In-plane angular dependence of superconducting gaps in FeSe probed by high resolution specific heat measurements

The influence of a rotating magnetic field (in the $ab-$plane) on the density of states has been investigated in the superconducting state of the nematic FeSe superconductor using high sensitivity specific heat measurements. As expected for (quasi-)nodal superconductors, oscillations in the specific heat ($C$) associated to the Doppler energy shift of Cooper pairs with momenta close to the gap minima are observed. In the $T_c = 9$~K crystal, $C(\phi$) displays a twofold symmetry at low temperature and low magnetic field confirming the nematic character of FeSe from thermodynamical measurements. As expected, a $\pi/2$ phase shift is observed for increasing temperatures (at $H=1$~T) but the gap structure abruptly changes above $\sim 1$~K in this sample. At low temperature, the maxima observed for $H||a-$axis at low $H$ split into lobes at $\pm 45^\circ$ when the magnetic field is increased indicating an anomalous field dependence of the gaps.