Spin-momentum locked interface in nearly metallic system detected using circularly polarized light

Photo-induced current is a phenomenon that occurs when incident photons have sufficient energy to excite electrons from the valence to the higher conduction band. When the incident light is circularly polarized, it may couple to the electron’s spins and excite electrons with a fixed spin direction. For materials with unique electronic properties that cause the electron spins to couple to its direction of motion, often referred to as spin-momentum locking, illumination with circular polarization can generate current where the electron spins are aligned in a fixed direction. This effect, known as the circular photogalvanic effect, has been observed mostly in semiconductor heterostructures with light energy close to the system bandgap.

Work by Hirose et al. discovered the circular photogalvanic effect in a Cu/Bi bilayer system. Irradiation of circularly polarized light in the visible range to the bilayer resulted in the generation of current only in a direction orthogonal to the light plane of incidence, indicating that the system under investigation possesses electronic states with spin-momentum locking. In contrast to previous studies, the light energy is much higher than the bandgap of the semimetal bismuth, which is a new class of material to exhibit this behavior. The strong spin-orbit coupling of bismuth and its unique interface state with metallic copper are believed to cause the light-induced generation of spin polarized current.

If this approach can be extended to metal systems to study the electronic structure of metallic interface states, it could be of interest to the field of spintronics due to the interest in strong spin-orbit coupling materials. This will allow the exploration of a variety of different metallic interfaces excited using circularly polarized light.

Source: “Circular photogalvanic effect in Cu/Bi bilayers,” by Hana Hirose, Naoto Ito, Masashi Kawaguchi, Yong-Chang Lau, and Masamitsu Hayashi, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5047418 .