Satofumi Souma

Nonequilibrium Spin Hall Accumulation in Ballistic Semiconductor Nanostructures

We demonstrate that flow of longitudinal unpolarized current through a ballistic two-dimensional electron gas with Rashba spin-orbit coupling will induce nonequilibrium spin accumulation which has opposite sign for the two lateral edges and it is, therefore, the principal observable signature of the spin Hall effect in two-probe semiconductor nanostructures. The magnitude of its out-of-plane component is gradually diminished by static disorder, while it can be enhanced by an in-plane transverse magnetic field. Moreover, our prediction of the longitudinal component of the spin Hall accumulation, which is insensitive to the reversal of the bias voltage, offers a smoking gun to differentiate experimentally between the extrinsic, intrinsic, and mesoscopic spin Hall mechanisms.

Spin Hall current driven by quantum interferences in mesoscopic Rashba rings.

We propose an all-electrical nanostructure where pure spin current is induced in the transverse voltage probes attached to a quantum-coherent ballistic one-dimensional ring when unpolarized charge current is injected through its longitudinal leads. Tuning of the Rashba spin-orbit coupling in a semiconductor heterostructure hosting the ring generates quasiperiodic oscillations of the predicted spin-Hall current due to spin-sensitive quantum-interference effects caused by the difference in the Aharonov-Casher phase accumulated by opposite spin states. Its amplitude is comparable to that of the spin-Hall current predicted for finite-size (simply connected) two-dimensional electron gases, while it gets reduced gradually in wide two-dimensional rings or due to spin-independent disorder.

Modulating unpolarized current in quantum spintronics: Visibility of spin-interference effects in multichannel Aharonov-Casher mesoscopic rings

The conventional unpolarized current injected into a {\em quantum-coherent} semiconductor ring attached to two external leads can be modulated from perfect conductor to perfect insulator limit via Rashba spin-orbit (SO) coupling. This requires that ballistic propagation of electrons, whose spin precession is induced by the Aharonov-Casher phase, takes place through a single conducting channel ensuring that electronic quantum state remains a pure separable one in the course of transport. We study the fate of such spin interference effects as more than one orbital conducting channel becomes available for quantum transport. Although the conductance of multichannel rings, in general, does not go all the way to zero at any value of the SO coupling, some degree of current modulation survives. We analyze possible scenarios that can lead to reduced visibility of spin interference effects that are responsible for the zero conductance at particular values of the Rashba interaction: (i) the transmitted spin states remain fully coherent, but conditions for destructive interference are different in different channels; (ii) the transmitted spins end up in partially coherent quantum state arising from entanglement to the environment composed of orbital degrees of freedom of the same particle to which the spin is attached.

Simulation-based design of a strained graphene field effect transistor incorporating the pseudo magnetic field effect

We present a numerical study on the performance of strained graphene-based field-effect transistors. A local strain less than 10% is applied over a central channel region of the graphene to induce the shift of the Dirac point in the channel region along the transverse momentum direction. The left and the right unstrained graphene regions are doped to be either n-type or p-type. By using the atomistic tight-binding model and a Greens function method, we predict that the gate voltage applied to the central strained graphene region can switch the drain current on and off with an on/off ratio of more than six orders of magnitude at room temperature. This is in spite of the absence of a bandgap in the strained channel region. Steeper subthreshold slopes below 60 mV/decade are also predicted at room temperature because of a mechanism similar to the band-to-band tunneling field-effect transistors.

Pure spin current induced by adiabatic quantum pumping in zigzag-edged graphene nanoribbons

We show theoretically that pure spin current can be generated in zigzag edged graphene nanoribbons through the adiabatic pumping by edge selective pumping potentials. The origin of such pure spin current is the spin splitting of the edge localized states, which are oppositely spin polarized at opposite edges. In the proposed device, each edge of the ribbon is covered by two independent time-periodic local gate potentials with a definite phase difference, inducing the edge spin polarized current. When the pumping phase difference is opposite in sign between two edges, the total charge currents is zero and the pure edge spin current is generated.

Influence of geometrical deformation and electric field on transport characteristics through carbon nanotubes

We study computationally the electronic transport properties through mechanically squashed zigzag carbon nanotubes (CNTs) under the uniform electric field perpendicular to the tube axis, based on the tight-binding molecular dynamics method for the structural analysis and the Landauer-Buttikers formalism for the transport analysis. Our simulations show that the band gaps of the zigzag carbon nanotubes exhibit nonlinear decrease as increasing the deformation ratio in the presence of the external perpendicular electric field, in contrast to the case of zero electric field, where the band gap decreases linearly as increasing the deformation ratio. Such properties allow us to tune the sensitivity of the electromechanical response in CNT devices by applying the external electric field.

Effect of uniaxial strain on the electronic transport in single layer graphene

We study the effect of the various types of strain on the electronic band structure and the transport characteristics in graphene. It has been found that the combination of shear and armchair uniaxial deformation is an effective way to open the band gap, meaning the efficient controllability of the electronic current through graphene‥

Analysis of electronic structure in quantum dot arrays for intermediate band solar cells

Intermediate band solar cells (IBSCs) have been proposed as highly efficient third generation photovoltaic devices. Quantum dot (QD) arrays produce mini-bands that are separated by a region of zero density of states from other states in the conduction band. Additional absorption from the valence band to the IB and the IB to the conduction band allows two photons with energies below the energy gap to be harvested in generating one electron-hole pair. We present a theoretical study of the electronic and optical properties of the IB formed by an InAs/GaAs QD arrays. The calculations are based on effective-mass approximation and finite element method. Theoretical results of the mini-band width variation with the period of the QD arrays in the z direction are presented.

Analysis of geometrical structure and transport property in InAs/Si heterojunction nanowire tunneling field effect transistors

Band-to-band tunneling (BTBT) field-effect transistors (FETs) is one of the promisng strategies in reducing the leakage current and improving the subthreshold characteristics compared with the conventional metal-oxide-semiconductor field-effect transistors (MOSFETs). However, BTBT-FETs have an intrinsic drawback of small on-current. We explore numerically the possibility of using the InAs/Si heterojunction nanowire (NW) to resolve such intrinsic difficulty in BTBT-FETs, and found that the use of the InAs/Si heterojunction nanowire is advantageous in increasing the on-current compared with the Si homojunction nanowires.

Fullband Simulation of Nano-Scale MOSFETs Based on a Non-equilibrium Green's Function Method

The analysis of multiband quantum transport simulation in double-gate metal oxide semiconductor field effects transistors (DGMOSFETs) is performed based on a non-equilibrium Greens function (NEGF) formalism coupled self-consistently with the Poisson equation. The empirical sp3s* tight binding approximation (TBA) with nearest neighbor coupling is employed to obtain a realistic multiband structure. The effects of non-parabolic bandstructure as well as anisotropic features of Si are studied and analyzed. As a result, it is found that the multiband simulation results on potential and current profiles show significant differences, especially in higher applied bias, from those of conventional effective mass model.