  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.
