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(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 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 indicating an anomalous field dependence of the gaps.