Magnetoconductance at tunnel junction contacts with disordered granular materials

We explore the magnetoconductance that can be generated at tunnel junction contacts with disordered normal (DN) materials. We use self-assembled 1,4-butanedithiol-linked gold nanoparticle films as a prototype DN material and different types of tunnel junction contacts, namely one native to a gold electrode/1,4-butanedithiol interface and another naturally occurring at aluminum/aluminum oxide layer. For control measurements, we also study normal metal and superconducting contacts above and below critical temperature Tc in place of tunnel junction contacts. We focus on a nanoparticle film regime where contact resistance is significant, i.e. when films are metallic and have low resistance. In this regime, superconducting contacts yield a zero-field magnetoconductance enhancement due to “reflectionless tunneling” below Tc. Normal metal contacts do not yield significant magnetoconductance. This is also true for superconducting contacts above Tc when the superconductor becomes a normal metal. When a tunnel junction is present between the normal metal and the DN material, a ~10% zero-field magnetoconductance suppression appears at temperatures below ~10K. We propose a mechanism for the tunnel junction mediated magnetoconductance based on coherent back scattering in disordered granular materials, percolation and a local nature of current flow across planar tunnel junctions.