Two-dimensional hole localization induced by Zn, Ni, and Mg dopings in Cu sites in La_1.85Sr_0.15Cu_1-xM_xO_y

Samples of La_1.85Sr_0.15Cu_1-xM_xO _y(M=Ni, Zn, and Mg) with a wide range of dopant concentration (0≤x≤0.30) were synthesized. X-ray diffraction analysis shows that the Zn doping results in a tetragonal-orthorhombic transition as x≥0.15, while both Ni- and Mg-doped samples still remain tetragonal up to x=0.3. Furthermore, the Ni, Zn, and Mg dopings all reduce the local Jahn-Teller distortion of the CuO₆ octahedron in a similar way. The metal-insulator transition at the region of higher doping level (x>0.1) is observed in all three doped systems. The observed metal-insulator transition can be well interpreted in the context of Anderson's theory of disorder-induced localization. For most of the semiconducting-like samples with higher doping level (x>0.1), the conductive behavior is dominated by two-dimensional variable-range-hopping (2D-VRH) with ρ(T)∝e^(T0/T)1/3. This suggests that all the dopings of Ni, Zn, and Mg in Cu sites cause the localization of the holes. In addition, a remarkable difference in the room-temperature resistivity for the heavily doped samples with the same dopant (Zn, Ni, Mg) concentrations is also observed. A possible explanation is provided for this phenomenon.