Transport Properties and Fluctuations in Underdoped GdBa2Cu3O x Thin Films

We have investigated the transport properties, resistivity and Hall effect, in a series of underdoped GdBa₂Cu₃O_xthin films grown by off-axis magnetron sputtering. We find a systematic correlation between the critical temperature T_cand the inverse Hall constantR_H^-1, related, in simple models, to the carrier concentration n. Our experimental thin film T_c(n) data are in good agreement with the temperature-doping phase diagram obtained for YBa₂Cu₃O_xsingle crystals. By measuring the activation energies in the liquid vortex phase, and by using a 2-dimensional model for vortex dynamics, we have extracted the penetration depth of these samples, and studied the relation between the carrier concentration and the superfluid density to probe the role of phase fluctuations on superconductivity.