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We have studied the chemical potential shift in the high-temperature superconductor Bi 2 Sr 2 Ca 1Ϫx R x Cu 2 O 8ϩy (RϭPr, Er͒, where the hole concentration is varied from 0.025 to 0.17 per Cu, by precise measurements of core-level photoemission spectra. The result shows that the shift is depressed in the underdoped region as in the case of La 2Ϫx Sr x CuO 4 ͑LSCO͒ but the depression is much weaker than in LSCO. The observed shift in the present system can be relatively well explained by numerical results on the doped two-dimensional Hubbard model and suggests that the change of the electronic structure induced by hole doping is less influenced by stripe fluctuations than in LSCO.

Single crystals of insulating Bi2Sr2RCu2O8 (R = Dy, Er, Y and Ca0.5Er0.5) and superconducting Bi2Sr2Ca1-xPrxCu2O8 (x = 0, 0.10, 0.25, 0.36 and 0.42) were prepared, and the thermopower S(T) along the CuO2 plane was measured from 4.2 to 420 K. All of the thermopowers including those for the insulators are roughly expressed as A-BT above a certain temperature Tscale, where A and B are positive constants. S(T)/S(Tscale) for all of the samples falls onto a single curve as a function of T/Tscale. Since Tscale is close to the pseudogap temperature for the superconductors, this scaling offers a rough estimate of the pseudogap temperature of high-Tc superconductors. The present study further suggests that the parent insulators have a pseudogap above room temperature.

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