Direct evidence for a pressure induced nodal superconducting gap in the Ba$_{0.65}$Rb$_{0.35}$Fe$_{2}$As$_{2}$ superconductor

In contrast to other unconventional superconductors, in the Fe-based superconductors (Fe-HTSs) both $d$-wave and extended $s$-wave pairing symmetries are close in energy. Probing the proximity between these different superconducting (SC) states and identifying experimental parameters that can tune them is of central interest. We report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth in the optimally doped nodeless $s$-wave Fe-HTS Ba$_{0.65}$Rb$_{0.35}$Fe$_{2}$As$_{2}$. Upon pressure, a strong decrease of the penetration depth is observed, while the SC transition temperature remains nearly constant. More importantly, the low-temperature behavior of the inverse squared magnetic penetration depth, which is a direct measure of the superfluid density, changes qualitatively from an exponential saturation at zero pressure to a linear-in-$T$ behavior at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the SC gap.