First order-like out-of-plane resistive transition in YBa2Cu3O7-delta for B exactly parallel to ab-planes

Melting transition of a flux line lattice//ab-planes has been studied ill t winned YBa2Cu3O7-delta single crystals using transport measurements along t he c-axis. In contrast to previously studied geometry with field and curren t in planes (W. K. Kwok et al., Phys. Rev, Lett. 72, 1088 (1994)), a step-l ike c-axis resistive transition has been found for exact field alignment al ong ab-planes. The angular dependence of rho(c), with rotation of the cryst al around the c-axis was studied. The results show that below the resistive step, the pinning force is anomalously increased when the field is aligned along ac(bc)-crystal facets.