Superconductivity in the underdoped region of the T'-structure cuprates based on an effective two-band model

Electronic states in the underdoped (including non-doped) region of the so-called T'-structure cuprates are studied based on the d-p model and a newly proposed effective two-band model for those compounds. We find d-wave superconducting transition for both models when the charge transfer gap is small enough which is consistent with the recent experimental results for the properly reduced NCO films etc. We also show that the two-band model gives almost similar result for the superconducting transition temperature and the static spin susceptibility with those obtained in the d-p model, which implies the two-band model can give a good description of the T'-cuprates. (paper)