Crossover from three-dimensional to two-dimensional vortex fluctuations in the irreversibility line of the Y1-xPrxBa2Cu3O7-delta (0<=x<=0.55) system

It is shown that the temperature dependence of the irreversibility line B(T*) of the Y1-xPrxBa2Cu3O6.97 (0 less than or equal to x less than or equal to 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 T-c to a more rapid dependence below T*/T-c approximate to 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 gamma and an upper limit for the crossover magnetic induction B-cr above which 2D vortex fluctuations dominate were determined.