title: Correlation between superfluid density and T(C) of underdoped YBa2Cu3O6+x near the superconductor-insulator transition.
abstract: We report measurements of the ab-plane superfluid density n(s) (magnetic penetration depth lambda) of heavily underdoped films of YBa2Cu3O6+x, with T(C)'s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO2 layers. At the lowest dopings, T(C) is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO2 bilayers. Our main finding is that T(C) is not proportional to n(s)(0); instead, we find T(C) proportional to ns(1/2.3+/-0.4). This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.
authors.affiliation: Department of Physics, Ohio State University, 191 Woodruff Avenue, Columbus, Ohio 43210, USA.
authors.forename: Yuri
authors.initials: Y
authors.lastname: Zuev
authors.forename: Mun Seog
authors.initials: MS
authors.lastname: Kim
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Lemberger