Disorder-Driven Transition from s± to s++ Superconducting Order Parameter in Proton Irradiated Ba(Fe1-xRhx)2As2 Single Crystals.

Microwave measurements of the London penetration depth and critical temperature Tc were used to show evidence of a disordered-driven transition from s± to s++ order parameter symmetry in optimally doped Ba(Fe1-xRhx)2As2 single crystals, where disorder was induced by means of 3.5 MeV proton irradiation. Signatures of such a transition, as theoretically predicted [V. D. Efremov et al., Phys. Rev. B 84, 180512(R) (2011)], are found as a drop in the low-temperature values of the London penetration depth and a virtually disorder-independent superconducting Tc. We show how these experimental observations can be described by multiband Eliashberg calculations in which the effect of disorder is accounted for in a suitable way. To this aim, an effective two-band approach is adopted, allowing us to treat disorder in a range between the Born approximation and the unitary limit. [ABSTRACT FROM AUTHOR]