Spontaneous Spin Polarization due to Lateral Spin Orbit Coupling in InAs Quantum Point Contacts
This dissertation reports the first experimental observation of spontaneous spin polarization due to lateral spin orbit coupling (LSOC) in side- gated (SG) quantum point contacts (QPCs). The QPC devices are fabricated on InAs/InGaAs quantum well structures using e-beam lithography. The low band gap InAs semiconductor was chosen because of its large intrinsic spin-orbit coupling. The side gates are realized by wet etching technique which is optimized to pattern the QPC devices. The width of the QPC is varied from 200 nm to 500 nm, while the length of the QPC is kept in the range 150-200 nm. The gradient in the lateral potential confinement in a side gated (SG) quantum point contact (QPC) causes a spin-orbit coupling (SOC). This LSOC induces a spontaneous spin polarization of opposite nature at the two edges of the QPC in the absence of any applied magnetic field. We have observed an anomalous conductance plateau at G @ 0.5 (2e2/h) (0.5 structure) in the SG QPCs fabricated on InAs/InGaAs QW structures. The 0.5 structure moves up in perpendicular magnetic field and approaches the normal conduction quantization at G = (2e2/h) in high magnetic field, whereas in-plane magnetic field has no effect on it. The evolution in magnetic field clearly indicates LSOC is responsible for the 0.5 structure. We believe it is the asymmetry in the confining potential of the QPC that leads to a net spin polarization giving the 0.5 structure. By electrically modulating the asymmetry of the QPC confinement, we have succeeded in making this structure appear and disappear. Such a QPC can conceivably be used as a spin polarizer or detector on demand by tuning the gate voltages. We also have proposed a dual-QPC device to experimentally validate the spin polarization by electrical means.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:spin polarization orbit coupling quantum point contact
Date of Publication:01/01/2007