Short ballistic Josephson coupling in micrometer-long tantalum/graphene/tantalum junction

Abstract

Enhancing the Josephson coupling strength inside superconductor-normal-superconductor junctions facilitates the versatile applications of Josephson devices. We adopted DC magnetron sputtered tantalum film for the superconducting contact of graphene Josephson junction which resulted in reinforcing the superconducting correlation. The transport properties of the micrometer scale graphene Josephson junctions both in normal and superconducting states are investigated. The strong Josephson coupling is manifested by two transport characteristics of Josephson junction. First, the ICRN product in scale of superconducting gap energy approaches to 2, which is close to the theoretical prediction for the Josephson junction in short and ballistic regime. Second, the temperature dependence of critical current shows a good agreement with the short Josephson junction characteristics suggested by Kulik and Omel'yanchek. This strong Josephson coupling over significantly long channel length of greater than micrometer scale can be attributed to the small superconducting gap energy that leads to long superconducting correlation length in graphene. Clear features of multiple Andreev reflections up to the third order also support that Josephson coupling persists over micrometer scale.