First-order melting transition observed from resistivity measurements in ultra-pure YBa₂Cu₃O₇-_δ single crystals with high twin boundary density

R(T) measurements have been performed on optimally and overdoped heavily twinned high-purity YBa2Cu3O7-delta single crystals, under a magnetic field B oriented parallel to the twin boundary planes (B parallel to c). The characteristic feature attributed to the flux line lattice melting transition has been observed in the overdoped case for B greater than or equal to 5 T.

The first-order transition was not found in the optimally doped sample, even when B was tilted with respect to the c-axis. Differences between these samples are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.