Helicity-dependent photocurrent induced by circularly polarized infrared light at atomic bilayer superstructure Si(111)-√3×√3-(Tl, Sn)

抄録

Spin injection into materials by transferring spin angular momentum of circularly polarized light (CPL) attracts much attention because it can be detected without an external electric/magnetic field at room temperature. In materials having strong spin-orbit interaction (SOI) with spin-splitting bands, asymmetric electron excitation occurs due to the spin-selective excitations by CPL. Such excited carriers form helicity-dependent photocurrent (HDP). HDP has been demonstrated in topological insulators and Rashba surface/interface systems so far [1-5]. Pure spin current can also be induced by CPL under some conditions, which can be detected electrically by the inverse spin Hall effect.

In this report, we observe HDP induced by irradiating infrared CPL laser (λ = 1550 nm) in an atomic bilayer material, Si(111)-√3×√3-(Tl, Sn) surface superstructure, which has spin-splitting surface bands due to giant Rashba effect [6]. We fabricated the sample by molecular beam epitaxy (MBE) method in an ultra-high vacuum (UHV) chamber and then measured it in situ electrically with irradiating the laser (Fig. 1(a)). HDP, detected with clamp electrodes at both ends of the sample (y-direction), increased with the laser spot going to both edges of the sample in the x-direction (Fig. 1(a)), and the sign of HDP reversed between right and left edges. HDP became larger when CPL was irradiated more obliquely, that is, the in-plane component of the injected spin increased (Fig. 1(b), (c)). These behaviors are unusual because in previous reports [1,7] enhancement and reversal of HDP at sample edges occur with out-of-plane spin component and HDP should decrease when CPL is obliquely illuminated. Here, we have successfully explained such behaviors as a superposition phenomena by precession of electrons’ spin due to the strong SOI and spin Hall effect at both edges of the sample.

References

[1] D. Fan et al., Phys. Rev. Res. 2, 023055 (2020). [2] K. N. Okada et al., Phys. Rev. B 93, 081403(R) (2016). [3] W. Wu et al., Opt. Express 30, 15085-15095 (2022). [4] S. D. Ganichev and W. Prettl, J. Phys. Cond. Mat. 15, R935 (2003). [5] I. Taniuchi et al., arXiv:2308.02485 (2023). [6] D. V. Gruznev et al., Phys. Rev. B 91, 035421 (2015). [7] J. Yu et al., Nano Lett. 17, 12, 7878–7885 (2017).