A generalized Anderson single-impurity model with off-site Coulomb
interactions is derived from the extended three-band Hubbard model, originally
proposed to describe the physics of the copper-oxides. Using the abelian
bosonization technique and canonical transformations, an effective Hamiltonian
is derived in the strong coupling limit, which is essentially analogous to the
Toulouse limit of the ordinary Kondo problem. In this limit, the effective
Hamiltonian can be exactly solved, with a mixed valence quantum critical point
separating two different Fermi liquid phases, {\it i.e.} the Kondo phase and
the empty orbital phase. In the mixed valence quantum critical regime, the
local moment is only partially quenched and X-ray edge singularities are
generated. Around the quantum critical point, a new type of non-Fermi liquid
behavior is predicted with an extra specific heat Cimp∼T1/4 and a
singular spin-susceptibility χimp∼T−3/4. At the same time, the
effective Hamiltonian under single occupancy is transformed into a
resonant-level model, from which the correct Kondo physical properties
(specific heat, spin susceptibility, and an enhanced Wilson ratio) are easily
rederived. Finally, a brief discussion is given to relate these theoretical
results to observations in UPdxCu5−x (x=1,1.5) alloys, which show
single-impurity critical behavior consistent with our predictions.Comment: 26 pages, revtex, no figure. Some corrections have been made, but the
basic results are kept. To be published in Physical Review