Spin-Orbital-Intertwined Nematic State in FeSe

Abstract

The importance of the spin-orbit coupling (SOC) effect in Fe-based superconductors (FeSCs) has recently been under hot debate. Considering the Hund's coupling-induced electronic correlation, the understanding of the role of SOC in FeSCs is not trivial and is still elusive. Here, through a comprehensive study of 77 Se and 57 Fe nuclear magnetic resonance, a nontrivial SOC effect is revealed in the nematic state of FeSe. First, the orbital-dependent spin susceptibility, determined by the anisotropy of the 57 Fe Knight shift, indicates a predominant role from the 3dxy orbital, which suggests the coexistence of local and itinerant spin degrees of freedom (d.o.f.) in the FeSe. Then, we reconfirm that the orbital reconstruction below the nematic transition temperature (Tnem ∼ 90 K) happens not only on the 3dxz and 3dyz orbitals but also on the 3dxy 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 d.o.f. below Tnem, which is ascribed to a Hund's coupling-induced electronic crossover on the 3dxy orbital. Finally, due to 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 simply spin-or orbital-driven nematicity. The present work not only reveals a nontrivial SOC effect in the nematic state but also sheds light on the mechanism of nematic transition in FeSe.

Singular magnetic anisotropy in the nematic phase of FeSe

“…Knight-shift anisotropy: experimental results When no stress is applied and the field is aligned with the Fe-Fe bonds (i.e., the orthorhombic a-or b-axis), NMR lines in the (nonmagnetic) orthorhombic phase are split into two peaks (Fig. 1a, c), in agreement with earlier studies of both LaFeAsO and FeSe 12,13,55,[59][60][61] . Upon applying uniaxial stress in both samples, the intensity of one peak grows at the expense of the other while the total intensity of the spectrum is conserved, as seen from Fig.…”

Absence of Y-pocket in 1-Fe Brillouin zone and reversed orbital occupation imbalance in FeSe

“…Regarding strong coupling picture 25,26 , the absence of magnetism could be explained within the context of an orbital weight redistribution scheme. As the magnetic moment is mainly derived from the d xy orbital 27,28 , the reduced d xy orbital weight due to strong orbitaldependent hybridization should weaken the overall magnetic instability 29 . Meanwhile, the opposite resistivity anisotropy of FeSe in comparison to pnictides can be explained by reversed orbital occupation imbalance.…”

Electron-nuclear hyperfine coupling in quantum kagome antiferromagnets from first-principles calculation and a reflection of the defect effect

“…We note that there are known examples, in which the HFC constant varies with T 24,25 . Therefore, in general one should examine first whether the application condition can be met.…”