ИСТИНА

Using a “self-flux” technique, we have grown large NaFe(1-x)Co(x)As single crystals of various composition with x = 0.02-0.05 and Tc ≈ 18–22 K, respectively. For all the experiments, the sample mounting was made in a dry argon atmosphere. Magnetotransport probes showed almost linear shifting of the resistive SC transition in the fields up to 16 T. We determined the slope dH_c2/dT ≈ -2.4 T/K, and estimate the upper critical field H_c2(0) ≈ 34 T using a single-band model. At 4.2 K, using a planar “break-junction” technique we formed tunneling junctions of SC–constriction–SC (ScS) type. Above Tc we reproducibly observed a residual non-linearity of the d(V)/dV spectra of ScS junctions that cannot be described by a junction overheating, and does not depend on the junction area and resistance. Such normal-state features could originate from the electron density of states (DOS) features in the vicinity if the Fermi level, or its renormalization by a resonant electron-phonon interaction.