in $\left(113\right)A$ (Ga,Mn)As layers grown by molecular-beam epitaxy is studied by means of superconducting quantum interference device magnetometry as well as by ferromagnetic resonance (FMR) and magnetooptical effects. Experimental results are described considering cubic and two kinds of uniaxial magnetic anisotropy. The magnitude of cubic and uniaxial anisotropy constants is found to be proportional to the fourth and second power of saturation magnetization, respectively. Similarly to the case of (001) samples, the spin reorientation transition from uniaxial anisotropy with the easy axis along the $\left[\overline{1}10\right]$ direction at high temperatures to the biaxial $⟨100⟩$ anisotropy at low temperatures is observed around 25 K. The determined values of the anisotropy constants have been confirmed by FMR studies. As evidenced by investigations of the polar magnetooptical Kerr effect, the particular combination of magnetic anisotropies allows the out-of-plane component of magnetization to be reversed by an in-plane magnetic field. Theoretical calculations within the $p\text{−}d$ Zener model explain the magnitude of the out-of-plane uniaxial anisotropy constant caused by epitaxial strain but do not explain satisfactorily the cubic anisotropy constant. At the same time the findings point to the presence of an additional uniaxial anisotropy of unknown origin. Similarly to the case of (001) films, this additional anisotropy can be explained by assuming the existence of a shear strain. However, in contrast to the (001) samples, this additional strain has an out of the (001) plane character.

We measured magnetotransport properties of PbS single crystals which exhibit the quantum linear magnetoresistance (MR) as well as the static skin effect that creates a surface layer of additional conductivity. The Shubnikov-de Haas oscillations in the longitudinal MR signify the peculiar role of spin-orbit coupling. In the angular-dependent MR, sharp peaks are observed when the magnetic field is slightly inclined from the longitudinal configuration, which is totally unexpected for a system with nearly spherical Fermi surface and points to an intricate interplay between the spin-orbit coupling and the conducting surface layer in the quantum transport regime.

Gate-voltage-dependent Hall coefficient $R_H$ is measured in high-mobility field-effect transistors of polycrystalline dinaphtho$\left[2,3\text{-b}:2^{\prime },3^{\prime }\text{-f}\right]$thieno[3,2-b]thiophene films. The value of $R_H$ evolves with density of accumulated charge $q$, precisely satisfying the free-electron formula $R_H=1/q$ near room temperature. The result indicates that the intrinsic charge transport inside the grains is bandlike in the vacuum-deposited high-mobility organic-semiconductor thin films that are of significant interest in industry. At lower temperatures, even Hall-effect mobility averaged over the whole polycrystalline film decreases due to the presence of carrier-trapping levels at the grain boundaries while the free-electron-like transport is preserved in the grains. With the separated description of the intergrain and the intragrain charge transport, it is demonstrated that the reduction in mobility with decreasing temperature often shown in organic thin-film transistors does not necessarily mean mere hopping transport.

We develop a theory for density, disorder, and temperature-dependent electrical conductivity of bilayer graphene in the presence of long-range charged impurity scattering and short-range defect scattering, establishing that both contribute significantly to determining bilayer transport properties. We find that although strong screening properties of bilayer graphene lead to qualitative differences with the corresponding single-layer situation, both systems exhibit the approximately linearly density-dependent conductivity at high density and the minimum conductivity behavior around the charge neutrality point due to the formation of inhomogeneous electron-hole puddles. The importance of short-range disorder in determining bilayer conductivity is a qualitative finding of our work.

We have measured the frequency $f_G$ of the gyrotropic mode of a magnetic vortex formed in individual soft ferromagnetic disks with diameters from 600 nm to $2\mu \text{m}$. For low excitation amplitudes, we observe fluctuations in $f_G$ as a function of applied magnetic field. The relationship between the applied field and the spatial displacement of the vortex core from the center of the disk indicates that the fluctuations are due to a distribution of nanoscale defects that pin the vortex core, which has a diameter of $\sim 10\text{nm}$. In the limit of high excitation amplitude, the gyrotropic frequency is independent of field, indicating that the core is depinned. In this limit $f_G=f_{ideal}$ where $f_{ideal}$ is the frequency predicted by analytical models and micromagnetic simulations of ideal vortex behavior. We also find both experimentally and in simulations that the average frequency shift $⟨\Delta f⟩=⟨f_{\text{max}}-f_{ideal}⟩$ is independent of disk diameter. From this observation we argue that $\Delta f$ for a particular fluctuation is proportional to the interaction energy $W_P$ of the vortex core with a single nanoscale defect and estimate the average energy to be $W_P/e\approx 2\text{eV}$ for the defects in these sputtered permalloy films.

Numerical approaches to Anderson localization face the problem of having to treat large localization lengths while being restricted to finite system sizes. We show that by finite-size scaling of the probability distribution of the local density of states (LDOS) this long-standing problem can be overcome. To this end we reexamine this method, propose numerical refinements, and apply it to study the dependence of the distribution of the LDOS on the dimensionality and coordination number of the lattice. Particular attention is given to the graphene lattice. We show that the system-size dependence of the LDOS distribution is indeed an unambiguous sign of Anderson localization, irrespective of the dimension and lattice structure. The numerically exact LDOS data obtained by us agree with a log-normal distribution over up to ten orders of magnitude and thereby fulfill a nontrivial symmetry relation previously derived for the nonlinear $\sigma$ model.

Using scanning tunneling spectroscopy (STS) at 5 K in $B$ fields up to 7 T, we investigate the local density of states of a two-dimensional electron system (2DES) created by Cs adsorption on $p$-type InSb(110). The 2DES, which in contrast to previous STS studies exhibits a 2D Fermi level, shows standing waves at $B=0\text{T}$ with corrugations decreasing with energy and with wave numbers in accordance with theory. In magnetic field percolating drift states are observed within the disorder broadened Landau levels. Due to the large electric field perpendicular to the surface, a beating pattern of the Landau levels is found and explained quantitatively by Rashba spin splitting within the lowest 2DES subband. The Rashba splitting does not contribute significantly to the standing wave patterns in accordance with theory.

We analyze the statistics of the electromagnetic radiation emitted from electrons pushed through a quantum point contact. We consider a setup implemented in a two-dimensional (2D) electron gas where the radiation manifests itself in terms of 2D-plasmons/photons emitted from electrons scattered at the point contact. The bosonic statistics of the photons compete with the fermionic statistics of the electrons; as a result, the quantum point contact emits nonclassical radiation with a statistics which can be tuned from bunching to anti-bunching by changing the driving voltage. Our perturbative calculation of the irreducible two-photon probability correlator provides us with information on the statistical nature of the emitted photons and on the underlying electronic current flow.

We describe in detail a counterexample to the topological classification of free fermion systems. We deal with a one-dimensional chain of Majorana fermions with an unusual $T$ symmetry. The topological invariant for the free fermion classification lies in $\mathbb{Z}$, but with the introduction of interactions the $\mathbb{Z}$ is broken to ${\mathbb{Z}}_8$. We illustrate this in the microscopic model of the Majorana chain by constructing an explicit path between two distinct phases whose topological invariants are equal modulo 8, along which the system remains gapped. The path goes through a strongly interacting region. We also find the field-theory interpretation of this phenomenon. There is a second-order phase transition between the two phases in the free theory, which can be avoided by going through the strongly interacting region. We show that this transition is in the two-dimensional Ising universality class, where a first-order phase transition line, terminating at a second-order transition, can be avoided by going through the analog of a high-temperature paramagnetic phase. In fact, we construct the full phase diagram of the system as a function of the thermal operator (i.e., the mass term that tunes between the two phases in the free theory) and two quartic operators, obtaining a first-order Peierls transition region, a second-order transition region, and a region with no transition.

A rf–superconducting quantum interference device (SQUID) flux qubit that is robust against fabrication variations in Josephson-junction critical currents and device inductance has been implemented. Measurements of the persistent current and of the tunneling energy between the two lowest-lying states, both in the coherent and incoherent regimes, are presented. These experimental results are shown to be in agreement with predictions of a quantum-mechanical Hamiltonian whose parameters were independently calibrated, thus justifying the identification of this device as a flux qubit. In addition, measurements of the flux and critical current noise spectral densities are presented that indicate that these devices with Nb wiring are comparable to the best Al wiring rf SQUIDs reported in the literature thus far, with a $1/f$ flux noise spectral density at 1 Hz of ${1.3}_{-0.5}^{+0.7}\mu {\Phi }_0/\sqrt{\text{Hz}}$. An explicit formula for converting the observed flux noise spectral density into a free-induction-decay time for a flux qubit biased to its optimal point and operated in the energy eigenbasis is presented.

Domain switching in the charge/orbital-ordered state of ${\text{La}}_{0.5}{\text{Sr}}_{1.5}{\text{MnO}}_4$, which can be manipulated by applying an electric field, was studied by in situ transmission electron microscopy. Dark-field images revealed the formation of micrometer-scale charge/orbital-ordered domains on cooling, which were separated by a clearly defined, meandering interface. Applying a uniaxial electric field aided the growth of a domain with its orbital chain (i.e., the direction of easy electron hopping) parallel to the applied field. These observations provide useful information for understanding of the switching mechanism of charge/orbital-ordered domains in manganite.

In this paper, we ask the question: how well can Coulomb blockade experiments correctly identify and distinguish between different topological orders in quantum Hall states? We definitively find the answer to be: quite poorly. In particular, we write the general expression for the spacing of resonance peaks in a simple form that explicitly displays its dependence on the conformal scaling dimensions of the systems’ edge modes. This form makes transparent the general argument that the Coulomb blockade peak spacings do not provide a strongly indicative signature of the topological order of the system since it is only weakly related to the braiding statistics. We bolster this general argument with examples for all the most physically relevant non-Abelian candidate states, demonstrating that they have Coulomb blockade doppelgängers–candidate states at the same filling fraction with identical Coulomb blockade signatures but dramatically different topological orders and braiding statistics.

Using microscopic theory, we investigate the properties of a spin current driven by magnetization dynamics. In the limit of smooth magnetization texture, the dominant spin current induced by the spin pumping effect is shown to be the diffusive spin current, i.e., the one arising from only a diffusion associated with spin accumulation. That is to say, there is no effective field that locally drives the spin current. We also investigate the conversion mechanism of the pumped spin current into a charge current by spin-orbit interactions, specifically the inverse spin Hall effect. We show that the spin-charge conversion does not always occur and that it depends strongly on the type of spin-orbit interaction. In a Rashba spin-orbit system, the local part of the charge current is proportional to the spin-relaxation torque, and the local spin current, which does not arise from the spin accumulation, does not play any role in the conversion. In contrast, the diffusive spin current contributes to the diffusive charge current. Alternatively, for spin-orbit interactions arising from random impurities, the local charge current is proportional to the local spin current that constitutes only a small fraction of the total spin current. Clearly, the dominant spin current (diffusive spin current) is not converted into a charge current. Therefore, the nature of the spin current is fundamentally different depending on its origin and thus the spin transport and the spin-charge conversion behavior need to be discussed together along with spin current generation.

Charged excitations in the fractional quantum Hall effect are known to carry fractional charges, as theoretically predicted and experimentally verified. Here we report on the dependence of the tunneling quasiparticle charge, as determined via highly sensitive shot noise measurements, on the measurement conditions, in the odd denominators states $v=1/3$ and $v=7/3$, and in the even denominator state $v=5/2$. In particular, for very weak backscattering probability and sufficiently small excitation energies (temperature and applied voltage), tunneling charges across a constriction were found to be significantly higher than the theoretically predicted fundamental quasiparticle charges.

While the thermal conductivity of the filled skutterudites has been of great interest it had not been calculated within a microscopic theory. Here a central force, Guggenheim-McGlashen, model with parameters largely extracted from first-principles calculations and from spectroscopic data, specific to ${\text{LaFe}}_4{\text{Sb}}_{12}$ or ${\text{CoSb}}_3$, is employed in a Green-Kubo/molecular dynamics calculation of thermal conductivity as a function of temperature. We find that the thermal conductivity of a filled solid is more than a factor of two lower than that of an unfilled solid, assuming the “framework” interatomic force parameters are the same between filled and unfilled solids, and that this decrease is almost entirely due to the cubic anharmonic interaction between filling and framework atoms. In addition, partially as a test of our models, we calculate thermal expansivity and isotropic atomic mean-square displacements using both molecular dynamics and lattice dynamics methods. These quantities are in reasonable agreement with experiment, increasing our confidence in the anharmonic parameters of our models. We also find an anomalously large filling-atom mode Gruneisen parameter that is apparently observed for a filled skutterudite and is observed in a clathrate.

A register of quantum bits with fixed transition frequencies and weakly coupled to one another through simple linear circuit elements is an experimentally minimal architecture for a small-scale superconducting quantum information processor. Presently, the known schemes for implementing two-qubit gates in this system require microwave signals having amplitudes and frequencies precisely tuned to meet a resonance condition, leaving only the signal phases as free experimentally adjustable parameters. Here, we report a minimal and robust microwave scheme to generate fast, tunable universal two-qubit gates: simply irradiate one qubit (the “control”) at the transition frequency of another (the “target”). The effective coupling between them is then switched on by tuning only the frequency of this single drive tone; the drive amplitude adjusts the effective coupling strength; and the drive phase selects the particular two-qubit gate implemented. This cross-resonance effect turns on linearly with the ratio of the drive amplitude $\Omega$ to the qubit-qubit detuning $\Delta$, as compared with earlier proposals that turn on as ${\left(\Omega /\Delta \right)}^4$.

We use magnetic long-range order as a tool to probe the Cooper-pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron-diffraction experiments, in particular, for the unusual behavior of the magnetic order below $T_c$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.

We present a comprehensive study of the conductance behavior of atomic-size contacts made of ferromagnetic metals (Co) or noble metals (Au) with ferromagnetic electrodes (Co). In order to separate the influence of the large electrodes from the influence of the contacts themselves, we used different sample geometries. These include combinations of nonmagnetic electrodes connected to magnetic bridges and vice versa as well as different orientations of the magnetic field. The magnetoresistance (MR) curves show very rich behavior with strong MR ratios (MRR). In all geometries the MRR values are of comparable size, reaching up to a few thousand percent in the tunneling regime. We study the possible influence of the micromagnetic order of the domains in the vicinity of the contact as well as ballistic MR, giant MR, tunnel MR, atomically enhanced anisotropic MR (AAMR), and magnetostriction. We conclude that AAMR is the most important origin for the MR at high magnetic fields $\left(\left|B\right|>2\text{T}\right)$, while magnetostriction, tunnel MR, and giant MR govern the low-field regime $\left(\left|B\right|<2\text{T}\right)$.

We have experimentally and theoretically demonstrated strong absorption of visible light in a thin nanostructured layer consisting of silver nanoparticles embedded in a dielectric matrix. Light absorption at the level of above 90% is recorded over a wide optical wavelength range (240–850 nm) and for a broad range of angles of light incidence $\left(0°–70°\right)$ in extremely thin films (160 nm). We suggest a generic principle for enhancement of light absorption in thin layers of artificial metamaterials and show that effective refractive indices of our samples measured with the help of ellipsometry can be adequately described by an effective-medium theory. We demonstrate that a substantial fraction of light can be trapped in the nanostructured film due to scattering by noble metal nanoparticles and total internal reflection.

We examine the star lattice Kitaev model whose ground state is a chiral spin liquid. We fermionize the model such that the fermionic vacua are toric-code states on an effective Kagome lattice. This implies that the Abelian phase of the system is inherited from the fermionic vacua and that time-reversal symmetry is spontaneously broken at the level of the vacuum. In terms of these fermions we derive the Bloch-matrix Hamiltonians for the vortex-free sector and its time-reversed counterpart and illuminate the relationships between the sectors. The phase diagram for the model is shown to be a sphere in the space of coupling parameters around the triangles of the lattices. The Abelian phase lies inside the sphere and the critical boundary between topologically distinct Abelian and non-Abelian phases lies on the surface. Outside the sphere the system is generically gapped except in the planes where the coupling parameters between the vertices on triangles are zero. These cases correspond to bipartite lattice structures and the dispersion relations are similar to that of the original Kitaev honeycomb model. In a further analysis we demonstrate the threefold non-Abelian ground-state degeneracy on a torus by explicit calculation.