Suppression of spin relaxation in submicron InGaAs wires

Controlled Rotation of Electrically Injected Spins in a Nonballistic 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 operate 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 nonballistic 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 nonballistic channels with relatively low SOC, relaxing two major constraints of the original sFET proposal.

Flying electron spin control gates

The control of "flying” (or moving) spin qubits is an important functionality for the manipulation and exchange of quantum information between remote locations on a chip. Typically, gates based on electric or magnetic fields provide the necessary perturbation for their control either globally or at well-defined locations. Here, we demonstrate the dynamic control of moving electron spins via contactless gates that move together with the spins. The concept is realized using electron spins trapped and transported by moving potential dots defined by a surface acoustic wave (SAW). The SAW strain at the electron trapping site, which is set by the SAW amplitude, acts as a contactless, tunable gate that controls the precession frequency of the flying spins via the spin-orbit interaction. We show that the degree of precession control in moving dots exceeds previously reported results for unconstrained transport by an order of magnitude and is well accounted for by a theoretical model for the strain contribution to the spin-orbit interaction. This flying spin gate permits the realization of an acoustically driven optical polarization modulator based on electron spin transport, a key element for on-chip spin information processing with a photonic interface.

Spin qubits are a platform for quantum computing. There are many advantages for quantum information processing if the spin qubit can move. Here, Helgers et al. use a surface acoustic wave to define a moving quantum dot and demonstrate the magneticfield-free control of the spin precession, bringing “flying” spin qubits a step closer.

Electrical Spin Injection and Detection in Silicon Nanowires with Axial Doping Gradient

The interest in spin transport in nanoscopic semiconductor channels is driven by both the inevitable miniaturization of spintronics devices toward nanoscale and the rich spin-dependent physics the quantum confinement engenders. For such studies, the all-important issue of the ferromagnet/semiconductor (FM/SC) interface becomes even more critical at nanoscale. Here we elucidate the effects of the FM/SC interface on electrical spin injection and detection at nanoscale dimensions, utilizing a unique type of Si nanowires (NWs) with an inherent axial doping gradient. Two-terminal and nonlocal four-terminal lateral spin-valve measurements were performed using different combinations from a series of FM contacts positioned along the same NW. The data are analyzed with a general model of spin accumulation in a normal channel under electrical spin injection from a FM, which reveals a distinct correlation of decreasing spin-valve signal with increasing injector junction resistance. The observation is attributed to the diminishing contribution of the d-electrons in the FM to the injected current spin polarization with increasing Schottky barrier width. The results demonstrate that there is a window of interface parameters for optimal spin injection efficiency and current spin polarization, which provides important design guidelines for nanospintronic devices with quasi-one-dimensional semiconductor channels.

Rapid Transition of the Hole Rashba Effect from Strong Field Dependence to Saturation in Semiconductor Nanowires

The electric field manipulation of the Rashba spin-orbit coupling effects provides a route to electrically control spins, constituting the foundation of the field of semiconductor spintronics. In general, the strength of the Rashba effects depends linearly on the applied electric field and is significant only for heavy-atom materials with large intrinsic spin-orbit interaction under high electric fields. Here, we illustrate in 1D semiconductor nanowires an anomalous field dependence of the hole (but not electron) Rashba effect (HRE). (i) At low fields, the strength of the HRE exhibits a steep increase with the field so that even low fields can be used for device switching. (ii) At higher fields, the HRE undergoes a rapid transition to saturation with a giant strength even for light-atom materials such as Si (exceeding 100 meV Å). (iii) The nanowire-size dependence of the saturation HRE is rather weak for light-atom Si, so size fluctuations would have a limited effect; this is a key requirement for scalability of Rashba-field-based spintronic devices. These three features offer Si nanowires as a promising platform for the realization of scalable complementary metal-oxide-semiconductor compatible spintronic devices.

Electrical spin injection and detection in high mobility 2DEG systems

In this review paper we present the current status of research related to the topic of electrical spin injection and detection in two-dimensional electron gas (2DEG) systems, formed typically at the interface between two III-V semiconductor compounds. We discuss both theoretical aspects of spin injection in case of ballistic transport as well as give an overview of available reports on spin injection experiments performed on 2DEG structures. In the experimental part we focus particularly on our recent work on all-semiconductor structures with a 2DEG confined at an inverted GaAs/(Al,Ga)As interface and with a ferromagnetic semiconductor (Ga,Mn)As employed as a source of spin-polarized electrons.

Room-temperature spin transport in InAs nanowire lateral spin valve

We report room temperature spin transport in an InAs nanowire device. A large spin signal of 35 kΩ and long spin diffusion length of 1.9 μm are achieved. We believe that these results open a practical way to design InAs NW based spintronic devices.

Modulating spin relaxation in nanowires with infrared light at room temperature

Spintronic devices usually rely on long spin relaxation times and/or long spin relaxation lengths for optimum performance. Therefore, the ability to modulate these quantities with an external agent offers unique possibilities. The dominant spin relaxation mechanism in most technologically important semiconductors is the D'yakonov-Perel' (DP) mechanism which may vanish if the spin carriers (electrons) are confined to a single conduction subband in a quantum wire. Here, we report modulating the DP spin relaxation rate (and hence the spin relaxation length) in self assembled 50 nm diameter InSb nanowires with infrared (IR) light at room temperature. In the dark, almost all the electrons in the nanowires are in the lowest conduction subband, resulting in near-complete absence of DP relaxation. This allows observation of spin-sensitive effects in the magnetoresistance. Under IR illumination, higher subbands get populated and the DP spin relaxation mechanism is revived, leading to a three-fold decrease in the spin relaxation length. Consequently, the spin sensitive effects disappear under illumination. This phenomenon may have applications in spintronic room-temperature IR photodetection.

Electrical spin injection and transport in semiconductor nanowires: challenges, progress and perspectives

Spintronic devices are of fundamental interest for their nonvolatility and great potential for low-power electronics applications. The implementation of those devices usually favors materials with long spin lifetime and spin diffusion length. Recent spin transport studies on semiconductor nanowires have shown much longer spin lifetimes and spin diffusion lengths than those reported in bulk/thin films. In this paper, we have reviewed recent progress in the electrical spin injection and transport in semiconductor nanowires and drawn a comparison with that in bulk/thin films. In particular, the challenges and methods of making high-quality ferromagnetic tunneling and Schottky contacts on semiconductor nanowires as well as thin films are discussed. Besides, commonly used methods for characterizing spin transport have been introduced, and their applicability in nanowire devices are discussed. Moreover, the effect of spin-orbit interaction strength and dimensionality on the spin relaxation and hence the spin lifetime are investigated. Finally, for further device applications, we have examined several proposals of spinFETs and provided a perspective of future studies on semiconductor spintronics.

Electrical spin injection and detection in silicon nanowires through oxide tunnel barriers

We demonstrate all-electrical spin injection, transport, and detection in heavily n-type-doped Si nanowires using ferromagnetic Co/Al(2)O(3) tunnel barrier contacts. Analysis of both local and nonlocal spin valve signals at 4 K on the same nanowire device using a standard spin-transport model suggests that high spin injection efficiency (up to ~30%) and long spin diffusion lengths (up to ~6 μm) are achieved. These values exceed those reported for spin transport devices based on comparably doped bulk Si. The spin valve signals are found to be strongly bias and temperature dependent and can invert sign with changes in the dc bias current. The influence of the nanowire morphology on field-dependent switching of the contacts is also discussed. Owing to their nanoscale geometry, ~5 orders of magnitude less current is required to achieve nonlocal spin valve voltages comparable to those attained in planar microscale spin transport devices, suggesting lower power consumption and the potential for applications of Si nanowires in nanospintronics.

Electrical spin injection into InN semiconductor nanowires

We report on the conditions necessary for the electrical injection of spin-polarized electrons into indium nitride nanowires synthesized from the bottom up by molecular beam epitaxy. The presented results mark the first unequivocal evidence of spin injection into III-V semiconductor nanowires. Utilizing a newly developed preparation scheme, we are able to surmount shadowing effects during the metal deposition. Thus, we avoid strong local anisotropies that arise if the ferromagnetic leads are wrapping around the nanowire. Using a combination of various complementary techniques, inter alia the local Hall effect, we carried out a comprehensive investigation of the coercive fields and switching behaviors of the cobalt micromagnetic spin probes. This enables the identification of a range of aspect ratios in which the mechanism of magnetization reversal is single domain switching. Lateral nanowire spin valves were prepared. The spin relaxation length is demonstrated to be about 200 nm, which provides an incentive to pursue the route toward nanowire spin logic devices.

Generalized path integral method for Elliott-Yafet spin relaxations in quantum wells and narrow wires

We generalized the semiclassical path integral method originally used in the D'yakonov-Perel' mechanism to study the spin relaxation of the Elliott-Yafet mechanism in low-dimensional systems. In quantum wells, the spin properties calculated by this method confirmed the experimental results. In two-dimensional narrow wires, size and impurity effects on the Elliott-Yafet relaxation were predicted, including the wire-width-dependent relaxation time, the polarization evolution on the sample boundaries, and the relaxation behavior during the diffusive-ballistic transition. These properties were compared with those of the D'yakonov-Perel' relaxation calculated under similar conditions. For ballistic narrow wires, we derived an exact relation between the Elliott-Yafet relaxation time and the wire width, which confirmed the above simulations.

Theory of spin noise in nanowires

We develop a theory of spin noise in semiconductor nanowires considered as prospective elements for spintronics. In these structures, spin-orbit coupling can be realized as a random function of a coordinate correlated on a spatial scale of the order of 10 nm. By analyzing different regimes of electron transport and spin dynamics, we demonstrate that the spin relaxation can be very slow, and the resulting noise power spectrum increases algebraically as the frequency goes to zero. This effect makes spin phenomena in nanowires best suitable for studies by rapidly developing spin-noise spectroscopy.

Lateral spin injection in germanium nanowires

Electrical injection of spin-polarized electrons into a semiconductor, large spin diffusion length, and an integration friendly platform are desirable ingredients for spin-based devices. Here we demonstrate lateral spin injection and detection in germanium nanowires, by using ferromagnetic metal contacts and tunnel barriers for contact resistance engineering. Using data measured from over 80 samples, we map out the contact resistance window for which lateral spin transport is observed, manifestly showing the conductivity matching required for spin injection. Our analysis, based on the spin diffusion theory, indicates that the spin diffusion length is larger than 100 mum in germanium nanowires at 4.2 K.

Density enhanced diffusion of dipolar excitons within a one-dimensional channel

We experimentally investigate the lateral diffusion of dipolar excitons in coupled quantum wells in two (2D) and one (1D) dimensions. In 2D, the exciton expansion obeys nonlinear temporal dynamics due to the repulsive dipole pressure at a high exciton density, in accordance with recent reports. In contrast, the observed 1D expansion behaves linearly in time even at high exciton densities. The corresponding 1D diffusion coefficient exceeds the one in 2D by far and depends linearly on the exciton density. We attribute the findings to screening of quantum well disorder by the dipolar excitons.

Enhancement of spin lifetime in gate-fitted InGaAs narrow wires

We investigated the spin lifetime in gate-fitted InGaAs narrow wires from magnetotransport measurement. Applying positive gate bias voltage, the spin lifetimes in narrow wires became more than one order longer than those obtained from a Hall bar sample with two-dimensional electron gas. This enhancement of spin lifetime in gated wires is the first experimental evidence of dimensional confinement and resonant spin-orbit interaction effect controlled by gate bias voltage. Spin relaxation due to the cubic Dresselhaus term is negligible in the present InGaAs wires.

Electrical generation of pure spin currents in a two-dimensional electron gas

Pure spin currents are generated and detected in micron-wide channels of a GaAs two-dimensional electron gas, using quantum point contacts in an in-plane magnetic field as injectors and detectors. The enhanced sensitivity to spin transport offered by a nonlocal measurement geometry enables accurate spin current measurements in this widely studied physical system. The polarization of the contacts is used to extract the quantum point contact g factor and provides a test for spontaneous polarization at 0.7 structure. The spin relaxation length in the channel is 30-50 microm over the magnetic field range 3-10 T, much longer than has been reported in GaAs two-dimensional electron gases but shorter than that expected from Dyakonov-Perel relaxation.

All-electrical detection of the relative strength of Rashba and Dresselhaus spin-orbit interaction in quantum wires

We propose a method to determine the relative strength of Rashba and Dresselhaus spin-orbit interaction from transport measurements without the need of fitting parameters. To this end, we make use of the conductance anisotropy in narrow quantum wires with respect to the directions of an in-plane magnetic field, the quantum wire, and the crystal orientation. We support our proposal by numerical calculations of the conductance of quantum wires based on the Landauer formalism which show the applicability of the method to a wide range of parameters.

Room-temperature ferromagnetism in Cu doped GaN nanowires

We report magnetism in Cu doped single-crystalline GaN nanowires. The typical diameter and the length of the Ga1-xCuxN nanowires (x = 0.01, 0.024) are 10-100 nm and tens of micrometers, respectively. The saturation magnetic moments are measured to be higher than 0.86 microB/Cu at 300 K, and the Curie temperatures are far above room temperature. Anomalous X-ray scattering and X-ray diffraction measurement make it clear that Cu atoms substitute the Ga sites, and they largely take part in the wurtzite network of host GaN. X-ray absorption and X-ray magnetic circular dichroism spectra at Cu L(2,3) edges show that doped Cu has local magnetic moment and the electronic configuration of it is mainly 3d9 but mixed with a small portion of trivalent component. It seems that the ionocovalent bonding nature of Cu 3d orbital with surrounding semiconductor medium makes Cu atom a mixed electron configuration and local magnetic moments. These outcomes suggest that the Ga1-xCuxN system is a room-temperature ferromagnetic semiconductor.

Charge and spin dynamics in a two-dimensional electron gas

A number of time-resolved optical experiments probing and controlling the spin and charge dynamics of the high-mobility two-dimensional electron gas in a GaAs/AlGaAs heterojunction are discussed. These include time-resolved reflectivity, luminescence, transient grating, magneto-optical Kerr effect, and electro-optical Kerr effect experiments. The optical experiments provide information on the carrier lifetimes and spin dephasing times, as well as on the carrier diffusion coefficient which directly gives the charge mobility. A combination of the two types of Kerr experiment proves to be useful in extracting both the carrier lifetimes and spin dephasing times in a single experiment.