Pressure dependence of the irreversibility line in Bi2Sr 2CaCu2O8+δ

One of the important problems of high-temperature superconductivity is to understand and ultimately to control fluxoid motion. We present the results of a new technique for measuring the pressure dependence of the transition to superconductivity in a diamond anvil cell. By measuring the third harmonic of the ac susceptibility, we determine the onset of irreversible flux motion. This enables us to study the effects of pressure on flux motion. The application of pressure changes interplanar spacing, and hence the interplanar coupling, without significantly disturbing the intraplanar superconductivity. Thus we are able to separate the effects of coupling from other properties that might affect the flux motion. Our results directly show the relationship between lattice spacing, effective-mass anisotropy, and the irreversibility line in Bi₂Sr₂CaCu₂O_8+δ. Our results also demonstrate that an application of 2.5 GPa pressure causes a four-fold decrease in the effective-mass anisotropy