Effect of Disorder on the Transport Properties of the High-T_c Superconductor Nd_(2-x)Ce_xCuO_(4+δ)

摘要

The temperature dependences ρ_(ab)(T) of Nd_(2-x)Ce_xCuO_(4+δ) single crystals with 0 ≤ x ≤ 0.20 are studied and analyzed on the basis of the concepts in the theory of disordered 2D systems. The results are compared with the data obtained for other copper-oxide HTSC. It is found that a transition to the superconducting state in the optimal doping region 0.14 ≤ x ≤ 0.18 occurs only in crystals with a fairly small degree of disorder (k_Fl ≥ 2, where l is the mean free path). This transition is compatible with the weak 2D-localization mode as long as the localization radius is longer than the characteristic size of a Cooper pair. The superconducting transition temperature in the optimal doping region increases monotonically with the parameter k_Fl characterizing the degree of disorder in the crystal. The degradation of superconducting properties upon a further increase in the doping level (x > 0.18) is apparently associated with a transition from 2D to 3D conductivity in the single crystal.