Structural and Fluctuation Induced Excess Conductivity in R:1113 Superconductors

We report here the fluctuation-induced excess conductivity

in RBaSrCu3O7-δ (R=Y, Gd, Nd, and La) superconductor.

It is found that Y and Gd samples are orthorhombic,

while the Nd and La samples are quasi-tetragonal. The oxygen

content is found to be close to 7 for all R:1113 samples. The

logarithmic plots of Δσ and reduced temperature Є reveal two

different exponents corresponding to crossover temperature

due to shifting the order parameter from (2D/1D) to (3D).

The critical temperature, mean field temperature, crossover

temperature, out of plane coherence length, and interlayer

coupling are decreased as the ionic size increases. While the

in plane and effective coherence lengths, and anisotropy are

increased.We have also estimated several physical parameters

such as upper critical magnetic fields in both a-b- and c-axis

(Bab and BC), and critical current density J (0 K), and their

values are found to be decrease as the ionic size increases. The

results are discussed in terms of oxygen rearrangement, localization

of carriers, coherence lengths, and anisotropy which

are produced as the ionic size increases.