Observation of vortex fluctuations in the cuprate superconductor ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

Using a micro-Hall probe, measurements of field of first flux penetration ${\mathit{H}}_{\mathit{p}}$ and the critical current density ${\mathit{J}}_{\mathit{c}}$ have been made near ${\mathit{T}}_{\mathit{c}}$ of superconducting ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. In the temperature region just below ${\mathit{T}}_{\mathit{c}}$=86.0 K; ${\mathit{T}}_{\mathit{s}}$<T<${\mathit{T}}_{\mathit{c}}$, it was found that ${\mathit{H}}_{\mathit{p}}$ and ${\mathit{J}}_{\mathit{c}}$ are strongly suppressed, leaving a reversible diamagnetic susceptibility. No similar effect is observed n ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ which suggests that the highly two-dimensional character of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ is responsible for its particular behavior. The results are compared with recent theoretical predictions.