Anderson transitions in Ln_1-xSr_xCoO₃ (Ln═Pr or Nd)

Abstract

Electrical conductivity measurements show that Ln_1-x Sr _x CoO₃, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x≈0 3. The d.c. conductivity of compositions with 0<x<0.l obeys the T−^1/4law up to 400 K and the a.c. conductivity data follow the ω^δ law with δ in the range 0.6–0.8. Surprisingly, however, δ seems to increase with increasing temperature. Thermopower measurements show clear evidence for a change in the mechanism of conduction (around 350 K) from variable range hopping to transport at E _c. All these transport properties suggest that Co⁴⁺ ions in Ln_1-x Sr _x CoO₃ (0<x<0.1), considered as moving in a lattice of Co³⁺, form a degenerate gas and that the states at the Fermi energy are Anderson localized.