Magnetic, superconducting and electron-boson properties of GdO(F)FeAs oxypnictides

We performed comprehensive studies of nearly optimal fluorine-substituted ${\mathrm{GdO}}_{1-x}{F}_x$ FeAs oxypnictide superconductors with $T_C=48-53K$. Specific heat measurements revealed a sharp peak at $T=3.5K$ that shifts to lower temperatures with magnetic field increase. This peak corresponds to an antiferromagnetic ordering in ${\mathrm{Gd}}^{3+}$ ion sublattice and may indicate coexistence between superconducting and magnetic orderings. Andreev transport through artificially made constriction demonstrated two channels for the carriers from the band(s) with the large superconducting gap as well as from those with the small gap. As expected, the presence of a transport channel with the bands mixing (${\Delta }_L+{\Delta }_S$) was not detected. Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we determined two superconducting gaps, ${\Delta }_S\approx 2.7\mathrm{meV}$, and ${\Delta }_L\approx 11.6\mathrm{meV}$. The reproducible fine structure in the dI(V)/dV spectra of the Andreev contacts (satellites of the main subharmonic gap structure for ${\Delta }_L$) was interpreted as caused by a resonant emission of bosons with the energy ${\epsilon }_0=12-15\mathrm{meV}$ $\approx {\Delta }_L+{\Delta }_S$ during the process of multiple Andreev reflections (MAR) for normal carriers in ${\Delta }_L$-band(s) transport channel.