SPIN FLUCTUATIONS AND RELAXATION OF LOCAL MOMENTS IN HEAVY-FERMION SYSTEMS

The Korringa relaxation-rate process is derived for a heavy-fermion system. The approach is based on a time-dependent perturbation theory, which avoids the use of the Ward identity and ignores possible effects due to 4f rare-earth concentration, employing instead the Keldysh formalism. The Yoshimori-Kasai Hamiltonian is used to model an electron spin resonance (ESR) experiment, and the spin relaxation rate is derived from Green functions obtained within the context of this model. The low-temperature equilibrium properties of the model are briefly discussed, and an expression for the enhanced mass is derived indicating the transition from the zero-temperature coherent state to the finite-temperature scattering state. The Korringa rate process is expressed as an integral equation, and the contribution due to the correlation of fluctuating impurity spins is identified as a resonance near the Fermi energy within the integrand of this equation. The results are compared to experimental ESR work on UBe13, CeCu2Si2 and CeAl3, and found to have good qualitative agreement. The model connects the exchange coupling parameter J to the observation of temperature-dependent non-linear behaviour of the spin relaxation rate in heavy-fermion systems.