Metastability line in the phase diagram of vortices in Bi2Sr2CaCu2O8

Nonlinear transport in the low-temperature vortex glass state of single crystal Bi2Sr2CaCu2O8 has been investigated with fast current pulses driven along the ab plane. Field cooled preparation shows a higher threshold current ~marking a jump or break in slope in the voltage response! than the zero field cooled ~ZFC! one, but it is found to be metastable and convertible to the ZFC response by a small field excursion. The metastability appears on the low temperature side of the peak in the temperature dependence of the ZFC threshold current. It is suggested that the onset of metastability signals a different thermodynamic ground state.