Going beyond the Chandrasekhar-Clogston limit in a flat-band superconductor

TL;DR

This paper demonstrates that in a flat-band superconductor model, the critical magnetic field limit can be exceeded due to enhanced pairing from a flat band, allowing spin-singlet superconductivity beyond the traditional Chandrasekhar-Clogston limit.

Contribution

The study introduces a model with a flat band that boosts pairing and surpasses the Chandrasekhar-Clogston limit, revealing new possibilities for high-field superconductivity.

Findings

Superconductivity can persist beyond the Chandrasekhar-Clogston limit in flat-band systems.

Additional flat-band pairing increases condensation energy without affecting paramagnetic susceptibility.

The model predicts nontrivial solutions for large spin-splitting fields.

Abstract

The Chandrasekhar-Clogston limit normally places stringent conditions on the magnitude of the magnetic field that can coexist with spin-singlet superconductivity, restricting the critical induced Zeeman shift to a fraction of the superconducting gap. Here, we consider a model system where the spin-singlet Cooper pairing in a dispersive band crossing the Fermi level is boosted by an additional flat-band located away from the Fermi level. The boosting of the pairing in the dispersive band allows for nontrivial solutions to the coupled gap equations for spin-splitting fields considerably larger than the superconducting gaps at zero field. Further, the additional Cooper pairing in the flat-band, away from the Fermi level, increases the superconducting condensation energy without affecting the paramagnetic susceptibility of the system, making the free energy favor the superconducting state. This opens up the possibility for spin-singlet superconductivity beyond the standard Chandrasekhar-Clogston limit.