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 Tc=9 K crystal, C(φ) displays a twofold symmetry at low temperature and low magnetic field confirming the nematic character of FeSe from thermodynamical measurements. As expected, a π/2 phase shift is observed for increasing temperatures (at H=1 T) but our measurements suggest that the gap structure abruptly changes above ≈1 K in this sample. At low temperature, the maxima observed for H||a axis at low H split into lobes at ±45 when the magnetic field is increased, which can be accounted for by assuming that the electron gap rapidly closes with field.