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Abstract:
We report a low-temperature specific heat study of high-quality single
crystals of the heavily hole-doped superconductor Ca0.32Na0.68Fe2As2.
This compound exhibits bulk superconductivity with a transition
temperature T-c approximate to 34 K, which is evident from the
magnetization, transport, and specific heat measurements. The zero-field
data manifest a significant electronic specific heat in the normal state
with a Sommerfeld coefficient gamma approximate to 53 mJ/mol K-2. Using
a multiband 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 magnitudes Delta(i) of 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.
