Fluctuation contribution to the diamagnetic susceptibility of YBa2Cu3O7-γ

Thermodynamic fluctuations above the transition temperature make a diamagnetic contribution to the diamagnetic susceptibility of a superconductor. The contribution can yield information on the number of components of the order parameter. The magnetic susceptibility of a powder sample of YBa2Cu3O7-γ has been measured with a Quantum Design Squid magnetometer at close intervals of temperature above the superconducting transition temperature. The background paramagnetic contribution to the susceptibility is varied by giving the same sample different post-annealing treatments. The susceptibility over the entire temperature range is fitted to the formula χ = χ0 + b/T + χfl, where the first two terms represent the background paramagnetic contribution and the third term the fluctuation contribution χfl. The fluctuation term is represented by a Lawrence-Doniach type of expression - CT/[ε (ε + Δ)]1/2. Here ε = ln(T/Tm) and Δ =[2 ξc(0)/d]2, where Tm is the mean-field superconducting transition temperature, ε (0) is the coherence length along the c axis and d the spacing between adjacent superconducting layers. The results with four runs with varying treatments of the sample, and hence varying background paramagnetic susceptibility, yielded nearly the same parameters for the fluctuation contribution. The coherence lengths εc(0) and εab(0) estimated from the data, assuming a single component order parameters, were found to be in agreement with estimates from other types of measurements. (author). 8 refs, 6 figs, 2 tabs