Crossover from three-dimensional to two-dimensional vortex fluctuations in the irreversibility line of the Y1-xPrxBa2Cu3O7- delta (0

Abstract

It is shown that the temperature dependence of the irreversibility line B(T*) of the ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.97}$ (0\ensuremath{\le}x\ensuremath{\le}0.55) system, inferred from magnetoresistance measurements, can be described by a Lindemann-type model of a vortex-solid--vortex-fluid phase transition triggered by vortex fluctuations. In this model, the previously observed transition in B(T*) from a power-law temperature dependence near ${\mathit{T}}_{\mathit{c}}$ to a more rapid dependence below T*/${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}0.6 can be accounted for in terms of a crossover from three-dimensional (3D) to two-dimensional (2D) vortex fluctuations. For different x values, a lower limit for the anisotropy ratio \ensuremath{\gamma} and an upper limit for the crossover magnetic induction ${\mathit{B}}_{\mathrm{cr}}$ above which 2D vortex fluctuations dominate were determined. \textcopyright{} 1996 The American Physical Society.