New Aspects of Quasi-Kondo Physics: Two-Level Kondo and Strongly Coupled Local Electron–Phonon Systems

New aspects of the two-level Kondo (TLK) effect and the strongly coupled local electron–phonon (LEP) system are studied by the Wilson numerical renormalization group method. It turns out that there exist strong-coupling fixed points for both systems which can be mapped on the antiferromagnetic pseudo-spin sd-model, if the bare coupling is chosen moderately strong. In the case of TLK model, it is found that there exsist two different strong-coupling regions one of which is found for the first time, and that there exists a zero-coupling region as well. In the case of LEP system, in which three low-lying phonon levels are taken into account, it is found that there exist a strong-coupling fixed point which corresponds to the effective potential of ionic motion with double-well character. An analysis of LEP system based on the poorman's scaling with Lee algebra of SU(3) is also discussed.