The temperature dependence of the in-plane resistivity ρ(T) is measured on epitaxial c-axis oriented single-layer Bi2Sr1.6La0.4CuOythin films at various oxygen concentrations. By successive annealing treatments, the oxygen content of the same film is changed from maximally overdoped to strongly underdoped non-superconducting state, passing through the optimal state with Tcmax= 30 K. The underdoped states show a downturn of the resistivity from the high T-linear behavior below a characteristic temperature T*, signature of the pseudogap effect. T* appears near optimally doped state and increases sharply with decreasing carrier concentration. Two other characteristic temperatures are observed in ρ(T) for underdoped states: the temperature TIof the inflection point in ρ(T) (TIto approximately 0.5 T*) and the temperature TMcorresponding to the onset of localization effects. A phase diagram T versus doping is established.
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