Investigation of the solid and the liquid phase of the vortex system in YBa2Cu3O7-δ single crystals

The solid and the liquid phase of the vortex system in YBa2Cu3O7-δ single crystals is investigated in this work. The solid phase is characterized by an anomalous field dependence of the current density. In single crystals with a high concentration of point-like defects the current density increases at low fields with increasing external field. To discuss different models a comparison between transport and shielding currents is made. To reduce heating at the current contacts in transport measurements a pulse method is used. Shielding currents are obtained from magnetization measurements with a commercial Vibrating Sample Magnetometer. From the comparison of both measurement methods models which are connected with a granular behavior of the sample and the relaxation model can be excluded. There exist several experimental hints that the behavior of the current density can be described by two mechanisms. The field region below the current maximum can be characterized by the theory of collective pinning and the field region above the current maximum by a plastic deformation of the vortex system. (orig.)