Pseudo fermion

We write the hamiltonian as in eq. (7.35) (section 2.6) in terms of a molecular field part and an interaction part S(f. Let m), m ), etc. denote molecular field states on a single ion, and introduce pseudo-fermion operators Ctm, Cm that create and annihilate states w> on the ion f, respectively. Then... 

Not truly a heavy-fermion system but used as endpoint in series of pseudo-ternary compounds. 

As a first step, it is important to prove the existence of hybridization gaps in heavy-fermion systems and then show that thermal excitations across this gap at elevated temperatures influence the physical properties in the observed way. The first experimental evidence of a hybridization gap has been given by Marabelli et al. (1986a) by far infrared optical reflectivity measurements at low temperatures and in more recent years this method has been extended to many other heavy-fermion compounds so that the author now believes that the hybridization (pseudo) gap is a general feature of all heavy fermions. 

A d-f hybridization model according to Brandow (1986) always yields a hybridization gap in the f quasiparticle density of states. The Fermi level can be in the gap or pseudo gap when the Luttinger theorem permits, as e.g. in SmBs, high-pressure SmS or YbBi2, or it can be in a quasiparticle band as in metallic intermediate-valent systems as YbCuAl, CePda or in heavy fermions like UPta, CeAla, CeCug, etc. Quite recently the same theoretical approach has been taken by Czycholl and Schweitzer (1992) and transport and magnetic properties of heavy fermions with a hybridization gap have been calculated in agreement with experiment. 

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