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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.