We study the Josephson coupling in ballistic graphene devices encapsulated with hexagonal boron nitride crystal by low temperature transport measurement. We adopted DC magnetron sputtered tantalum film for the transparent superconducting contact of graphene Josephson junction which has long superconducting correlation length to reinforce the relative Josephson coupling strength.
The transport properties of the micrometer scale graphene Josephson junctions both in normal and superconducting states are investigated. The normal transport measurement demonstrates that the ballistic transport of our graphene devices by high ballistic transmission coefficient and Fabry-Perot resonance. 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. We also investigate the short/long crossover-like behavior of Josephson coupling. The suppression of Josephson coupling strength in 2μm long device may attribute to the imperfect transmission of Cooper pair from tantalum to graphene due to the incorporated impurities during the sputtering process.