Volovik effect in a highly anisotropic multiband superconductor: Experiment and theory

We present measurements of the specific heat coefficient γ (C/T) in the low-temperature limit as a function of an applied magnetic field for the Fe-based superconductor BaFe₂(As_0.7P_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 √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 electronlike Fermi surface sheets.