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Tunneling in heavy-fermion junctions

In this paper I briefly overview recent theoretical and experimental advances in understanding of tunneling processes between normal metals and metals containing electrons which occupy partially filled f-orbitals. In heavy-fermion materials the effective mass of the quasiparticles far exceeds the bare electron mass due to strong hybridization between conduction and f-orbital states. Kondo lattices form a class of heavy-fermion systems in which an average occupation number of f-electron states is close to an integer. Therefore, the tunneling into a Kondo lattice necessarily involves co-tunneling process of a tip electron into an f-electron state of a Kondo lattice. This co-tunneling process is manifested in the Fano-lineshape of differential conductance as a function of an applied voltage, which has been routinely observed in recent experiments on various Kondo lattice systems. To illustrate these ideas, I discuss the problem of the tunneling junction when the single particle states in the tip are also a product of hybridization between conduction and f-states, i.e. tunneling between two heavy-fermion materials.