Topological properties of noncentrosymmetric superconductors TIr2B2 (T=Nb,Ta)

A recent experiment reported two new noncentrosymmetric superconductors $\mathrm{Nb}{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$ and $\mathrm{Ta}{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$ with respective superconducting transition temperatures of 7.2 and 5.2 K and further suggested their superconductivity to be unconventional [K. G\'ornicka et al., Adv. Funct. Mater. 31, 2007960 (2021)]. Here, based on first-principles calculations and symmetry analysis, we propose that $T{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}\phantom{\rule{0.28em}{0ex}}(T=\mathrm{Nb},\mathrm{Ta})$ are topological Weyl metals in the normal state. In the absence of spin-orbit coupling (SOC), we find that $\mathrm{Nb}{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$ has 12 Weyl points, and $\mathrm{Ta}{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$ has four Weyl points, i.e., the minimum number under time-reversal symmetry; meanwhile, both of them have a nodal net composed of three nodal lines. In the presence of SOC, a nodal loop on the mirror plane evolves into two hourglass Weyl rings, along with the Weyl points, which are dictated by the nonsymmorphic glide mirror symmetry. Besides the rings, $\mathrm{Nb}{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$ and $\mathrm{Ta}{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$ have 16 and 20 pairs of Weyl points, respectively. The surface Fermi arcs are explicitly demonstrated. On the (110) surface of $\mathrm{Ta}{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$, we find extremely long surface Fermi arcs ($\ensuremath{\sim}0.6\phantom{\rule{0.28em}{0ex}}{\AA{}}^{\ensuremath{-}1}$) located 1.4 meV below the Fermi level, which should be readily probed in experiment. Combined with the intrinsic superconductivity and the nontrivial bulk Fermi surfaces, $T{\mathrm{Ir}}_{2}{\mathrm{B}}_{2}$ may, thus, provide a very promising platform to explore the three-dimensional topological superconductivity.