Thermopower of high-T_c cuprates

We have studied the thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} which undergo an antiferromagnetic transition near room temperature and Bi_2Sr_{2-x}La_xCuO_{6+z} for which a broad spectrum of doping is possible. The thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} is seen to exhibit an anomaly at the Neel temperature, whereas the resistivity is not. The extrapolated zero-temperature intercept of the thermopower, which is known to be positive for hole-doped cuprate superconductors, is found to become negative for Bi_2Sr_{2-x}La_xCuO_{6+z} above some critical doping-level. The data strongly suggest that the thermopower of high-T_c cuprates contains a large amount of extra contribution in addition to the usual diffusion thermopower. We discuss origins of the extra contribution in the thermopower.