Temperature-dependent hybridization gaps: A cause of phonon anomalies in f electron systems?

There is evidence that a number of heavy-fermion/mixed-valent materials show strongly renormalized hybridization gaps either at the Fermi-energy or close to the Fermi-energy. In the former case, a heavy-fermion semiconducting state ensues and in the later case, the system remains metallic at low temperatures. Due to the temperature-dependence of the electronic correlations, the magnitudes of the hybridization gaps decrease with increasing temperatures. The existence of a temperature-dependent low-energy electronic energy scale opens up the possibility that the Born-Oppenheimer approximation may fail and that there may be a resonant coupling between the phonons and the electronic excitations. It is argued that such a mechanism may be the cause of the anomalous phonon mode observed in alpha-uranium at high temperatures.