Anomalous Proximity Effect in UnderdopedYBa2Cu3O6+xJosephson Junctions

Josephson junctions were photogenerated in underdoped thin films of the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ family using a near-field scanning optical microscope. The observation 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 is readily perturbed into the superconducting state. The critical current of the junctions was found to be consistent with the conventional Josephson relationship. This result appears to constrain the applicability of SO(5) theory to explain the phase diagram of high critical temperature superconductors.