Asymmetry of a complex gap near the Fermi level, determined from measurements of the thermopowerin La1−xCaxMn1−yFeyO3

Abstract

The thermopower, S, of La0.7Ca0.3Mn1−yFeyO3 (y = 0–0.09), measuredfor T = 25–310 K andmagnetic fields of B = 0–10 T, exhibits strong sensitivity to doping with Fe. The weak increase ofS observed in the undoped material with lowering temperature is enhanced considerably in the sample withy = 0.03 untila steep decrease takes place in the vicinity of the paramagnetic–ferromagnetic transition temperature,TC.S increases withfurther growth of y and its maximum follows the decrease ofTC from 228 K aty = 0.03 to 119 K aty = 0.09. Application of amagnetic field reduces S strongly. The experimental data are analysed quantitatively with apercolation model using results of our recent investigations of theShklovskii–Efros-like variable-range hopping conductivity in the same set ofLa1−xCaxMn1−yFeyO3 samples with evidence for a soft Coulomb gap and a rigid gap in the densityof states near the Fermi level (Laiho et al 2002 J. Phys.: Condens. Matter 148043). In the paramagnetic insulating phase the temperature dependence ofS is determined by competing contributions from two types of asymmetry of the gap: a smallshift of its centre with respect to the Fermi level and cubic non-parabolicity.Numerical values of the shift and the cubic asymmetry terms are determined.