Abstract
From the measurement and analysis of the specific heat of high-quality K1- xNaxFe2As2 single crystals we establish the presence of large T-2 contributions with coefficients alpha(sc) approximate to 30 mJ/mol K-3 at low T for both x = 0 and x = 0.1. Together with the observed root B behavior of the specific heat in the superconducting state both findings give evidence of d-wave superconductivity on almost all Fermi-surface sheets with an average gap amplitude of Delta(0) in the range of 0.4-0.8 meV. The derived Delta(0) and observed T-c agree well with the values calculated within Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime.