Vortex lattice melting and Hc2 in underdoped YBa2Cu3Oy

Vortices in a type-II superconductor form a lattice structure that melts when the thermal displacement of the vortices is an appreciable fraction of the distance between vortices. In an anisotropic high-T-c superconductor, such as YBa2Cu3Oy, the magnetic field value where this melting occurs can be much lower than the mean-field critical field H-c2. We examine this melting transition in YBa2Cu3Oy with oxygen content y from 6.45 to 6.92, and we perform a quantitative analysis of this transition in the cuprates by fitting the data to a theory of vortex-lattice melting. The quality of the fits indicates that the transition to a resistive state is indeed the vortex lattice melting transition, with the shape of the melting curves being consistent with the known change in penetration depth anisotropy from underdoped to optimally doped YBa2Cu3Oy. We establish these fits as a valid technique for finding H-c2(T = 0) from higher-temperature data when the experimentally accessible fields are not sufficient to melt the lattice at zero temperature (near optimal doping). From the fits we extract Hc2(T = 0) as a function of hole doping. The unusual doping dependence of H-c2(T = 0) points to some form of electronic order competing with superconductivity around 0.12 hole doping.