Electronic specific heat in BaFe$_{2-x}$Ni$_x$As$_2$
  We have systematically studied the low-temperature specific heat of the BaFe$_{2-x}$Ni$_x$As$_2$ single crystals covering the whole superconducting dome. Using the nonsuperconducting heavily overdoped x = 0.3 sample as a reference for the phonon contribution to the specific heat, we find that the normal-state electronic specific heats in the superconducting samples may have a nonlinear temperature dependence, which challenges previous results in the electron-doped Ba-122 iron-based superconductors. A model based on the presence of ferromagnetic spin fluctuations may explain the data between x = 0.1 and x = 0.15, suggesting the important role of Fermi-surface topology in understanding the normal-state electronic states. 
Dongliang Gong, Tao Xie, Xingye Lu, Cong Ren, Lei Shan, Rui Zhang,   Pengcheng Dai, Yi-feng Yang, Huiqian Luo, Shiliang Li
cond-mat.supr-con cond-mat.str-el