Mobility of solid vortex matter in 'shaking' ac magnetic fields of variable amplitude

The vortex solid in high temperature superconductors exhibits several regimes and dynamical behaviors. A temporarily symmetric magnetic ac field (e.g. sinusoidal, square, triangular) can increase the vortex lattice mobility and a temporarily asymmetric one (e.g. sawtooth) can decrease it. In this work, we study the effect on the mobility of the vortex solid as a function of the amplitude of an ac symmetric 'shaking' field when it is applied to previously prepared high and low mobility configurations. This study was carried out in high quality twinned YBCO single crystals and vortex mobility was studied through ac susceptibility measurements