Precision measurement of the in-plane penetration depth ab(T) in YBa2Cu3O7- using grain-boundary Josephson junctions

We have determined the temperature dependence of the ab-plane penetration depth ab in epitaxial YBa2Cu3O7- films by measuring the magnetic-field dependence of the critical current of YBa2Cu3O7- bicrystal grain-boundary Josephson junctions. Using this dc technique the change of the penetration depth with varying temperature has been measured between 4.2 and 60 K with a resolution of 0.2. We found a linear ab(T) dependence at temperatures below about 10 K. This linear dependence is consistent with lines nodes of the gap function.