The superconducting gap structure in iron-based high-temperature superconductors (Fe-HTSs) is non-universal. In contrast to other unconventional superconductors, in the Fe-HTSs both d-wave and extended s-wave pairing symmetries are close in energy. Probing the proximity between these very different superconducting states and identifying experimental parameters that can tune them is of central interest. Here 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 in the zero-temperature limit is observed, while the superconducting transition temperature remains nearly constant. More importantly, the low-temperature behaviour 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-temperature behaviour at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the superconducting gap.
Bibliographical noteFunding Information:
Experimental work was performed at the Swiss Muon Source (SµS) Paul Scherrer Institute, Villigen, Switzerland. Z.G. acknowledges the support by the Swiss National Science Foundation. R.M.F. and J.K. were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award number DE-SC0012336. A.S. acknowledges support from the SCOPES grant No. IZ73Z0_128242. G.P. is supported by the Humboldt Research Fellowship for Postdoctoral Researchers.
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