We have observed very strong suppression of the transition to zero resistan
ce by a magnetic field in underdoped YBa2Cu3Oy Single crystals (T-c = 60 K)
. It follows from the comparison of transport and magnetic data that the ma
in part of the resistivity tail corresponds to the vortex liquid state. We
demonstrate that resistance of the vortex liquid cannot be described using
a simple Bardeen-Stephen model. (C) 2000 Elsevier Science B.V. All rights r
eserved.