Direct evidence of hidden local spin polarization in a centrosymmetric superconductor LaO0.55 F0.45BiS2

Shi-Long Wu,Kazuaki Taguchi,Taichi Okuda,Masashi Arita,Koji Miyamoto,Isao Tanaka,Masanori Nagao,Satoshi Watauchi,Tomoki Yoshikawa,Yoshifumi Ueda,Akio Kimura,Kazuki Sumida

doi:10.1038/s41467-017-02058-2

Shi-Long Wu, Kazuaki Taguchi + Show 10 more

Open Access

https://doi.org/10.1038/s41467-017-02058-2

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Abstract

Conventional Rashba spin polarization is caused by the combination of strong spin–orbit interaction and spatial inversion asymmetry. However, Rashba–Dresselhaus-type spin-split states are predicted in the centrosymmetric LaOBiS2 system by recent theory, which stem from the local inversion asymmetry of active BiS2 layer. By performing high-resolution spin- and angle-resolved photoemission spectroscopy, we have investigated the electronic band structure and spin texture of superconductor LaO0.55F0.45BiS2. Here we present direct spectroscopic evidence for the local spin polarization of both the valence band and the conduction band. In particular, the coexistence of Rashba-like and Dresselhaus-like spin textures has been observed in the conduction band. The finding is of key importance for fabrication of proposed dual-gated spin-field effect transistor. Moreover, the spin-split band leads to a spin–momentum locking Fermi surface from which superconductivity emerges. Our demonstration not only expands the scope of spintronic materials but also enhances the understanding of spin–orbit interaction-related superconductivity.

Highlights

Conventional Rashba spin polarization is caused by the combination of strong spin–orbit interaction and spatial inversion asymmetry
The theoretical works suggested that LaOBiS2 and the related compounds can be such systems possessing R-2 and/or D-2 due to the breaking of local inversion symmetry in each BiS2 bilayer and the opposite polar fields caused by ionic bonding between (BiS2)− bilayer and (La2O2)2+ layer
Theoretical study of either film[9] or bulk LaOBiS21 further pointed out that spin texture of conduction band at each X point in Brillouin zone must be non-helical originating from D-2 effect whereas the valence band possesses helical spin texture originating from R-2 effect

Summary

Introduction

Conventional Rashba spin polarization is caused by the combination of strong spin–orbit interaction and spatial inversion asymmetry. Rashba–Dresselhaus-type spin-split states are predicted in the centrosymmetric LaOBiS2 system by recent theory, which stem from the local inversion asymmetry of active BiS2 layer. The theoretical works suggested that LaOBiS2 and the related compounds can be such systems possessing R-2 and/or D-2 due to the breaking of local inversion symmetry in each BiS2 bilayer and the opposite polar fields caused by ionic bonding between (BiS2)− bilayer and (La2O2)2+ layer. Since the projected local spin polarization on each real-space sector of BiS2 bilayer in LaOBiS2 crystals holds opposite orientation, so called spin-layer locking effect, has been theoretically predicted in the LaOBiS2 film at first[9] which could offer advantages for the design of new generation of spin-field effect transistors (SFET)[9]. Heretofore, Rashba superconductors[13–16] with mixed singlet and triplet pairings have been limited to non-centrosymmetric compounds or surface systems

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