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

TL;DR

This study uses high-resolution specific heat measurements to explore the in-plane angular dependence of superconducting gaps in FeSe, revealing nematic behavior and complex gap structure changes with temperature and magnetic field.

Contribution

It provides detailed thermodynamic evidence of the nematic character and gap anisotropy in FeSe through angular-dependent specific heat measurements.

Findings

Observed twofold symmetry in specific heat confirming nematicity

Detected abrupt change in gap structure above ~1 K

Identified anomalous field dependence of superconducting gaps

Abstract

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.