Heavy Fermion systems

G. Aeppli and C. Broholm, Magnetic correlations in heavy-fermion systems neutron scattering from single crystals 123... 

A.J. Arko, P.S. Riseborough, A.B. Andrews, J.J. Joyce, A.N. Tahvildar-Zadeh and M. Jarrell, Photoelectron spectroscopy in heavy fermion systems Emphasis on single crystals 265... 

Typical for heavy-fermion systems is the large value for re that can take values which are several orders of magnitude larger than what is found in normal metals. It implies that the effective mass has a large volume dependence as does the effective Fermi temperature. 

Physics described by the model with so many parameters is very rich and the model is able particularly to treat heavy fermion systems. To study the model many approaches were suggested (see reviews [2-5]). They are successful for particular regions of the parameter space but no one is totally universal. In this paper we apply to PAM the generating functional approach (GFA) developed first by Kadanoff and Baym [6] for conventional systems and generalized for strongly correlated electron systems [7-10]. In particular it has been applied to the Hubbard model with arbitrary U in the X-operators formalism [10]. The approach makes it possible to derive equations for the electron Green s function (GF) in terms of variational derivatives with respect to fluctuating fields. 

The first example of a heavy electron system CeAl3, is a good material to study the heavy fermion systems because it presents an extreme case of these properties (it does not become superconducting at low temperatures). Then it should be useful to study the basic properties of these materials. 

Heavy fermion systems becoming superconductors at low temperatures, CeCu2Si2, > U2PtC2. 

Heavy fermion systems with a magnetic low temperature ground state UCdii, U2Zn y. 

At this point we want to stress that the heavy fermion systems (22) are related to a very large spin enhanced susceptibility or localized magnetic moments, very narrow bands of elementary excitations at the Fermi level, and a new type of pairing, in the case where they become superconducting, at low temperatures even if they were spin fluctuators above the critical superconducting transition temperature Tg. 

Measured values of n are as large as 100-200 in heavy fermion systems (17). 

Here A% and Aorb are the hyperfme field due to spin and orbital angular momentum, Xs and Xvv are the spin and Van Vleck susceptibility, respectively, and pe is the Bohr magneton. In a heavy fermion system, where 4/ or 5/ electrons play principal roles, the state is specified by j = l +. 7, so that Ks is associated with the ground state (not purely due to spin and temperature dependent in general). AVv is induced from the transition between the ground state and the excited state, and is temperature independent. Each of As and Aorb includes the spin and orbital hyperfme fields. 

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