Volovik effect in a highly anisotropic multiband superconductor:   experiment and theory
  We present measurements of the specific heat coefficient \gamma(= C/T) in the low temperature limit as a function of an applied magnetic field for the Fe-based superconductor BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$. We find both a linear regime at higher fields and a limiting square root $H$ behavior at very low fields. The crossover from a Volovik-like $\sqrt{H}$ to a linear field dependence can be understood from a multiband calculation in the quasiclassical approximation assuming gaps with different momentum dependence on the hole- and electron-like Fermi surface sheets. 
Y. Wang, J.S. Kim, G. R. Stewart, P.J. Hirschfeld, S. Graser, S.   Kasahara, T. Terashima, Y. Matsuda, T. Shibauchi and I. V...
cond-mat.supr-con