Temperature dependence of hybridization gaps in metallic heavy-fermion systems.

There is evidence that a number of heavy-fermion/mixed-valence materials show hybridization gaps either at the Fermi energy or close to it. In the former case, a heavy-fermion semiconducting state ensues, and in the latter case, the system remains metallic at low temperatures. In either case, there are significant indications that the electronic structure is extremely temperature dependent. In particular, there is evidence from spectroscopic and transport properties that the gap closes at high temperatures and also that the heavy-quasiparticle bands disappear at high temperatures. The magnitudes of the gaps scale with the effective quasiparticle masses. We present a phenomenological model that exhibits a temperature dependence which is consistent with the above behavior. The model is based on a periodic array of Anderson impurities in which the electron correlations are represented by the coupling to bosons with Einstein spectra. The model can be approximately solved in a systematic manner. The solution consists of semi-analytic expressions which represent the temperature dependences of the coherent and incoherent structures in the electronic excitation spectra. We shall compare the hybridization gaps predicted by the theory for the metallic case and those inferred from photoemission experiments on UPd2Al3.