First-order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields

We present an exhaustive analysis of transport measurements performed in tw inned YBa2Cu3O7 single crystals which establishes that the vortex solid-liq uid transition is first order when the magnetic held H is applied at an ang le theta away from the direction of the twin planes. We show that the resis tive transitions are hysteretic and the V-I curves are nonlinear, displayin g a characteristic S shape at the melting line H-m(T), which scales as epsi lon(theta)H-m(T,theta), where epsilon(theta) is the anisotropy factor. Thes e features are gradually lost when the critical point H*(theta) is approach ed. Above H*(theta) the V-I characteristics show a linear response in the e xperimentally accessible V-l window, and the transition becomes reversible. Finally we show that the first order phase transition takes place between a highly correlated Vortex liquid in the field direction and a solid state of unknown symmetry. As a consequence, our data support the scenario for a ver-tex-line melting in twinned YBa2Cu3O7 crystals in contrast to a vortex sublimation as recently suggested for untwinned La2-xSrxCuO4, YBa2Cu3O7 and Bi2Sr2CaCu2O8 [T. Sasagawa et al., Phys. Rev. Lett. 80, 4297 (1998)].