Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in \eta-Carbide Type Ti_4Ir_2O

We report the synthesis, crystal structure, and superconductivity of Ti_4Ir_2O. The title compound crystallizes in an \eta-carbide type structure of the space group Fd\bar3m (No. 227), with lattice parameters a=b=c=11.6194(1) Å. The superconducting temperature T_\rm c is found to be 5.1–5.7 K. Most surprisingly, Ti_4Ir_2O hosts an upper critical field of 16.45 T, which is far beyond the Pauli paramagnetic limit. Strong coupled superconductivity with evidences for multigap is revealed by the measurements of heat capacity and upper critical field. First-principles calculations suggest that the density of states near the Fermi level originates from the hybridization of Ti-3d and Ir-5d orbitals, and the effect of spin-orbit coupling on the Fermi surfaces is prominent. Large values of the Wilson ratio (R_\rm W \sim 3.9), the Kadowaki–Woods ratio A/\gamma^2 \sim 9.0 \times 10^-6 µ\Omega\cdotcm/(mJ\cdotmol^-1\cdotK^-1)^2, and the Sommerfeld coefficient (\gamma = 33.74 mJ\cdotmol^-1\cdotK^-2) all suggest strong electron correlations (similar to heavy fermion systems) in Ti_4Ir_2O. The violation of Pauli limit is possibly due to a combination of strong-coupled superconductivity and large spin-orbit scattering. With these intriguing behaviors, Ti_4Ir_2O serves as a candidate for unconventional superconductor.