Bernhard Kramer

A three-terminal spin filter induced by spin-orbit interaction in the presence of an antidot

The generation of a spin-polarized electric current is one of the key issues in the field of semiconductor spintronics. We show by numerical simulation that a three-terminal conductor with an antidot potential at the junction can be used as a spin filter. In addition, one can control the direction of spin polarization by changing the Fermi energy with respect to the height of antidot potential. This is understood by the interplay between spin-orbit coupling and the curvature of the potential. We also investigate the effect of disorder and an asymmetric confinement potential in order to clarify the validity of the spin filter.

Spin-projected unrestricted Hartree-Fock ground states for harmonic quantum dots

We report the results for the ground state energies and wave functions obtained by projecting spatially unrestricted Hartree-Fock states to eigenstates of the total spin and the angular momentum for harmonic quantum dots with N12 interacting electrons including a magnetic field. The ground states with the correct spatial and spin symmetries have lower energies than those obtained by the unrestricted method. The chemical potential as a function of a perpendicular magnetic field is obtained. Signature of an intrinsic spin blockade effect is found.

Unconventional conductance plateau transitions in quantum Hall wires with spatially correlated disorder

Quantum transport properties in quantum Hall wires in the presence of spatially correlated random potential are investigated numerically. It is found that the potential correlation reduces the localization length associated with the edge state, in contrast to the naive expectation that the potential correlation increases it. The effect appears as the sizable shift of quantized conductance plateaus in long wires, where the plateau transitions occur at energies much higher than the Landau band centers. The scale of the shift is of the order of the strength of the random potential and is insensitive to the strength of magnetic fields. Experimental implications are also discussed.

Conductance-plateau transitions in quantum Hall wires with spatially correlated random magnetic fields

Quantum transport properties in quantum Hall wires in the presence of spatially correlated disordered magnetic fields are investigated numerically. It is found that the correlation drastically changes the transport properties associated with the edge state, in contrast to the naive expectation that the correlation simply reduces the effect of disorder. In the presence of correlation, the separation between the successive conductance-plateau transitions becomes larger than the bulk Landau-level separation determined by the mean value of the disordered magnetic fields. The transition energies coincide with the Landau levels in an effective magnetic field stronger than the mean value of the disordered magnetic field. For a long wire, the strength of this effective magnetic field is of the order of the maximum value of the magnetic fields in the system. It is shown that the effective field is determined by a part where the stronger magnetic-field region connects both edges of the wire.

Mesoscopic Hall Effect driven by Chiral Spin Order

A Hall effect due to the spin chirality in mesoscopic systems is investigated numerically. We consider 4‐terminal Hall systems including local spins with vortex domain wall geometry, where strong spin chirality appears near the vortex center. The Fermi energy of the conduction electrons is assumed to be comparable to the exchange coupling energy where the adiabatic approximation cannot be applied. Our results show Hall effect where voltage drop and spin current arise in transverse direction.

Spin Polarized Transport and Spin Relaxation in Quantum Wires

We give an introduction to spin dynamics in quantum wires. After a review of spin-orbit coupling (SOC) mechanisms in semiconductors, the spin diffusion equation with SOC is introduced. We discuss the particular conditions in which solutions of the spin diffusion equation with vanishing spin relaxation rates exist, where the spin density forms persistent spin helices. We give an overview of spin relaxation mechanisms, with particular emphasis on the motional narrowing mechanism in disordered conductors, the D’yakonov–Perel’ spin relaxation. The solution of the spin diffusion equation in quantum wires shows that the spin relaxation becomes diminished when reducing the wire width below the spin precession length L SO. This corresponds to an effective alignment of the spin-orbit field in quantum wires and the formation of persistent spin helices whose form as well as amplitude is a measure of the particular SOCs, the linear Rashba and the linear Dresselhaus coupling. Cubic Dresselhaus coupling is found to yield in diffusive wires an undiminished contribution to the spin relaxation rate, however. We discuss recent experimental results which confirm the reduction of the spin relaxation rate. We next review theoretical proposals for creating spin-polarized currents in a T-shape structure with Rashba-SOC. For relatively small SOC, high spin polarization can be obtained. However, the corresponding conductance has been found to be small. Due to the self-duality of the scattering matrix for a system with spin-orbit interaction, no spin polarization of the current can be obtained for single-channel transport in two-terminal devices. Therefore, one has to consider at least a conductor with three terminals. We review results showing that the amplitude of the spin polarization becomes large if the SOC is sufficiently strong. We argue that the predicted effect should be experimentally accessible in InAs. For a possible experimental realization of InAs spin filters, see [1].

Three-terminal spin filter based on spin-orbit coupling in the presence of anti-dot

We have numerically investigated the spin-dependent electronic transport in a multi-terminal conductor with an anti-dot at the junction in the presence of spin-orbit coupling. It is shown that the direction of spin polarization can be flipped by changing the Fermi energy with respect to the height of anti-dot potential in a three-terminal conductor. This is understood by the interplay between spin-orbit coupling and the curvature of the potential. The spin Hall conductance in a four-terminal conductor with an anti-dot is also investigated.

Spin Hall Current Induced By Electric Field Pulse

We have numerically investigated the spin Hall current induced by an electric field pulse in a two dimensional electron system without impurities in the presence of Rashba spin‐orbit coupling. By solving the time‐dependent Schrodinger equation, the dynamics of a wave packet has been calculated in a system subject to a time‐dependent potential that represents an electric field pulse. It is found that a spin Hall current is generated when the external electric field is switched on by tilting the potential. This spin Hall current reaches its maximum value and then decreases to zero after the electric field is switched off. The maximum value is proportional to the duration of the electric field pulse. The stationary charge current decreases with increasing strength of the spin‐orbit coupling.