Specific Heat To Hc2: Evidence for Nodes or Deep Minima in the   Superconducting Gap of Under- and Overdoped Ba(Fe1-xCox)2As2
  Low temperature specific heat, C, in magnetic fields up to Hc2 is reported for underdoped Ba(Fe0.955Co0.045)2As2 (Tc=8 K) and for three overdoped samples Ba(Fe1-xCox)2As2 (x=0.103, 0.13, and 0.15, Tc=17.2, 16.5, and 11.7 K respectively). Previous measurements of thermal conductivity (as a function of temperature and field) and penetration depth on comparable composition samples gave some disagreement as to whether there was fully gapped/nodal behavior in the under-/overdoped materials respectively. The present work shows that the measured behavior of the specific heat gamma (proportional to C/T as T->0, i. e. a measure of the electronic density of states at the Fermi energy) as a function of field approximately obeys gamma proportional to H**(0.5 +- 0.1), similar to the Volovik effect for nodal superconductors, for both the underdoped and the most overdoped Co samples. However, for the two overdoped compositions x=0.103 and 0.13, the low field (H < 10 T) data show a Volovik-like behavior of gamma proportional to H**(0.3-0.4), followed by an inflection point, followed at higher fields by gamma proportional to H**1. We argue that within the 2-band theory of superconductivity, an inflection point may occur if the interband coupling is dominant. 
J. S. Kim, B. D. Faeth, Y. Wang, P.J. Hirschfeld, G. R. Stewart, K.   Gofryk, F. Ronning, A. S. Sefat, K. Y. Choi and K. H. Kim
cond-mat.supr-con