Optical spectral weights and the ferromagnetic transition temperature of colossal-magnetoresistance manganites: Relevance of double exchange to real materials

We present a detailed quantitative comparison of double-exchange models to experimental data on the colossal magnetoresistance manganese perovskites. Our results settle a controversy by showing that physics beyond double-exchange is important even in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3,$ which has been regarded as a conventional double-exchange system. We show that the crucial quantity for comparisons of different calculations to each other and to data is the conduction band kinetic energy K, which is insensitive to the details of the band structure and can be experimentally determined from optical conductivity measurements. The seemingly complicated dependence of $T_c$ on the Hund’s coupling J and carrier concentration n is shown to reflect the variation of K with J, n and temperature. We present results for the optical conductivity which allow interpretation of experiments and show that a feature previously interpreted in terms of the Hund’s coupling was misidentified. We also correct minor errors in the phase diagram presented in previous work.