Scaling of the superfluid density in strongly underdoped YBa2Cu3O6+y: Evidence for a Josephson phase

Recent measurements on extremely underdoped YBa2Cu3O6+y [Phys. Rev. Lett. 99, 237003 (2007)] have allowed the critical temperature T-c, superfluid density rho(s0)equivalent to rho(s)(T < T-c), and dc conductivity sigma(dc)(T greater than or similar to T-c) to be determined for a series of electronic dopings for T-c similar or equal to 3-17 K. The general scaling relation rho(s0)/8 similar or equal to 4.4 sigma T-dc(c) is observed, extending the validity of both the ab-plane and c-axis scaling an order of magnitude and creating a region of overlap. This suggests that strongly underdoped materials may constitute a Josephson phase; as the electronic doping is increased a more uniform superconducting state emerges.