FIELD-INDUCED SUPPRESSION OF THE VORTEX LATTICE MELTING TRANSITION INTWINNED YBA2CU3O7-DELTA

We present magneto-resistance data for a high quality, twinned YBa Cu3 O7-delta crystal, taken with the current applied along the ab plane. T he crystal was examined at a number of angles to an applied magnetic f ield, of up to 20T, in order to observe the influence of correlated an d point-like disorder on the vortex dynamics. When the applied field w as orientated at 15 degrees to the crystalline c-axis (theta=15 degree s), for fields below H=12T, we observed the kink in rho(T) associated with vortex lattice melting. We found that when the field exceeded th is value, there was a complete suppression of this kink and the rho(T) curves resembled those of crystals with extensive point disorder. Thi s suppression in melting occurs abruptly between 11T and 12T. The melt ing transition is recovered when the angle between the c-axis and the field is increased. An analysis of features around 12T in rho(T) and r ho(theta) has been performed for a number of fields and angles.