Nematicity and in-plane anisotropy of superconductivity in $beta$-FeSe detected by $^{77}$Se nuclear magnetic resonance

The recent study of $^{77}$Se nuclear magnetic resonance (NMR) in a $beta$-FeSe single crystal proposed that ferro-orbital order breaks the $90^circ$ $C_4$ rotational symmetry, driving nematic ordering. Here, we report an NMR study of the impact of small strains generated by gluing on nematic state and spin fluctuations. We observe that the local strains strongly affect the nematic transition, considerably enhancing its onset temperature. On the contrary, no effect on low-energy spin fluctuations was found. Furthermore we investigate the interplay of the nematic phase and superconductivity. Our study demonstrates that the twinned nematic domains respond unequivalently to superconductivity, evidencing the twofold $C_2$ symmetry of superconductivity in this material. The obtained results are well understood in terms of the proposed ferro-orbital order.