Normal-state spin gap of Bi-based superconductors

The thermoelectric power (TEP) S(T) and resistivity ρ(T) were measured for underdoped and overdoped Bi_1.8Pb_0.2Sr₂Ca_n-1Cu_nO_2n+4+δ (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 T_g for these samples, as observed in underdoped samples of YBa₂Cu₃O_7-δ and YBa₂Cu₄O₈. Such an anomaly in S(T) below T_g 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 (T_g) 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 Bi₂O₂ 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.