  A generalization of the Anderson model that includes pseudo-Jahn-Teller
impurity coupling is proposed to describe distortions of an endohedral impurity
in a carbon nanotube. Treating the distortion within mean-field theory,
spontaneous axial symmetry breaking is found when the vibronic coupling
strength g exceeds a critical value g$_c$. The effective potential in the
symmetry-broken state is found to have O(2) symmetry, in agreement with
numerical calculations. For metallic zigzag nanotubes endohedrally-doped with
transition metals in the dilute limit, the low-energy properties of the system
may display two-channel Kondo behavior; however, strong vibronic coupling is
seen to exponentially suppress the Kondo energy scale.
