Introduction of a Josephson field effect transistor (JoFET) concept sparked active research on proximity effects in semiconductors. Induced superconductivity and electrostatic control of critical current has been demonstrated in two-dimensional gases
in InAs, graphene and topological insulators, and in one-dimensional systems including quantum spin Hall edges. Recently, interest in superconductor-semiconductor interfaces was renewed by the search for Majorana fermions, which were predicted to reside at the interface. More exotic non-Abelian excitations, such as parafermions (fractional Majorana fermions) or Fibonacci fermions may be formed when fractional quantum Hall edge states interface with superconductivity. In this paper we develop transparent superconducting contacts to high mobility two-dimensional electron gas (2DEG) in GaAs and demonstrate induced superconductivity across several microns. Supercurrent in a ballistic junction has been observed across 0.6 $mu$m of 2DEG, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields ($>16$ Tesla) in NbN contacts enables investigation of a long-sought regime of an interplay between superconductivity and strongly correlated states in a 2DEG at high magnetic fields.
We discover a new topological excitation of two dimensional electrons in the quantum Hall regime. The strain dependence of resistivity is shown to change sign upon crossing filling-factor-specified boundaries of reentrant integer quantum Hall effect
(RIQHE) states. This observation violates the known symmetry of electron bubbles thought to be responsible for the RIQHE. We demonstrate theoretically that electron bubbles become elongated in the vicinity of charge defects and form textures of finite size. Calculations confirm that texturing lowers the energy of excitations. These textures form hedgehogs (vortices) around defects having (lacking) one extra electron, resulting in striking strain-dependent resistivity that changes sign on opposite boundaries of the RIQHE. At low density these textures form an insulating Abrikosov lattice. At densities sufficient to cause the textures to overlap, their interactions are described by the XY-model and the lattice melts. This melting explains the sharp metal-insulator transition observed in finite temperature conductivity measurements.
Topological superconductors which support Majorana fermions are thought to be realized in one-dimensional semiconducting wires coupled to a superconductor. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize qua
ntum gates that are topologically protected from local sources of decoherence. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor/superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with a radio-frequency f in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height hf/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge-2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf/e, suggesting that the supercurrent is carried by charge-e quasiparticles. This is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago.
Spontaneous breaking of rotational symmetry and preferential orientation of stripe phases in the quantum Hall regime has attracted considerable experimental and theoretical effort over the last decade. We demonstrate experimentally and theoretically
that the direction of high and low resistance of the two-dimensional (2D) hole gas in the quantum Hall regime can be controlled by an external strain. Depending on the sign of the in-plane shear strain, the Hartree-Fock energy of holes or electrons is minimized when the charge density wave (CDW) is oriented along [110] or [1-10] directions. We suggest that shear strains due to internal electric fields in the growth direction are responsible for the observed orientation of CDW in pristine electron and hole samples.
Conventional computer electronics creates a dichotomy between how information is processed and how it is stored. Silicon chips process information by controlling the flow of charge through a network of logic gates. This information is then stored, mo
st commonly, by encoding it in the orientation of magnetic domains of a computer hard disk. The key obstacle to a more intimate integration of magnetic materials into devices and circuit processing information is a lack of efficient means to control their magnetization. This is usually achieved with an external magnetic field or by the injection of spin-polarized currents. The latter can be significantly enhanced in materials whose ferromagnetic properties are mediated by charge carriers. Among these materials, conductors lacking spatial inversion symmetry couple charge currents to spin by intrinsic spin-orbit (SO) interactions, inducing nonequilibrium spin polarization tunable by local electric fields. Here we show that magnetization of a ferromagnet can be reversibly manipulated by the SO-induced polarization of carrier spins generated by unpolarized currents. Specifically, we demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor.
We examine energy spectra of Si quantum dots embedded into Si_{0.75}Ge_{0.25} buffers using atomistic numerical calculations for dimensions relevant to qubit implementations. The valley degeneracy of the lowest orbital state is lifted and valley spli
tting fluctuates with monolayer frequency as a function of the dot thickness. For dot thicknesses <6 nm valley splitting is found to be >150 ueV. Using the unique advantage of atomistic calculations we analyze the effect of buffer disorder on valley splitting. Disorder in the buffer leads to the suppression of valley splitting by a factor of 2.5, the splitting fluctuates with ~20 ueV for different disorder realizations. Through these simulations we can guide future experiments into regions of low device-to-device fluctuations.
We investigate effects of lateral confinement on spin splitting of energy levels in 2D hole gases grown on [311] GaAs. We found that lateral confinement enhances anisotropy of spin splitting relative to the 2D gas for both confining directions. Unexp
ectedly, the effective $g$-factor does not depend on the 1D energy level number $N$ for $B|[0bar{1}1]$ while it has strong $N$-dependence for $B|[bar{2}33]$. Apart from quantitative difference in the spin splitting of energy levels for the two orthogonal confinement directions we also report qualitative differences in the appearance of spin-split plateaus, with non-quantized plateaus observed only for the confinement in $[0bar{1}1]$ direction. In our samples we can clearly associate the difference with anisotropy of spin-orbit interactions.
A rapidly developing field of spintronics is based on the premise that substituting charge with spin as a carrier of information can lead to new devices with lower power consumption, non-volatility and high operational speed. Despite efficient magnet
ization detection, magnetization manipulation is primarily performed by current-generated local magnetic fields and is very inefficient. Here we report a novel non-volatile hybrid multiferroic memory cell with electrostatic control of magnetization based on strain-coupled GaMnAs ferromagnetic semiconductor and a piezoelectric material. We use the crystalline anisotropy of GaMnAs to store information in the orientation of the magnetization along one of the two easy axes, which is monitored via transverse anisotropic magnetoresistance. The magnetization orientation is switched by applying voltage to the piezoelectric material and tuning magnetic anisotropy of GaMnAs via the resulting stress field.
We report the observation of negative magnetoresistance in the ferromagnetic semiconductor GaMnAs at low temperatures ($T<3$ K) and low magnetic fields ($0< B <20$ mT). We attribute this effect to weak localization. Observation of weak localization p
rovides a strong evidence of impurity band transport in these materials, since for valence band transport one expects either weak anti-localization due to strong spin-orbit interactions or total suppression of interference by intrinsic magnetization. In addition to the weak localization, we observe Altshuler-Aronov electron-electron interactions effect in this material.
