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Abstract

We study the electronic part of the thermal conductivity kappa of
metals. We present two methods for calculating kappa, a quantum
Monte-Carlo method and a method where the phonons but not the electrons
are treated semiclassically (SC). We compare the two methods for a
model of alkali-doped C-60, A(3)C(60), and show that they agree well.
We then mainly use the SC method, which is simpler and easier to
interpret. We perform SC calculations for Nb for large temperatures T
and find that kappa increases with T as kappa(T)=a+bT, where a and b
are constants, consistent with a saturation of the mean free path, l,
and in good agreement with experiment. In contrast, we find that for
A(3)C(60), kappa(T) decreases with T for very large T. We discuss
qualitatively the reason for this in the limit of large T. We give a
quantum-mechanical explanation of the saturation of l for Nb and derive
the Wiedemann-Franz law in the limit of T < W, where W is the
bandwidth. In contrast, due to the small W of A(3)C(60), the assumption
T < W can be violated. We show that this leads to kappa(T)similar to
T-3/2 for very large T and a strong violation of the Wiedemann-Franz
law.