Variable-range hopping conductivity and absence of a true metal–insulator transition in La[sub 0.7-δ] Ca[sub 0.3]Mn[sub 1-y]Fe[sub y]O[sub 3].

Resistivity of La[sub 0.7-δ]Ca[sub 0.3]Mn[sub 1-y]Fe[sub y]O[sub 3] (LCMFO) with y = 0-0.05, δ = 0 and 0.017 is investigated between 4.2-350 K, showing that La vacancies increase the effect of Fe doping by influencing electronic states through additional microscope disorder. The Shklovskii-Efros variable-range hopping conductivity, governed by generation of a soft parabolic gap in the spectrum of localized states due to Coulomb interaction between charge carriers, is observed both above and well below the ferromagnetic-to-paramagnetic transition temperature. Our results give evidence for absence of a true metal-insulator transition both in undoped material and in LCMFO. [ABSTRACT FROM AUTHOR]