All-electrical measurements of direct spin Hall effect in GaAs with Esaki diode electrodes

We report on measurements of direct spin Hall effect in a lightly n-doped GaAs channel. As spin detecting contacts we employed highly efficient ferromagnetic Fe/(Ga,Mn)As/GaAs Esaki diode structures. We investigate bias and temperature dependence of the measured spin Hall signal and evaluate the value of total spin Hall conductivity and its dependence on channel conductivity and temperature. From the results we determine skew scattering and side jump contribution to the total spin hall conductivity and compare it with the results of experiments on higher conductive n-GaAs channels[Phys. Rev. Lett. 105,156602(2010)]. As a result we conclude that both skewness and side jump contribution cannot be fully independent on the conductivity of the channel.Comment: 14 pages, 4 figure

Few electron double quantum dot in an isotopically purified 28^{28}Si quantum well

We present a few electron double quantum dot (QD) device defined in an isotopically purified $^{28}$Si quantum well (QW). An electron mobility of $5.5 \cdot 10^4 cm^2(Vs)^{-1}$ is observed in the QW which is the highest mobility ever reported for a 2D electron system in $^{28}$Si. The residual concentration of $^{29}$Si nuclei in the $^{28}$Si QW is lower than $10^{3} ppm$, at the verge where the hyperfine interaction is theoretically no longer expected to dominantly limit the $T_{2}$ spin dephasing time. We also demonstrate a complete suppression of hysteretic gate behavior and charge noise using a negatively biased global top gate.Comment: 4 pages, 3 figure

Shot Noise Induced by Nonequilibrium Spin Accumulation

When an electric current passes across a potential barrier, the partition process of electrons at the barrier gives rise to the shot noise, reflecting the discrete nature of the electric charge. Here we report the observation of excess shot noise connected with a spin current which is induced by a nonequilibrium spin accumulation in an all-semiconductor lateral spin-valve device. We find that this excess shot noise is proportional to the spin current. Additionally, we determine quantitatively the spin-injection-induced electron temperature by measuring the current noise. Our experiments show that spin accumulation driven shot noise provides a novel means of investigating nonequilibrium spin transport.Comment: 5 pages and Supplemental Materia

Controlled rotation of electrically injected spins in a non-ballistic spin field-effect transistor

Electrically controlled rotation of spins in a semiconducting channel is a prerequisite for the successful realization of many spintronic devices, like, e.g., the spin field effect transistor (sFET). To date, there have been only a few reports on electrically controlled spin precession in sFET-like devices. These devices operated in the ballistic regime, as postulated in the original sFET proposal, and hence need high SOC channel materials in practice. Here, we demonstrate gate-controlled precession of spins in a non-ballistic sFET using an array of narrow diffusive wires as a channel between a spin source and a spin drain. Our study shows that spins traveling in a semiconducting channel can be coherently rotated on a distance far exceeding the electrons mean free path, and spin-transistor functionality can be thus achieved in non-ballistic channels with relatively low SOC, relaxing two major constraints of the original sFET proposal.Comment: Main text: 21 pages, 4 figures. Supplementary Information:11 pages, 4 figure

Enhanced spin-orbit coupling in core/shell nanowires

The spin-orbit coupling (SOC) in semiconductors is strongly influenced by structural asymmetries, as prominently observed in bulk crystal structures that lack inversion symmetry. Here, we study an additional effect on the SOC: the asymmetry induced by the large interface area between a nanowire core and its surrounding shell. Our experiments on purely wurtzite GaAs/AlGaAs core/shell nanowires demonstrate optical spin injection into a single free-standing nanowire and determine the effective electron g-factor of the hexagonal GaAs wurtzite phase. The spin relaxation is highly anisotropic in time-resolved micro-photoluminescence measurements on single nanowires, showing a significant increase of spin relaxation in external magnetic fields. This behavior is counterintuitive compared to bulk wurtzite crystals. We present a model for the observed electron spin dynamics highlighting the dominant role of the interface-induced SOC in these core/shell nanowires. This enhanced SOC may represent an interesting tuning parameter for the implementation of spin-orbitronic concepts in semiconductor-based structures

Wurtzite quantum wires with strong spatial confinement: polarization anisotropies in single wire spectroscopy

We report GaAs/AlGaAs nanowires in the one-dimensional (1D) quantum limit. The ultrathin wurtzite GaAs cores between 20-40\,nm induce large confinement energies of several tens of meV, allowing us to experimentally resolve up to four well separated subband excitations in microphotoluminescence spectroscopy. Our detailed experimental and theoretical polarization-resolved study reveals a strong diameter-dependent anisotropy of these transitions: We demonstrate that the polarization of the detected photoluminescence is governed by the symmetry of the wurtzite 1D quantum wire subbands on the one hand, but also by the dielectric mismatch of the wires with the surrounding material on the other hand. The latter effect leads to a strong attenuation of perpendicularly polarized light in thin dielectric wires, making the thickness of the AlGaAs shell an important factor in the observed polarization behavior. Including the dielectric mismatch to our k.p-based simulated polarization-resolved spectra of purely wurtzite GaAs quantum wires, we find an excellent agreement between experiment and theory

Magnetic Force Sensing Using a Self-Assembled Nanowire

We present a scanning magnetic force sensor based on an individual magnet-tipped GaAs nanowire (NW) grown by molecular beam epitaxy. Its magnetic tip consists of a final segment of single-crystal MnAs formed by sequential crystallization of the liquid Ga catalyst droplet. We characterize the mechanical and magnetic properties of such NWs by measuring their flexural mechanical response in an applied magnetic field. Comparison with numerical simulations allows the identification of their equilibrium magnetization configurations, which in some cases include magnetic vortices. To determine a NW's performance as a magnetic scanning probe, we measure its response to the field profile of a lithographically patterned current-carrying wire. The NWs' tiny tips and their high force sensitivity make them promising for imaging weak magnetic field patterns on the nanometer-scale, as required for mapping mesoscopic transport and spin textures or in nanometer-scale magnetic resonance

Spin relaxation in wurtzite nanowires

We theoretically investigate the D'yakonov-Perel' spin-relaxation properties in diffusive wurtzite semiconductor nanowires and their impact on the quantum correction to the conductivity. Although the lifetime of the long-lived spin states is limited by the dominant k-linear spin-orbit contributions in the bulk, these terms show almost no effect in the finite-size nanowires. Here, the spin lifetime is essentially determined by the small k-cubic spin-orbit terms and nearly independent of the wire radius. At the same time, these states possess in general a complex helical structure in real space that is modulated by the spin-precession length induced by the k-linear terms. For this reason, the experimentally detected spin relaxation largely depends on the ratio between the nanowire radius and the spin-precession length as well as the type of measurement. In particular, it is shown that while a variation of the radius hardly affects the magnetoconductance correction, which is governed by the long-lived spin states, the change in the spin lifetime observed in optical experiments can be dramatic. We compare our results with recent experimental studies on wurtzite InAs nanowires