Upper Critical Field and Tunneling Spectroscopy of Underdoped Na(Fe,Co)As Single Crystals

A comprehensive study of superconducting properties of underdoped NaFe [Formula: see text] Co [Formula: see text] As single crystals by a combination of upper critical field measurements and incoherent multiple Andreev reflection effect (IMARE) spectroscopy is presented. The [Formula: see text] temperature dependences are measured at magnetic fields up to 16 T with in-plane and out-of-plane field directions and considered within single-band and two-band models in order to estimate the [Formula: see text] value. In IMARE spectroscopy probes, the magnitude, characteristic ratio, and temperature dependence of the superconducting order parameters ([Formula: see text]) are determined locally and directly. A possible k-space anisotropy of the large superconducting gap is demonstrated. We show that usage of a quadruple of [Formula: see text] coupling constants obtained in the IMARE experiment can significantly reduce the number of free parameters required to fit the [Formula: see text] dependence within a two-band approach (from six to two).