Superconductivity from undressing. II. Single-particle Green’s function and photoemission in cuprates

Experimental evidence indicates that the superconducting transition in high-$T_c$ cuprates is an “undressing” transition. Microscopic mechanisms giving rise to this physics were discussed in the first paper of this series. Here we discuss the calculation of the single-particle Green’s function and spectral function for Hamiltonians describing undressing transitions in the normal and superconducting states. A single parameter ${\rm Y}$ describes the strength of the undressing process, and drives the transition to superconductivity. In the normal state, the spectral function evolves from predominantly incoherent to partly coherent as the hole concentration increases. In the superconducting state, the “normal” Green’s function acquires a contribution from the anomalous Green’s function when ${\rm Y}$ is nonzero; the resulting contribution to the spectral function is positive for hole extraction, and negative for hole injection. It is proposed that these results explain the observation of sharp quasiparticle states in the superconducting state of cuprates along the $(\pi ,0)$ direction, and their absence along the $(\pi ,\pi )$ direction.