Scaling behaviour and crossover of resistive transition in YBa 2Cu 3O 7- x

Abstract

Our resistivity measurements on YBa 2Cu 3O 7- x ceramics, in the limit I=0, confirm a second-order phase transition at a critical temperature T c(0)< T cs where T cs is the critical temperature of the superconducting phase transition. We assume a percolation phase transition as it has been proposed in usual disordered granular superconductors for the onset of a superconducting order. Moreover, we show the existence of a crossover temperature T ∗( I). For the temperatures T > T ∗( I) > T c(0), σ∼ t -γ with γ⋍0.9 and t = (T-T c(I))/T c(0) where T c( I) corresponds to the critical temperature shift due to I. This conductivity critical behaviour is identical to the one observed as T→ T c(0) + and I=0. For the temperature range T c( I)< T⪌ T ∗ ( I), σ∼ A( I) t −γ̃ where the critical exponent value γ̃⋍1.3 appears to be in good agreement with the one computed elsewhere for the pure 3D XY system. We suggest that phase fluctuations account for the excess conductivity above the percolation threshold and we propose an intergranular coherence transition at T c( I).