Angle-resolved specific heat in iron-based superconductors: The case for a nodeless extended s-wave gap

We consider the variation in the field-induced component of the specific heat C(H) with the direction of the applied field in Fe pnictides within quasiclassical Doppler-shift approximation with special emphasis to recent experiments on FeSe0.4Te0.6. We show that for extended s-wave gap with no nodes, C(H) has cos 4 phi component, where phi is the angle between H and the direction between hole and electron Fermi surfaces. The maxima of C(H) are at pi/4, 3 pi/4, etc., if the applied field is smaller than H-0 <= 1 T, and at phi = 0, pi/2, etc., if the applied field is larger than H-0. The angle dependence of C(H), the positions of the maxima, and the relative magnitude of the oscillating component are consistent with the experiments performed in the field of 9 T >> H-0. We show that the observed cos 4 phi variation does not hold if the s-wave gap has accidental nodes along the two electron Fermi surfaces.