Abstract
Upper critical fields Bc2(0) have been measured in dc fields up to 12 T for YBa2Cu3O7−δ samples with 60 and 90 K superconductivity. The data yield (dBc2/dT)T=Tc =1.49 T/K for the 90 K sample and 0.35 T/K for the 60 K sample. The estimated Bc2(0) values are 98 and 15 T for the 90 and 60 K samples, respectively. The weak link effect is associated with the low Tc phase regions in the sample which are caused by shear transformation in the system.