Large critical fields in superconducting Ti$_{4}$Ir$_2$O from spin-orbit coupling

TL;DR

This study explains the large critical magnetic fields in Ti$_4$Ir$_2$O superconductors by showing how strong spin-orbit coupling and a Van Hove singularity near the Fermi level enhance the Pauli limiting field, defying typical cubic symmetry expectations.

Contribution

The paper combines DFT and analytic modeling to reveal the role of nonsymmorphic symmetry, SOC, and Van Hove singularities in enhancing the critical field in Ti$_4$Ir$_2$O superconductors.

Findings

Strong SOC near X points leads to vanishing effective g-factor.

Van Hove singularity accounts for ~65% of the DOS near the Fermi level.

Enhanced critical field results from momentum-dependent gap suppression.

Abstract

The recently synthesized $\eta$-carbide-type superconductors exhibit large critical fields. A notable example is Ti$_4$Ir$_2$O, for which the upper critical field strongly violates the Pauli paramagnetic limit, behavior that 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$\overline{3}$m 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 $\sim$65\% of the total density of states (DOS), occurring near the chemical potential. We propose that the strong SOC and enhanced DOS 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.