On the nature of the Peak Effect in YBCO crystals

G. PASQUINI; V. BEKERIS

Mumbay

Workshop; 12.Xth International Vortex State Studies Workshop (IVW-10); 2005

The Peak Effect  (PE) in the critical current density in both low and high temperature superconductors has been subject of a large amount of experimental and theoretical work in the last years. The origin and nature of the anomalous increase of critical current with temperature or magnetic field are still controversial issues. Experiments of neutron diffraction done in traditional superconductors show a clear change in the structure factor of the vortex lattice (VL) that is ascribed to an order-disorder transition. Recent experiments of Bitter decoration in NbSe2, however, show that the disordered VL  is  a policrystal.  On the other hand, there is strong experimental evidence that, in the case of BSCCO, the so called “second peak” in Jc(B) has its origin in a structural/dimensional transition.  In the case of YBCO, however, crucial discussions describing a dynamic or a static picture are not settled. In that region of field and temperature both the mobility of the vortex lattice (VL)  and the measured  critical current are found to be dependent on the dynamical history. In this work we explore the static and dynamic behaviour of the vortex lattice in YBCO crystals with the DC magnetic field tilted out of the twin boundaries. By performing sensitive linear and non-linear AC susceptibility measurements we compare the behaviour of the measured critical current and the effective pinning potential wells in the region of the PE. Assisted by large enough ac fields, vortices perform inter-valley motion and the non-linear penetration depth is directly related to the measured critical current density, while for the lower amplitudes, intra-valley motion dominates and the shape of the effective pinning potential well is sensed. In the first case, an anomalous decrease with temperature of the vortex mobility is clearly observed. In the second case, no evidence of  any anomaly in the temperature dependence of  the effective pinning potential curvature is observed throughout  the PE region. From our present results, the PE seems to be directly related with a sudden change in the creep rate that determines the PE anomaly in the effective critical current when inter-valley motion of vortices prevails.  Our data support a dynamic scenario for this phenomena in YBCO crystals.