Breaking the Pauli Limit: Anomalous Pseudospin in the Superconductor Ti₄Ir₂O

The recently synthesized eta-carbide-type superconductors exhibit large critical fields, with the most notable example being Ti₄Ir₂O, for which the upper critical field strongly violates the Pauli paramagnetic limit. This is unusual for cubic materials that preserve inversion symmetry. Here, by combining density functional theory (DFT) and analytic modeling, we provide an explanation for this enhanced Pauli limiting field. We show that the nonsymmorphic Fd-3m symmetry implies that the electronic states near the X points exhibit strong spin-orbit coupling (SOC), which leads to a vanishing effective g-factor and enables the enhanced Pauli limiting field. Furthermore, our DFT results reveal a Van Hove singularity (VHS) peak near the X points, accounting for 65% of the total density of states (DOS), occurring near (13 meV below) the chemical potential. We propose that the strong SOC and the enhanced density of states in the vicinity of the X-points provide the origin of the observed enhanced critical field. This leads to a prediction that the magnetic field will lead to a strongly momentum-dependent gap suppression. The gap due to electronic states away from (near to) the X-points will be rapidly (slowly) suppressed by fields.