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

The thermopower, S , of La 0.7 Ca 0.3 Mn 1− y Fe y O 3 ( y = 0–0.09), measured for T = 25–310 K and magnetic fields of B = 0–10 T, exhibits strong sensitivity to doping with Fe. The weak increase of S observed in the undoped material with lowering temperature is enhanced considerably in the sample with y = 0.03 until a steep decrease takes place in the vicinity of the paramagnetic–ferromagnetic transition temperature, T C . S increases with further growth of y and its maximum follows the decrease of T C from 228 K at y = 0.03 to 119 K at y = 0.09. Application of a magnetic field reduces S strongly. The experimental data are analysed quantitatively with a percolation model using results of our recent investigations of the Shklovskii–Efros-like variable-range hopping conductivity in the same set of La 1− x Ca x Mn 1− y Fe y O 3 samples with evidence for a soft Coulomb gap and a rigid gap in the density of states near the Fermi level (Laiho et al 2002 J. Phys.: Condens. Matter 14 8043). In the paramagnetic insulating phase the temperature dependence of S is determined by competing contributions from two types of asymmetry of the gap: a small shift 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.