Superconductivity of underdoped PrFeAs(O,F): A point-contact Andreev-reflection spectroscopy and nuclear magnetic resonance study.

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while $^{75}$As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_{c}$ and expected to mediate the superconducting pairing.