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Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in $η$-Carbide type Ti$_4$Ir$_2$O

We report the synthesis, crystal structure, and superconductivity of Ti$_4$Ir$_2$O. The title compound crystallizes in an $η$-carbide type structure of the space group $Fd\overline{3}m$ (No. 227), with lattice parameters $a=b=c=11.6194(1)$ Å. The superconducting $T_c$ is found to be 5.1 $\sim$ 5.7 K. Most surprisingly, Ti$_4$Ir$_2$O 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-3$d$ and Ir-5$d$ orbitals, and the effect of spin-orbit coupling on the Fermi surfaces is prominent. Large values of the Wilson ratio ($R_W$ $\sim$ 3.9), the Kadowaki-Woods ratio ($A/γ^2$ $\sim$ 9.0 $\times$ 10$^{-6}$ $μΩ$ cm/(mJ mol$^{-1}$ K$^{-1}$)$^2$), and the Sommerfeld coefficient ($γ$ = 33.74 mJ mol$^{-1}$ K$^{-2}$) all suggest strong electron correlations (similar to heavy fermion systems) in Ti$_4$Ir$_2$O. The violation of Pauli limit is possibly due to a combination of strong-coupled superconductivity, large spin-orbit scattering, and electron correlation. With these intriguing behaviors, Ti$_4$Ir$_2$O serves as a candidate for unconventional superconductor.