• Buechner, B.
• Bombor, D.
• Drechsler, S. -L.
• Wolter, A. U. B.
• Rosner, H.
• Abdel-Hafiez, M.
• Grinenko, Vadim
• Krupskaya, Y.
• Johnston, S.
• Wurmehl, S.
• Hess, C.
• Harnagea, L.

Abstract

We report a low-temperature specific heat study of high-quality single crystals of the heavily hole doped superconductor Ca$_{0.32}$Na$_{0.68}$Fe$_2$As$_2$. This compound exhibits bulk superconductivity with a transition temperature $T_c34$\,K, which is evident from the magnetization, transport, and specific heat measurements. The zero field data manifests a significant electronic specific heat in the normal state with a Sommerfeld coefficient $53$ mJ/mol K$^{2}$. Using a multi-band Eliashberg analysis, we demonstrate that the dependence of the zero field specific heat in the superconducting state is well described by a three-band model with an unconventional s$_$ pairing symmetry and gap magnitudesof approximately 2.35, 7.48, and -7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling,which contributes significantly to the relatively high value of $T_c$. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-$T_c$ iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions. ; Comment: 8 Figures, Submitted to PRB

Topics

• impedance spectroscopy
• surface
• compound
• single crystal
• strength
• iron
• magnetization
• superconductivity
• superconductivity
• specific heat