Transport properties of an intermediate-valence model ofTl2Mn2O7

Abstract

The appearance of colossal magnetoresistance (CMR) in ${\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ has stimulated many recent studies of the pyrochlore family of compounds ${\mathrm{A}}_{2}{\mathrm{B}}_{2}{\mathrm{O}}_{7}$. The double exchange model of Zener does not describe the CMR in ${\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$, because its metallic conductivity cannot be explained by doping. Here we employ Hubbard operators to reformulate the intermediate valence model used by Ventura and Alascio [Phys. Rev. B 56, 14533 (1997)] to describe the electronic structure and transport properties of this compound. Following Foglio and Figueira [Phys. Rev. B 62, 7882, (2000)] we use approximate one-electron Green's functions to calculate the thermopower and the static and dynamic conductivity of ${\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ for several magnetic fields. A qualitative agreement was obtained with the experimental measurements of those properties. Although the agreement is far from perfect, these quantities are fairly well described by the same set of system parameters.