Role of Fermi surface anisotropy in the study of gap anisotropy using magnetic-field-angle dependence of thermal oscillations in AyFe2-xSe2 superconductors

We present a numerical study of the field-angle resolved oscillations of the thermal conductivity and specific heat under rotated magnetic field in the $A_y$Fe${}_{2-x}$Se${}_2$ [$A$=K,Rb,Cs,(Tl,K)] superconductors, using realistic two-band Fermi surface parameterization. Our key finding is that even for isotropic pairing on an anisotropic Fermi surface, the thermodynamic quantities exhibit substantial oscillatory behavior in the superconducting state, even much below the upper critical field. Furthermore, in multiband systems the competition of anisotropies between two Fermi surfaces can cause a double sign reversal of oscillations as a function of temperature, irrespective of gap anisotropy. Our findings put severe constraints on simple interpretations of field-angle resolved measurements widely used to identify the angular structure of the superconducting gap.