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 with the Doppler energy shift of Cooper pairs with momenta close to the gap minima are observed. In the ${T}_{c}=9$ K crystal, $C(\ensuremath{\phi}$) displays a twofold symmetry at low temperature and low magnetic field confirming the nematic character of FeSe from thermodynamical measurements. As expected, a $\ensuremath{\pi}/2$ phase shift is observed for increasing temperatures (at $H=1$ T) but our measurements suggest that the gap structure abruptly changes above $\ensuremath{\approx}1$ K in this sample. At low temperature, the maxima observed for $H||a$ axis at low $H$ split into lobes at $\ifmmode\pm\else\textpm\fi{}{45}^{\ensuremath{\circ}}$ when the magnetic field is increased, which can be accounted for by assuming that the electron gap rapidly closes with field.