Ir-Sb Binary System: Unveiling Nodeless Unconventional Superconductivity Proximate to Honeycomb-Vacancy Ordering

Vacancies play a crucial role in solid-state physics, but their impact on materials with strong electron-electron correlations has been underexplored. A recent study on the Ir-Sb binary system, Ir$_{16}$Sb$_{18}$ revealed a novel extended buckled-honeycomb vacancy (BHV) order. Superconductivity is induced by suppressing the BHV ordering through high-pressure growth with excess Ir atoms or isovalent Rh substitution, although the nature of superconducting pairing has remained unexplored. Here, we conduct muon spin rotation experiments probing the temperature-dependence of the effective magnetic penetration depth $\lambda_{eff}\left(T\right)$ in Ir$_{1-{\delta}}$Sb (synthesized at 5.5 GPa with $T_{\rm c}$ = 4.2 K) and ambient pressure synthesized optimally Rh-doped Ir$_{1-x}$Rh$_{x}$Sb ($x$=0.3, $T_{\rm c}$ = 2.7 K). The exponential temperature dependence of the superfluid density $n_{\rm s}$/m$^{*}$ at low temperatures indicates a fully gapped superconducting state in both samples. Notably, the ratio of $T_{\rm c}$ to the superfluid density is comparable to previously measured unconventional superconductors. A significant increase in $n_{\rm s}$/m$^{*}$ in the high-pressure synthesized sample correlates with $T_{\rm c}$, a hallmark feature of unconventional superconductivity. We further demonstrate a similar effect induced by chemical pressure (Rh substitution) and hydrostatic pressure in Ir$_{1-x}$Rh$_{x}$Sb, highlighting that the dome-shaped phase diagram is a fundamental feature of the material. These findings underscore the unconventional nature of the observed superconductivity, and classifies IrSb as the first unconventional superconducting parent phase with ordered vacancies.