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 77Se and 57Fe 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 57Fe 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.
Highlights
Electronic correlation is widely accepted as a predominant physical origin for the production of the various quantum matter states in high-temperature superconductors (HTS), including cuprate superconductors [1] and Fe-based superconductors (FeSCs) [2,3,4]
A remarkable splitting of the nuclear magnetic resonance (NMR) spectra is observed below Tnem for both nuclei, which is ascribed to the formation of two orthogonal nematic domains with Hka and Hkb [32,33,34,40,41]
The spin-orbit coupling (SOC) always leads to a twist of spin and orbital d.o.f., the sizable spin-space anisotropy observed in the present study indicates a nontrivial SOC effect in the nematic state
Summary
Electronic correlation is widely accepted as a predominant physical origin for the production of the various quantum matter states in high-temperature superconductors (HTS), including cuprate superconductors [1] and Fe-based superconductors (FeSCs) [2,3,4]. SOC can intertwine spin and orbital degrees of freedom (d.o.f.) in solids and induce intriguing physical properties such as nontrivial topological band structure [23,24,25,26] and exotic superconducting pairing [27,28,29]. A nontrivial topological band structure has been successfully verified in iron-chalcogenide superconductors [19,20,21] that confirms the importance of SOC on the band structure in FeSCs. A natural follow-up question is how to understand the SOC effect on the various quantum matter states, such as nematic and superconducting states, due to electronic correlation in FeSCs [13,14,15,16,17,18]
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