Deviations from Matthiessen rule and resistivity saturation effects in Gd and Fe

摘要

According to earlier first-principles calculations, the spin-disordercontribution to the resistivity of rare-earth metals in the paramagnetic stateis strongly underestimated if Matthiessen's rule is assumed to hold. Tounderstand this discrepancy, the resistivity of paramagnetic Fe and Gd isevaluated by taking into account both spin and phonon disorder. Calculationsare performed using the supercell approach within the linear muffin-tin orbitalmethod. Phonon disorder is modeled by introducing random displacements of theatomic nuclei, and the results are compared with the case of fictitiousAnderson disorder. In both cases the resistivity shows a nonlinear dependenceon the square of the disorder potential, which is interpreted as a resistivitysaturation effect. This effect is much stronger in Gd than in Fe. Thenon-linearity makes the phonon and spin-disorder contributions to theresistivity non-additive, and the standard procedure of extracting thespin-disorder resistivity by extrapolation from high temperatures becomesambiguous. An "apparent" spin-disorder resistivity obtained through suchextrapolation is in much better agreement with experiment compared to theresults obtained by considering only spin disorder. By analyzing the spectralfunction of the paramagnetic Gd in the presence of Anderson disorder, theresistivity saturation is explained by the collapse of a large area of theFermi surface due to the disorder-induced mixing between the electronic andhole sheets.