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, Ir₁₆Sb₁₈ 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 λeff(T) in Ir_1-δSb (synthesized at 5.5 GPa with Tc = 4.2 K) and ambient pressure synthesized optimally Rh-doped Ir₁₋ₓRhₓSb (x=0.3, Tc = 2.7 K). The exponential temperature dependence of the superfluid density nₛ/m* at low temperatures indicates a fully gapped superconducting state in both samples. Notably, the ratio of Tc to the superfluid density is comparable to previously measured unconventional superconductors. A significant increase in nₛ/m* in the high-pressure synthesized sample correlates with Tc, a hallmark feature of unconventional superconductivity. We further demonstrate a similar effect induced by chemical pressure (Rh substitution) and hydrostatic pressure in Ir₁₋ₓRhₓSb, 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.