Two-gap model for underdoped cuprate superconductors

TL;DR

This paper introduces a two-gap model for underdoped cuprate superconductors, explaining the coexistence of incoherent and coherent pairing phenomena and their evolution from BCS to boson-fermion behavior.

Contribution

The paper proposes a novel two-gap model that captures the momentum-dependent pseudogap and the transition from BCS to boson-fermion regimes in underdoped cuprates.

Findings

Gap forms near M points with incoherent pairing due to fluctuations.

Pairing near nodal points achieves phase coherence at lower temperatures.

Model describes a continuous evolution from BCS to boson-fermion behavior with doping.

Abstract

Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface respectively. We find that a gap forms near the M points resulting into incoherent pairing due to strong fluctuations. Instead the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).