THE TUNNELING CURRENT IN HEAVY-FERMION SYSTEMS

The role of f-electron charge fluctuations in heavy-fermion systems is discussed, and the tunnelling current as a probe of charge spectral density near the Fermi level is calculated within perturbation theory using the transfer Hamiltonian approach. It is shown that in the case of integer valence systems only c electrons from a wide conduction band contribute to the tunnelling current. One finds only a weak reflection of the strong mass enhancement in the I(U) curves, but the tunnelling conductance (and its derivative) can still be used for the study of spin excitations in the liquid formed by f spins. An examination is then made of the mixed-valence-type heavy-fermion compounds. In this case f electrons dominate in the current, and in some regimes the exact results for the arbitrary Coulomb interaction between the c and f electrons are derived. It is shown that because of the 'shaking up' of the polaron cloud the f electron during the tunnelling process the current acquires a non-analytic dependence on electric potential and temperature. Both narrow f-band and c-f hybridization peaks are studied in detail.