High‐Field Superconductivity Beyond the Pauli Limit in NaMo ₆ Se ₈ Driven by Strong Spin‐Orbit Interactions

ABSTRACT We report a study of crystal and electronic structures, and superconductivity of the Chevrel‐phase (CP) compound NaMo 6 Se 8 . Na + insertion results in a lattice expansion relative to the parent Mo 6 Se 8 , and substantially enhances the density of states at the Fermi level within a rigid‐band shift picture. Bulk superconductivity with a sharp transition at = 9.7 K is evidenced by electrical transport, magnetic, and thermodynamic measurements. The compound displays an exceptionally large upper critical field µ 0 H c2 (0) = 33.6 T, almost twice the Pauli paramagnetic limit and placing it among selenide CP superconductors with the highest‐ H c2 values. Strong spin‐orbit scattering offsets the Pauli paramagnetic effect, leading to an enhancement of the effective Pauli‐limiting field. Specific‐heat data reveal a low‐lying Einstein phonon mode associated with Na‐cation oscillating, and elucidate the intermediate‐coupling two‐gap BCS superconductivity. The narrow‐band electronic structures, enhanced spin‐orbit interactions, and Na‐induced low‐lying Einstein phonons synergistically promote T c and µ 0 H c2 . Our work establishes NaMo 6 Se 8 as a compelling high‐field superconductor and highlights its promising relevance for high‐field superconducting magnets.