Pressure dependence of the irreversibility line in Bi2Sr2CaCu2O8+delta

One of the important problems of high-temperature superconductivity is to u nderstand 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 harmon ic of the ac susceptibility, we determine the onset of irreversible flux mo tion. This enables us to study the effects of pressure on flux motion. The application of pressure changes interplanar spacing, and hence the interpla nar coupling, without significantly disturbing the intraplanar superconduct ivity. Thus we are able to separate the effects of coupling from other prop erties that might affect the flux motion. Our results directly show the rel ationship between lattice spacing, effective-mass anisotropy, and the irrev ersibility line in Bi2Sr2CaCu2O8+delta Our results also demonstrate that an application of 2.5 GPa pressure causes a four-fold decrease in the effecti ve-mass anisotropy.