NORMAL-STATE SPIN GAP OF BI-BASED SUPERCONDUCTORS

The thermoelectric power (TEP) S(T) and resistivity rho(T) were measur ed for underdoped and overdoped Bi1.8Pb0.2Sr2Can-1 Cu(n)Q(2n+4+delta) (n = 1,2) samples with different oxygen contents. A specific heat meas urement was also conducted for the n = 2 system. We found that S(T) ex hibited a significant enhancement below a characteristic temperature T -g for these samples, as observed in underdoped samples of YBa2Cu3O7-( delta) and YBa2CU4O8 Such an anomaly in S(T) below Tg 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 sys tems. A downturn behavior in rho(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 hi gh temperatures for the heavily overdoped samples of the n = 1 system, while all the overdoped samples of the n = 2 system had a negative sl ope 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 insulati ng in the whole overdoped region of the n = 2 system. In addition, spe cific heat measurements for the n = 2 system reveal that a decrease in oxygen content results in the occurrence of phase segregation.