Time reversal invariant single gap superconductivity with upper critical field larger than Pauli limit in NbIr$_2$B$_2$

Abstract

Recently, compounds with noncentrosymmetric crystal structure have attracted much attention for providing a rich playground in search for unconventional superconductivity. NbIr$_2$B$_2$ is a new member to this class of materials harboring superconductivity below $T_{\rm c} = 7.3(2)$~K and very high upper critical field that exceeds Pauli limit. Here we report on muon spin rotation ($\mu$SR) experiments probing the temperature and field dependence of effective magnetic penetration depth in this compound. Our transverse-field -$\mu$SR results suggest a fully gaped $s$-wave superconductvity. Further, the estimated high value of upper critical field is also supplemented by high field transport measurements. Remarkably, the ratio $T_{\rm c}$/$\lambda^{-2}(0)$ obtained for NbIr$_2$B$_2$ ($\sim$2) is comparable to those of unconventional superconductors. Zero-field $\mu$SR data reveals no significant change in the muon spin relaxation rate above and below $T_{\rm c}$, evincing that time-reversal symmetry is preserved in the superconducting state. The presented results will stimulate theoretical investigations to obtain a microscopic understanding of the origin of superconductivity with preserved time reversal symmetry in this unique noncentrosymmetric system.