Spin-Orbital-Intertwined Nematic State in FeSe

In this talk, I would like to discuss our recent progress on understanding the mechanism of electronic nematicity in FeSe superconductor. Through a comprehensive study of ⁷⁷Se and ⁵⁷Fe nuclear magnetic resonance (NMR) [1], a nontrivial SOC effect is revealed in the nematic state of FeSe. First, the orbital-dependent spin susceptibility, determined by the anisotropy of the ⁵⁷Fe Knight shift, indicates a predominant role from the 3d_xy orbital, which suggests the coexistence of local and itinerant spin degrees of freedom in the FeSe. Then, we reconfirm that the orbital reconstruction below the nematic transition temperature (T_nem) happens not only on the 3d_xz and 3d_yz orbitals but also on the 3d_xy orbital, which is beyond a trivial ferro-orbital order picture. Moreover, our results also indicate the development of a coherent coupling between the local and itinerant spin degrees of freedom below T_nem, which is ascribed to a Hund’s coupling-induced electronic crossover on the 3d_xy orbital. Finally, because of a nontrivial SOC effect, sizable in-plane anisotropy of the spin susceptibility emerges in the nematic state, suggesting a spin-orbital-intertwined nematicity rather than a simple spin- or orbital-driven nematicity. In addition, I will also introduce new progress on high-pressure ⁵⁷Fe NMR study of FeSe.

References:
[1] J. Li et al., Phys. Rev. X 10, 011034(2020)