  Recent measurements on extremely-underdoped YBa2Cu3O6+y [Phys. Rev. Lett. 99,
237003 (2007)] have allowed the critical temperature (T_c), superfluid density
[rho_0 (T << T_c)] and dc conductivity [sigma_dc (T ~ T_c)] to be determined
for a series of electronic dopings for T_c ~ 3 - 17 K. The general scaling
relation rho_0/8 ~ 4.4 sigma_dc T_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 severely underdoped materials may constitute a
Josephson phase; as the electronic doping is increased a more uniform
superconducting state emerges.
