Direct Evidence of Unconventional Superconductivity in Doped Kagome system RV$_3$Sb$_5$
The superconducting pairing symmetry of kagome metals remains a central unresolved question largely because phase-sensitive experiments capable of distinguishing between different possible order parameters have been difficult to implement. The spatial and energetic characteristics of impurity bound states measured by spectroscopic-imaging scanning tunneling microscopy encode information on the superconducting order parameter. Here, we use impurity-bound state spectroscopy to investigate optimally doped RbV$_3$Sb$_{5}$ where the charge-density wave instability is fully suppressed. We find that the superconducting state is fully gapped and exhibits two distinct energy scales, consistent with a multiband order parameter. Atomic-scale spectroscopy around nonmagnetic defects reveals pronounced particle-hole asymmetric bound states. Comparison with theoretical calculations demonstrates that these bound states are incompatible with conventional $s$-wave and sign-changing $s^{\pm}$ pairing. Our calculations also show that the data are fully consistent with a chiral order parameter. Our results establish impurity-bound-state spectroscopy as a powerful phase-sensitive probe of superconductivity in kagome materials and provide strong evidence that optimally doped RbV$_3$Sb$_{5}$ realizes a fully gapped chiral superconducting state.