Singularity in self-energy and composite fermion excitations of interacting electrons

We study the coherent excitations of a composite fermion operator f _iσ(^-1⁾niσ^̄, where f_iσ is the fermion operator for interacting electrons and n_iσ̄ is the number operator of electrons with the opposite spin. In the two-impurity Anderson model, we show that the excitation of this composite fermion has a finite spectral weight near the Fermi energy in the regime dominated by intersite spin exchange coupling where the Kondo fixed point is prevented. From scattering off this coherent composite fermion mode, the excitation of the regular fermion f_iσ develops a pseudogap and its self-energy is singular. Conversely, when the regular fermion develops Kondo resonance in the Kondo resonance regime, the excitation of the composite fermion develops a pseudogap instead. We argue that the composite fermion could develop a Fermi surface but "hidden" from charge excitations in lattice generalizations.