Pairing symmetry of heavy fermion superconductivity in the two-dimensional Kondo-Heisenberg lattice model

TL;DR

This paper investigates how the pairing symmetry in heavy fermion superconductivity within a 2D Kondo-Heisenberg lattice model depends on the Fermi surface structure, revealing transitions between s-wave and d-wave symmetries driven by coupling ratios.

Contribution

It demonstrates the dependence of pairing symmetry on Fermi surface topology and coupling ratios, providing a theoretical explanation for observed phase transitions in heavy fermion superconductors.

Findings

s-wave pairing favored at small J_H/J_K

extended s-wave favored at intermediate J_H/J_K

d-wave pairing becomes dominant at larger J_H/J_K

Abstract

In the two-dimensional Kondo-Heisenberg lattice model away from half-filled, the local antiferromagnetic exchange coupling can provide the pairing mechanism of quasiparticles via the Kondo screening effect, leading to the heavy fermion superconductivity. We find that the pairing symmetry \textit{strongly} depends on the Fermi surface (FS) structure in the normal metallic state. When $J_{H}/J_{K}$ is very small, the FS is a small hole-like circle around the corner of the Brillouin zone, and the s-wave pairing symmetry has a lower ground state energy. For the intermediate coupling values of $J_{H}/J_{K}$, the extended s-wave pairing symmetry gives the favored ground state. However, when $J_{H}/J_{K}$ is larger than a critical value, the FS transforms into four small hole pockets crossing the boundary of the magnetic Brillouin zone, and the d-wave pairing symmetry becomes more favorable. In that regime, the resulting superconducting state is characterized by either nodal d-wave or nodeless d-wave state, depending on the conduction electron filling factor as well. A continuous phase transition exists between these two states. This result may be related to the phase transition of the nodal d-wave state to a fully gapped state, which is recently observed in Yb doped CeCoIn$_{5}$.