Normal-state spin gap of Bi-based superconductors

The thermoelectric power (TEP) S(T) and resistivity ρ(T) were measured for underdoped and overdoped (Formula presented)(Formula presented)(Formula presented)(Formula presented)(Formula presented)(Formula presented) (n=1,2) samples with different oxygen contents. A specific heat measurement was also conducted for the n=2 system. We found that S(T) exhibited a significant enhancement below a characteristic temperature (Formula presented) for these samples, as observed in underdoped samples of (Formula presented)(Formula presented)(Formula presented) and (Formula presented)(Formula presented)(Formula presented). Such an anomaly in S(T) below (Formula presented) suggests that an energy gap exists in the normal states of underdoped and overdoped Bi-based superconductors (n=1 or 2). The normal-state gap temperature ((Formula presented)) increases with decreasing oxygen content in both n=1 and 2 systems. A downturn behavior in ρ(T) with the opening of a normal-state gap was also observed for some overdoped samples. On the other hand, the experimental data also showed that S(T) had a positive slope at high temperatures for the heavily overdoped samples of the n=1 system, while all the overdoped samples of the n=2 system had a negative slope in S(T) within the whole measured temperature range. This suggests that the (Formula presented)(Formula presented) layers are metallic and make a contribution to the TEP in the heavily overdoped region of the n=1 system, but are insulating in the whole overdoped region of the n=2 system. In addition, specific heat measurements for the n=2 system reveal that a decrease in oxygen content results in the occurrence of phase segregation.