Density of States and Spectral Function of a Superconductor out of a Quantum-Critical Metal

We analyze the validity of a quasiparticle description of a superconducting state above a metallic quantum-critical point (QCP). A normal state at a QCP is a non-Fermi liquid with no coherent quasiparticles. A superconducting order gaps out low-energy excitations, except for a sliver of states for non-s-wave gap symmetry, and at a first glance, restores coherent quasiparticle behavior. We argue that this does not necessarily hold as the fermionic self-energy may remain singular above the gap edge. This singularity gives rise to markedly non-BCS behavior of the density of states and to the appearance of a nondispersing mode at the gap edge in the spectral function. We analyze the set of quantum-critical models with an effective dynamical four-fermion interaction V(Omega) proportional to 1/Omega(gamma), where Omega is a frequency of a boson, which mediates the interaction. We show that coherent quasiparticle behavior in a superconducting state holds for gamma < 1/2, but breaks down for larger gamma. We discuss signatures of quasiparticle breakdown and compare our results with the photoemission data for Bi2201 and Bi2212.