title: Superfluid density in a highly underdoped YBa_{2}Cu_{3}O_{6+y} superconductor.
abstract: The superfluid density rho_{s}(T) identical with1/lambda;{2}(T) has been measured at 2.64 GHz in highly underdoped YBa_{2}Cu_{3}O_{6+y}, at 37 dopings with T_{c} between 3 and 17 K. Within limits set by the transition width DeltaT_{c} approximately 0.4 K, rho_{s}(T) shows no evidence of critical fluctuations as T-->T_{c}, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_{s} displays the behavior expected for a quantum phase transition in the (3+1)-dimensional XY universality class, with rho_{s0} proportional, variant(p-p_{c}), T_{c} proportional, variant(p-p_{c});{1/2}, and rho_{s}(T) proportional, variant(T_{c}-T);{1} as T-->T_{c}.
authors.affiliation: Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6.
authors.forename: D M
authors.initials: DM
authors.lastname: Broun
authors.forename: W A
authors.initials: WA
authors.lastname: Huttema
authors.forename: P J
authors.initials: PJ
authors.lastname: Turner
authors.forename: S
authors.initials: S
authors.lastname: Ozcan
authors.forename: B
authors.initials: B
authors.lastname: Morgan
authors.forename: Ruixing
authors.initials: R
authors.lastname: Liang
authors.forename: W N
authors.initials: WN
authors.lastname: Hardy
authors.forename: D A
authors.initials: DA
authors.lastname: Bonn