Vortex phases in YBa2Cu3Oy (y=6.5) for B parallel to CuO: impact of regular and random pinning arrays

Using high angular resolution (up to 10(-4) degree) resistive measurements we studied a YBa2Cu3O6.5 single crystal for the magnetic field (B < 28 T) direction closely parallel (within the lock-in angle of 0.3degrees) to the crystal CuO-planes. Our data show that the vortices become confined between the CuO-planes below a well-defined temperature T-cf. Using a pseudo-flux transformer geometry for electrical transport measurements we demonstrate directly a smectic-like vortex movement for temperatures below T-S. Our measurements reveal a rich phase diagram for magnetic field orientation Bparallel toCuO, originating from the inter-play between pinning by the CuO-planes, point-like disorder from oxygen vacancies and temperature fluctuations. Commensurability between the inter-vortex distance and the CuO-layer period causes a pronounced oscillatory behaviour dominated by in-plane vortex movement. (C) 2004 Elsevier B.V. All rights reserved