Theory of high Pauli limiting fields in BiS₂-based materials

Layered BiS₂-based superconductors with non-symmorphic space group P4/nmm exhibit very high critical fields, 2-7 times larger than the Pauli limiting field. Experimental data suggest that the highest critical fields can be raised by partial S-Se substitution. This work focuses on detailed lattice structures of some Bi(S/Se)₂-based superconductors to explain this phenomenon. The dependency of the energy dispersion curves on lattice parameters, spin-orbit coupling, and relative atomic positions is evaluated with density functional theory (DFT) computations. These are used to determine the Pauli field using a new ab initio approach. The DFT bands of the experimental structure are fitted with Slater-Koster tight-binding parameters to determine the key underlying interactions that lead to high critical field values.