Superconductivity in BaIrSi2: A 5d electron system with a noncentrosymmetric crystal structure

The compound $\mathrm{BaIrS}{\mathrm{i}}_{2}$ was successfully synthesized under high pressure at 6 GPa. $\mathrm{BaIrS}{\mathrm{i}}_{2}$ crystalizes in an orthorhombic symmetry with the noncentrosymmetric space group $C{222}_{1}$ and the lattice parameters $a=15.0492(1)\phantom{\rule{0.16em}{0ex}}\AA{}, b=8.0311(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, and $c=8.0314(1)\phantom{\rule{0.16em}{0ex}}\AA{}$. $\mathrm{BaIrS}{\mathrm{i}}_{2}$ shows superconductivity below a critical temperature ${T}_{c}\ensuremath{\sim}6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and an upper critical field ${\ensuremath{\mu}}_{0}{H}_{c2}(0)\ensuremath{\sim}6\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ with a Ginzburg-Landau parameter ${\ensuremath{\kappa}}_{\mathrm{GL}}\ensuremath{\sim}87$. Specific-heat measurements suggest that $\mathrm{BaIrS}{\mathrm{i}}_{2}$ is an intermediate-coupling superconductor with a moderate electron-phonon coupling constant ${\ensuremath{\lambda}}_{ep}\ensuremath{\sim}0.8$. The Sommerfeld constant is ${\ensuremath{\gamma}}_{n}\ensuremath{\sim}5.5\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{mole}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{2}$. Phonon mediation seems to be dominant for Cooper pairing in $\mathrm{BaIrS}{\mathrm{i}}_{2}$. The ab initio calculations revealed that $\mathrm{BaIrS}{\mathrm{i}}_{2}$ has four Fermi surfaces split by spin-orbit coupling. The Cooper pairing seems to occur between electrons without spin degeneracy.