Low temperature vortex phase diagram of Bi2Sr2CaCu2O8 : a magnetic penetration depth study

We report measurements of the magnetic penetration depth \lambda_m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero-field cooled sample, results in a non-monotonic \lambda_m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.