First-order phase transition of the vortex lattice in twinned YBa 2 Cu 3 O 7 single crystals in tilted magnetic fields

We present an exhaustive analysis of transport measurements performed in twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ single crystals which establishes that the vortex solid-liquid transition is first order when the magnetic field H is applied at an angle $\ensuremath{\theta}$ away from the direction of the twin planes. We show that the resistive transitions are hysteretic and the $V\ensuremath{-}I$ curves are nonlinear, displaying a characteristic S shape at the melting line ${H}_{m}(T),$ which scales as $\ensuremath{\varepsilon}(\ensuremath{\theta}{)H}_{m}(T,\ensuremath{\theta}),$ where $\ensuremath{\varepsilon}(\ensuremath{\theta})$ is the anisotropy factor. These features are gradually lost when the critical point ${H}^{*}(\ensuremath{\theta})$ is approached. Above ${H}^{*}(\ensuremath{\theta})$ the $V\ensuremath{-}I$ characteristics show a linear response in the experimentally accessible $V\ensuremath{-}I$ 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 vortex-line melting in twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ crystals in contrast to a vortex sublimation as recently suggested for untwinned ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4},{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ [T. Sasagawa et al., Phys. Rev. Lett. $80,$ 4297 (1998)].