Resonant Andreev reflection in a FIDI'D' system where F is spin-polarized and DI'D' is a D-wave superconductor containing a grain boundary

Abstract

In this thesis, I present a theoretical study of the conductance characteristics between a spin-polarized material, such as ferromagnet or a half-metallic ferromagnet, and a high-Tc superconductor (assumed to be d-wave) containing a {100}|{110} grain boundary perpendicular to the current direction. Parameters in this FIDI'D' system include the spin-polarization P of F, barrier parameters Z of "I" and Z' of "I'", and thickness L of the {100} grain in the current direction (assumed shorter than the inelastic mean free path, so ballistic treatment is valid). Because of the midgap states which exist at the DI' D' interface, this system can exhibit a resonant constructive interference between the Andreev reflections at the FID and DI'D' interfaces. As a result the net Andreev reflection is always 100% at P = 0 for all Z and L, but at zero energy only, (i.e., at eV = 0) with the width of this resonance peak narrower for lager Z and/or L. If P is increased from zero, this resonance peak is gradually suppressed. This result may be useful in the device application of using Andreev reflection to measure P of F. In the usual FIS junctions, Andreev-reflection amplitudes are large for small Z only (at P = 0), and yet a large effective Z due to discontinuities of material parameters between F and S may be unavoidable. It is clear that only large effective Andreev-reflection amplitude at P = 0 allows an accurate determination of P in F.