Temperature dependence of hybridization gaps in metallic heavy-fermion systems

Abstract

There is evidence that a number of heavy-fermion/mixed-valence materials showhybridization gaps either at the Fermi energy or close to it. In the former case, aheavy-fermion semiconducting state ensues, and in the latter case, the system remainsmetallic at low temperatures. In either case, there are significant indications that theelectronic structure is extremely temperature dependent. In particular, there is evidencefrom spectroscopic and transport properties that the gap closes at high temperatures andalso that the heavy-quasiparticle bands disappear at high temperatures. Themagnitudes of the gaps scale with the effective quasiparticle masses. We present aphenomenological model that exhibits a temperature dependence which is consistentwith the above behavior. The model is based on a periodic array of Andersonimpurities in which the electron correlations are represented by the coupling to bosonswith Einstein spectra. The model can be approximately solved in a systematicmanner. The solution consists of semi-analytic expressions which represent thetemperature dependences of the coherent and incoherent structures in the electronicexcitation spectra. We shall compare the hybridization gaps predicted by thetheory for the metallic case and those inferred from photoemission experiments onUPd2Al3.