Resistive transition and upper critical field in underdoped YBa2Cu3O6+x single crystals

A superconducting transition in the temperature dependence of the ab-plane resistivity of underdoped YBa2Cu3O6+x crystals in the range T(c)less than or similar to 30 K has been investigated. Unlike the case of samples with the optimal level of doping, the transition width increased insignificantly with magnetic field, and in the range T less than or similar to 13 K it decreased with increasing magnetic field. The transition point T-c(B) was determined by analyzing the fluctuation conductivity. The curves of B-c2(T) measured in the region T/T(c)greater than or similar to 0.1 did not show a tendency to saturation and had a positive second derivative everywhere, including the immediate neighborhood of T-c. The only difference among the curves of B-c2 (T) for different crystal states is the scales of T and B, so they can be described in terms of a universal function, which fairly closely follows Alexandrov's model of boson superconductivity. (C) 1999 American Institute of Physics. [S1063-7761(99)02301-X].