Irreversibility line of monocrystalline Bi2Sr2CaCu2O8: Experimental evidence for a dimensional crossover of the vortex ensemble

Abstract

The irreversibility line ${\mathit{B}}_{\mathit{m}}$(T) of monocrystalline superconducting ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ has been measured in magnetic fields H parallel to the c axis of the crystal. For low inductions B near ${\mathit{T}}_{\mathit{c}}$, we find a parabolic temperature dependence, ${\mathit{B}}_{\mathit{m}}$≊${\mathit{B}}_0$(${\mathit{T}}_{\mathit{c}}$/T-1${)}^2$, while for larger inductions, ${\mathit{B}}_{\mathit{m}}$ grows exponentially with ${\mathit{T}}^{\mathrm{-}1}$. We argue that the two regimes reflect the three- and the quasi-two-dimensional character of the respective vortex fluctuations. In both regimes, a Lindemann-type melting criterion yields quantitative expressions for ${\mathit{B}}_{\mathit{m}}$(T) which reproduce the experimental data very well.