Normal-state spin gap of Bi-based superconductors

Abstract

The thermoelectric power (TEP) S(T) and resistivity {rho}(T) were measured for underdoped and overdoped Bi{sub 1.8}Pb{sub 0.2}Sr{sub 2}Ca{sub n{minus}1}Cu{sub n}O{sub 2n+4+{delta}} (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{sub g} for these samples, as observed in underdoped samples of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and YBa{sub 2}Cu{sub 4}O{sub 8}. Such an anomaly in S(T) below T{sub 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{sub g}) increases with decreasing oxygen content in both n=1 and 2 systems. 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 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 Bi{sub 2}O{sub 2} layers are metallic and make a contribution to the TEP in themore » 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. {copyright} {ital 1997} {ital The American Physical Society}« less