Hall effect in underdoped GdBa2Cu3O7-d thin films: evidence for a crossover line in the pseudogap regime

We report on measurements of the resistivity and Hall coefficient in underdoped GdBa2Cu3O7-d epitaxial thin films grown by off-axis magnetron sputtering. The films have been lithographically patterned allowing precise measurements of the temperature dependencies of the inverse Hall constant RH-1 and of the Hall angle theta_H. We find that RH-1 is linear in temperature between 300K and the pseudogap temperature T*, whereas cot(theta_H) displays a perfect T2 temperature dependence between typically 300 and 100K. We observe for all the samples that the temperature at which the temperature dependence of cot(theta_H) deviates from the T2 behavior is correlated to the temperature at which RH displays a peak. This characteristic temperature, found to lie between Tc and T*, does not depend markedly on the doping level and defines a new crossover line in the temperature versus doping phase diagram. We tentatively relate these findings to recent high frequency conductivity and Nernst effect experimental results, and we briefly discuss the possible consequences for competing theories for the pseudogap state of the cuprates.