Yoji Kunihashi

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.

Direct determination of spin–orbit interaction coefficients and realization of the persistent spin helix symmetry

The spin-orbit interaction plays a crucial role in diverse fields of condensed matter, including the investigation of Majorana fermions, topological insulators, quantum information and spintronics. In III-V zinc-blende semiconductor heterostructures, two types of spin-orbit interaction--Rashba and Dresselhaus--act on the electron spin as effective magnetic fields with different directions. They are characterized by coefficients α and β, respectively. When α is equal to β, the so-called persistent spin helix symmetry is realized. In this condition, invariance with respect to spin rotations is achieved even in the presence of the spin-orbit interaction, implying strongly enhanced spin lifetimes for spatially periodic spin modes. Existing methods to evaluate α/β require fitting analyses that often include ambiguity in the parameters used. Here, we experimentally demonstrate a simple and fitting parameter-free technique to determine α/β and to deduce the absolute values of α and β. The method is based on the detection of the effective magnetic field direction and the strength induced by the two spin-orbit interactions. Moreover, we observe the persistent spin helix symmetry by gate tuning.

Electrical manipulation of spins in the Rashba two dimensional electron gas systems

We present our theoretical and experimental studies on manipulation of electron spins based on the Rashba spin-orbit interaction (SOI) in semiconductor heterostructures. Quantum well (QW) thickness dependence of the Rashba SOI strength α is investigated in InP/InGaAs/InAlAs asymmetric QWs by analyzing weak antilocalization. Two different QW thicknesses show inverse Ns dependence of |α| in the same heterostructures. This inverse Ns dependence of |α| is explained by the k⋅p perturbation theory. We confirm that narrow wires are effective to suppress the spin relaxation. Spin interference effects due to spin precession are experimentally studied in small array of mesoscopic InGaAs rings. This is an experimental demonstration of a time reversal Aharonov–Casher effect, which shows that the spin precession angle in an InGaAs channel can be controlled by an electrostatic gate.

Proposal of spin complementary field effect transistor

Spin complementary field effect transistor is proposed on the basis of gate-controlled persistent spin helix (PSH) states. Uniaxial effective magnetic field in the PSH state creates coherent spin propagation with or without precession. By the gate control of the Rashba spin-orbit interaction, the PSH state can be reversed to the inverted PSH state. Switching between two PSH states enables complementary output depending on the channel direction. Our proposed device could be a reconfigurable minimum unit of the spin-based logic circuit.

Drift transport of helical spin coherence with tailored spin–orbit interactions

Most future information processing techniques using electron spins in non-magnetic semiconductors will require both the manipulation and transfer of spins without their coherence being lost. The spin–orbit effective magnetic field induced by drifting electrons enables us to rotate the electron spins in the absence of an external magnetic field. However, the fluctuations in the effective magnetic field originating from the random scattering of electrons also cause undesirable spin decoherence, which limits the length scale of the spin transport. Here we demonstrate the drift transport of electron spins adjusted to a robust spin structure, namely a persistent spin helix. We find that the persistent spin helix enhances the spatial coherence of drifting spins, resulting in maximized spin decay length near the persistent spin helix condition. Within the enhanced distance of the spin transport, the transport path of electron spins can be modulated by employing time-varying in-plane voltages.

Drift-Induced Enhancement of Cubic Dresselhaus Spin-Orbit Interaction in a Two-Dimensional Electron Gas

We investigated the effect of an in-plane electric field on drifting spins in a GaAs quantum well. Kerr rotation images of the drifting spins revealed that the spin precession wavelength increases with increasing drift velocity regardless of the transport direction. A model developed for drifting spins with a heated electron distribution suggests that the in-plane electric field enhances the effective magnetic field component originating from the cubic Dresselhaus spin-orbit interaction.

Comparison of electrical and optical detection of spin injection in L10-FePt/MgO/GaAs hybrid structures

We have investigated comparative experiments for spin injection into semiconductor in an ordered L10-FePt/MgO/n-GaAs hybrid structure using electrical and optical detection methods. Spatial-resolved Kerr rotation microscope image clearly demonstrates accumulation of perpendicularly oriented spins in an n-GaAs channel at zero magnetic field. On the other hand, electrical three-terminal Hanle measurement shows shorter spin lifetime than that of the optical measurement. It suggests that the spin lifetime obtained from three-terminal Hanle method originates from spins at the MgO/GaAs interface but not in the bulk GaAs channel.

Bias dependence of spin injection/transport properties of a perpendicularly magnetized FePt/MgO/GaAs structure

We demonstrate injection and transport of perpendicularly spin-polarized electrons in an FePt/MgO/n-GaAs structure. Spin-polarized electrons were injected from a perpendicularly magnetized FePt layer into an n-GaAs layer through a MgO barrier and detected by spatially resolved Kerr rotation microscopy. By measuring the Hanle effect, we reveal that the injected/extracted spin polarizations drastically vary with bias voltages. A spin lifetime of 3.5 ns is obtained that is consistent with the result from pump–probe measurements. This direct observation of perpendicularly polarized spin injection and lateral transport is one step toward realizing future spintronic devices.

Spin coherence enhanced by in-plane electric field-induced spin-orbit interaction

Summary form only given. We investigated the dependence of spatial spin distribution on bias voltage in a GaAs quantum well with Kerr rotation microscopy. The spin precession frequency of drifting spins depends on in-plane electric fields, which indicates that effective magnetic fields induced by spin-orbit interactions are modulated by in-plane electric fields. By comparing an experimental result and a theoretical simulation based on the Monte Carlo approach, we confirmed that the in-plane electric field dependence of effective magnetic fields can be attributed to the k-cubic term of the Dresselhaus spin-orbit interaction. We found that spin coherent length can be elongated by balancing a Rashba SOI and all the terms of a Dresselhaus spin-orbit interaction. This technique will provide a new way of both suppressing spin relaxation and controlling the spin precession frequency when using the cubic Dresselhaus SOI.

Estimation of Spin-orbit Interaction Parameters with Drifting Spins in Semiconductor Quantum Wells

We investigated theoretically the spin dynamics of drifting electrons in high mobility two-dimensional electron gases with a semiclassical Monte Carlo approach. Our theoretical procedure enabled us to estimate the strengths of Rashba, linear Dresselhaus and cubic Dresselhaus spin-orbit interactions (SOI) from the crystal orientation dependence of precession frequency of drifting spins without applying any external magnetic field. When applying high in-plane electric fields, we obtained the quasi-one dimensional transports of electron spins whose spin precession frequency accurately reflected the spin splitting energy induced by SOIs. This theoretical method can be applied to experimental results of drifting spins. Our results will provide a useful tool for the accurate detection of SOI parameters.