Since spin and charge are both carried by electrons (or holes) in a solid, it is natural to assume that charge and spin diffusion coefficients will be the same. Drift-diffusion models of spin transport typically assume so. Here, we show analytically
that the two diffusion coefficients can be vastly different in quantum wires. Although we do not consider quantum wells or bulk systems, it is likely that the two coefficients will be different in those systems as well. Thus, it is important to distinguish between them in transport models, particularly those applied to quantum wire based devices
We propose and analyze a novel dual-gate Spin Field Effect Transistor (SpinFET) with half-metallic ferromagnetic source and drain contacts. The transistor has two gate pads that can be biased independently. It can be switched ON or OFF with a few mV
change in the differential bias between the two pads, resulting in extremely low dynamic power dissipation during switching. The ratio of ON to OFF conductance remains fairly large (~ 60) up to a temperature of 10 K. This device also has excellent inverter characteristics, making it attractive for applications in low power and high density Boolean logic circuits.
We show that a Spin Field Effect Transistor, realized with a semiconductor quantum wire channel sandwiched between half-metallic ferromagnetic contacts, can have Fano resonances in the transmission spectrum. These resonances appear because the ferrom
agnets are half-metallic, so that the Fermi level can be placed above the majority but below the minority spin band. In that case, the majority spins will be propagating, but the minority spins will be evanescent. At low temperatures, the Fano resonances can be exploited to implement a digital binary switch that can be turned on or off with a very small gate voltage swing of few tens of microvolts, leading to extremely small dynamic power dissipation during switching. An array of 500,000 x 500,000 such transistors can detect ultrasmall changes in a magnetic field with a sensitivity of 1 femto-Tesla/sqrt{Hz}, if each transistor is biased near a Fano resonance.
