Transport Anomalies of High-TcOxides above Room Temperature

Transport properties of La 2- x Sr x CuO 4 , YBa 2 Cu 3 O 6+ x and Bi 1.6 Pb 0.5 Sr 1.9- y La y Cu 1.05 O z have been studied over wide temperature regions. Drastic temperature variations of the Hall coefficients R H found in the underdoped region of the hole carriers suggest that there exists a crossover-like change of the electronic state above room temperature. Temperature dependence of other transport properties studied at high temperatures is also discussed in relation to this anomalous behavior of R H . These results elucidate the formation process of the abnormal metallic state of high- T c oxides at low temperatures, the existence of which characterizes the metal-insulator transition of the Cu-oxide system as a new-type Mott-Hubbard transition.