Nanoscale Ferromagnet-Superconductor Devices for Detection of Crossed Andreev Reflection

Andreev reflection (AR) is a charge transfer process occurring at a normal metal-superconductor interface whereby an incident electron (hole) of energy less than the superconducting gap may be converted to supercurrent with reflection of a hole (electron) of opposite momentum and spin. Recent interest has focused on nonlocal or crossed Andreev reflection (CAR) in a nanoscale device consisting of two normal metal leads contacting a superconductor with spatial separation of the order of the superconducting coherence length. CAR presents a possible source of solid state quantum entanglement, via the reflection of a hole (electron) produced by AR of an electron (hole) at one N/S junction at the second, forming an entangled Andreev pair and equivalent to transmission of two opposite spin quasiparticles into the separate leads. By use of ferromagnetic leads strongly spin dependent CAR and competing electron cotunelling (EC) may be controlled in a practical device. Based on design studies, micromagnetic simulation and computer modeling we have fabricated such nanoscale devices consisting of sputter deposited Fe or Co leads patterned by electron beam lithography ranging in size from 100 nm-3 mum, with in-plane separation of 100-300 nm, in point contact with an optically patterned 5 mum Al superconducting electrode. These devices were cooled to a minimum of 500 mK using an adiabatic demagnetization refrigerator to detect nonlocal voltages characteristic of CAR and differentiable under applied magnetic field from competing EC. Here we present our initial results.