Superconducting transition properties of grain boundaries in MgB2 films

Abstract

The intergrain interface of unconventional superconductors suppresses the order parameter and, thus, serves as a bottleneck for the supercurrent. Such a characteristic of the grain interface is a major limiting factor for the lossless-current-carrying capacity of superconducting transmission lines on the one hand, but can also provide a clue to the development of Josephson elements on the other. We investigated the critical transition properties of the MgB2 intergrain nanobridges patterned by using a focused ion beam. The nanobridge containing a single intergrain interface had nominal dimensions of 200 E 300 nm in width and length. We studied the current-voltage characteristics, the temperature-dependent critical current, and the normal state resistance. The measured MgB2 interface properties showed a wide spectrum of the coupling strength, ranging from a weak Josephson coupling to a very strong coupling comparable to an intragranular one. The origins of such a variety of intergrain coupling strengths were analyzed in conjunction with the lattice structure at the grain interface.