Anomalous dependence of the dissipation upon magnetic field angle in layered high T-c superconductors

We have investigated the dependence of the electrical dissipation upon the magnetic field angle in narrow single-crystal bismuth strontium calcium copper oxide (BSCCO) whiskers of the 2:2:1:2 phase. In the ohmic flux flow regime, which occurs at lower currents and for temperatures close to T-co, we find unusual angular dependences. As the magnetic field angle is varied in the be-plane close to H parallel to b we observe a single minimum in the resistance, two minima with a central maximum, three minima and two maxima, or a single minimum with shoulders, depending upon the temperature. Scaling shows that these features depend only upon the magnitude of the magnetic field component parallel to the e-axis. We propose the following explanation. At very low fields the dissipation is determined by thermally excited pancake vortices. As an applied c-axis field is increased vortices enter in the very mobile vortex gas phase and the dissipation rapidly increases. As the field is further increased the vortices enter the solid phase and the dissipation falls due to reduced mobility. At far higher fields the dissipation increases with increasing vortex numbers.