Thermopower of overdoped Ba:KBiO and Ni-substituted 214 cuprates

We have measured the thermoelectric power and resistivity of two different superconductor series as the doping level is increased into the overdoped regime where T _c decreases below its maximum value: Ba _1-xK _xBiO ₃ single crystals with K content up to x=0.55, and La _2-xSr _xCu _1-yNi _yO _4-δ ceramics with Sr content up to x=0.57. For overdoped samples in both series, thermopowers that are small and negative are found. In overdoped BaKBiO, the thermopower is wholly negative from T _c up to the limit of measurement near room temperature, and the temperature coefficient of thermopower is also negative. All the thermopower versus temperature plots for BaKBiO show a characteristic bend around 150 K, with a smaller temperature coefficient above 150 K. As Ni is substituted for Cu in the 214 cuprate series, there is a substantial increase in the thermopower for Sr concentration x=0.15, but no such increase is seen in the overdoped samples x∼0.5.