1Songshan Lake Materials Laboratory, Dongguan 523808, China 2Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China 3School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract：We report the synthesis, crystal structure, and superconductivity of Ti$_4$Ir$_2$O. The title compound crystallizes in an $\eta$-carbide type structure of the space group $Fd\bar{3}m$ (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$_4$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 ($R_{\rm W} \sim 3.9$), the Kadowaki–Woods ratio [$A/\gamma^2 \sim 9.0 \times 10^{-6}$ $µ\Omega\cdot$cm/(mJ$\cdot$mol$^{-1}\cdot$K$^{-1}$)$^2$], and the Sommerfeld coefficient ($\gamma = 33.74$ mJ$\cdot$mol$^{-1}\cdot$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 and large spin-orbit scattering. With these intriguing behaviors, Ti$_4$Ir$_2$O serves as a candidate for unconventional superconductor.
. [J]. 中国物理快报, 2022, 39(2): 27401-.
Bin-Bin Ruan, Meng-Hu Zhou, Qing-Song Yang, Ya-Dong Gu, Ming-Wei Ma, Gen-Fu Chen, and Zhi-An Ren. Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in $\eta$-Carbide Type Ti$_4$Ir$_2$O. Chin. Phys. Lett., 2022, 39(2): 27401-.
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