Resonance Model for Pair Correlations in Heavy Fermion Systems

The free energy and static spin-correlation function of a two site resonance model akin to the single site Schotte model are calculated. Thermodynamic functions are discussed and model parameters are linked to those of the microscopic two site Kondo model. Introduction Free energy and thermodynamics Single site resonance (SSR-) models [I, 21 are employed to generate analytical expressions for the thermodynamic functions of HF metals. The unique energy scale A = keTK, appearing in these models represents the strong coupling scale of the microscopic Kondo impurity problem. Since the exact solution of the latter (as far as free energy is concerned) the excellent semi-quantitative virtues of this SSRphenomenology have become even more evident. We introduce, at present, the simplest two site resonance (TSR-) model in order to cope with short ranged interactions in the paramagneti~ phase of HF compounds. The competition between a magnetic order parameter and Kondo effect has already been treated within SSR + molecular field [3]. In contrast to this, the TSR-model incorporates interaction without breaking the symmetry. The Hamiltonian is -PZ = wS1S2 + E (Sf + S;) P = (~BT)' . (1) A Heisenberg interaction W = w/p competes with a resonance p (E) in the total angular momentum channel j = S1 + S2 of two quantum spins S = 112. The resonant distribution p (E) = x-ld6 (el@) Ids is derived from a phase shift 6 ( E ) , the energy dependence of which is: