Electron–boson coupling in superconductors studied by a self-formed nanofilament device

Abstract

A novel self-textured nanofilament technique based on the resistive-switching effect in nano-scale transition-metal oxide films has been developed. While traditional tunneling and point-contact approaches to the electron–boson interaction spectroscopy in a strong-coupled superconductor are realized on different samples, the novel method allows one to get related data on the same device. Conductance traces of S/I/Pt junctions with two superconductors of interest (S = Nb and MgB2), a platinum counter-electrode and a double-oxide interlayer (I = Al2O3/TiO2) with a controllable interface transparency reveal superconducting-gap and phonon-induced features which disappear above the superconducting-to-normal state transition. In both cases, we have observed BCS gap features with comparatively strong broadening. In MgB2-based samples, nonlinearities in differential conductance-vs-voltage characteristics far above the energy gap were detected in two modes, a point-contact regime without any significant barrier at the superconductor/Pt interface and the tunneling one. In Nb-based junctions we have succeeded to realize only the first regime. The observed features arise at energies corresponding to maximums in the phonon density of states of niobium and magnesium diboride. Their amplitude and general shape agree with theoretical predictions of the electron–phonon coupling effect in the superconductors studied. We argue that the proposed nanoscale methodology provides a simple and promising way for studying an interaction responsible for Cooper pairing in superconducting materials.