Fluctuation-enhanced conductivity in YBa2Cu2Ox (6.5 <= x <= 6.95) and Tl2Ba2Ca2Cu3O10 +/-delta

The fluctuation-induced conductivity (Delta sigma(T)) near T-c of a sole YB a2Cu3Ox (YBCO) film with various precisely controlled oxygen contents (6.5 less than or equal to x less than or equal to 6.95) was studied and compare d with those obtained from its Tl-based counterparts. For YBCO films with x > 6.7, Delta sigma (T) displays a distinct 3D to 2D crossover as the tempe rature approaches T-c. On the other hand, as T -->T-c, Delta sigma (T) of b oth Tl-based and YBCO film with x less than or equal to 6.7 exhibits a reve rsed 2D to 3D crossover. It is suggestive that the coupling between the CuO 2 layers may have changed significantly with reducing oxygen. (C) 2000 Else vier Science B.V. All rights reserved.