Inverse-proximity effect and 2D superconductivity of Ta/Au bilayer thin films

Abstract

This study investigated the inverse-proximity effect (IPE) and two-dimensional (2D) superconductivity in α-phase Ta films and Ta/Au bilayers. These bilayers were deposited onto silicon substrates using a direct current (DC) sputtering system. The findings reveal that higher substrate temperatures and increased film thickness facilitate the formation of α-Ta up to a temperature limit of 300°C, resulting in higher Tc around 3.5–3.7 K. The fundamental superconductivity of α-Ta thin films is similar to that of a conventional s-wave BCS superconductor. The IPE in the Ta/Au bilayer films was observed, where the Tc increases and decreases with the Au overlayer thickness. This behavior was quantitatively described by considering a model accounting for the reduced superconducting fluctuations induced by the highly conductive Au layer. Furthermore, the presence of 2D superconductivity in the Ta film and Ta/Au bilayer was examined through critical field analysis, vortex motions, and Berezinskii-Kosterlitz-Thouless (BKT) transition detection. Finally, the results were summarized in an H-T phase diagram illustrating the presence of IPE and 2D superconductivity in a Ta/Au bilayer. This study provides additional insights into 2D superconductivity within normal/superconductor (N/S) bilayers, laying the groundwork for future research to optimize film properties to align more closely with theoretical expectations.