Failure of the empirical OCT law in the Bi2Sr2CuO6 + δ compound

We have studied the evolution of the thermoelectric power S(T) with oxygen doping of single-layered Bi2Sr2CuO 6 + δ thin films and ceramics in the overall superconducting (Tc, S290 K) phase diagram. While the universal relation between the room temperature thermopower S290 K and the critical temperature is found to hold in the strongly overdoped region (δ > 0.14), a strong violation is observed in the underdoped part of the phase diagram. The observed behaviour is compared with other cuprates and the different scenarios are discussed.