Anisotropy of magnetic vortex pinning in high-T_c superconductors (invited) (abstract)

There is no consensus as to the nature of the "irreversibility line" or the dynamic depinning transition boundary in H-T plane. The angle-resolved ac χ measurements allow us to probe the anisotropy of this boundary. We review remarkable new observations in single crystals of YBaCuO: one is the splitting of the loss peak in the out-of-phase ac susceptibility as a function of tilt angle between magnetic field and crystal axis, suggesting that vortices conform to the layered structure to form a modulated "staircase" shape. Another is the observation of a new irreversibility collapse transition in H-T plane at low fields near Tc. The new regime may indicate a separate boundary in the magnetic phase diagram, below which the pinning barriers are thermally renormalized so that pinning is significantly reduced or negligible. We discuss these observations in the context of intrinsic pinning and collective effects.