Anomalous proximity effect in underdoped YBa2Cu3O6+x Josephson junctions

Josephson junctions were photogenerated in underdoped thin films of the YBa 2Cu3O6+x family using a near-field scanning optical microscope. The observa tion of the Josephson effect for separations as large as 100 nm between two wires;indicates the existence of an anomalously large proximity effect and shows that the underdoped insulating material in the gap of the junction i s readily perturbed into the superconducting state. The critical current of the junctions was found to be consistent with the conventional Josephson r elationship. This result appears to constrain the applicability of SO(5) th eory to explain the phase diagram of high critical temperature superconduct ors.