Failure of the empirical OCT law in the compound

We have studied the evolution of the thermoelectric power S(T) with oxygen doping of single-layered thin films and ceramics in the overall superconducting (, ) phase diagram. While the universal relation between the room temperature thermopower and the critical temperature is found to hold in the strongly overdoped region , 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.