Near-flat-band-driven violation of Pauli limit in heavy fermion superconductors

TL;DR

This paper demonstrates that near-flat hybridized bands in heavy-fermion superconductors significantly enhance the critical magnetic field beyond the Pauli limit, driven by band structure effects and pairing symmetry.

Contribution

It provides a microscopic analysis showing how near-flat hybridized bands increase Pauli-limiting fields, highlighting the role of band curvature and pairing symmetry in heavy-fermion superconductors.

Findings

Critical field Bc exceeds Pauli limit near half filling.

Enhanced Clogston-Chandrasekhar ratio rCC due to flat bands.

Sensitivity of Pauli limiting to band curvature and pairing symmetry.

Abstract

Heavy-fermion superconductors often display upper critical fields that exceed the conventional Pauli paramagnetic limit, indicating that strong correlations and hybridized quasiparticle bands play an essential role in the paramagnetic pair-breaking process. Within the two-dimensional Kondo-Heisenberg model, we perform a self-consistent mean-field analysis of spin-singlet s-, extended-s-, and d-wave pairing under Zeeman fields, and compute the critical field Bc, the transition temperature Tc, and the Clogston-Chandrasekhar ratio rCC. We find that rCC increases sharply as the conduction filling approaches half filling. This enhancement arises from the weakly dispersive region of the lower hybridized band, where the strongly reduced Fermi velocity diminishes the normal-state paramagnetic energy and stabilizes superconductivity. At fixed filling, the distinct JH dependences among the three pairing channels reflect the sensitivity of Pauli limiting to both band curvature and the structure of the order parameter. These results provide microscopic evidence that proximity to a near-flat hybridized band offers a robust route to enhanced Pauli-limiting fields in heavy-fermion superconductors.