Seminários

There is evidence that a number of heavy-fermion/mixed-valent materials show 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. 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 magnitude of the gaps scale with effective quasiparticle masses. We present a 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. The model can be approximately solved in a systematic manner. The solution consists of semi-analytic expressions which represents the temperature-dependence 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. Furthermore, the model points to the possibility of a resonant between the bosonic modes and the gapped electronic excitations. This might be responsible for the anomalous phonon spectrum observed in alpha-uranium at the elevated temperatures (450 K).