Spin-charge separation and simultaneous spin and charge Kondo effect

We study the spin-charge separation in a Kondo-like model for an impurity with a spin and a charge (isospin) degree of freedom coupled to a single conduction channel (the ``spin-charge'' Kondo model). We show that the spin and charge Kondo effects can occur simultaneously at any coupling strength. In the continuum (wide-band or weak coupling) limit, the Kondo screening in each sector is independent, while at finite bandwidth and strong coupling the lattice effects lead to a renormalization of the effective Kondo exchange constants; nevertheless, universal spin and charge Kondo effects still occur. We find similar behavior in the two-impurity Anderson model with positive and negative electron-electron interaction and in the two-impurity Anderson-Holstein model with a single phonon mode. We comment on the applicability of such models to describe the conductance of deformable molecules with a local magnetic moment.