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Helicity-dependent photocurrent

2017-05-26 10:28:06 | 日記
One of the most critical steps towards spin-functionalized electronics and optoelectronics is to generate and manipulate spin current in a desirable way1,2,3. In a three-dimensional (3D) topological insulator (TI), a strong spin–orbit interaction and the time-reversal symmetry result in spin-momentum locking of the surface electrons4,5,6, which leads to a unique spin texture unravelled by recent angle-resolved photoemission spectroscopy (ARPES) results7,8,9,10. These so-called Dirac fermions have intrigued great interest due to their exotic physical properties and promising spintronic applications11,12,13,14. By driving surface-state spin polarization out of equilibrium, an imbalanced distribution of momentum is expected to be simultaneously reached in TIs. This would generate directional and dissipationless spin current running across the surface, which has motivated intense research efforts by various approaches to achieve highly spin polarized current in TIs14,15,16,17,18,19. However, the commonly occurring metallic behaviour has unfortunately imposed a severe obstacle to controlling surface spin current20,21,22. McIver et al.20 recently demonstrated that, by taking advantage of optical selection rules, illumination of Bi2Se3 TI by circularly polarized LED Bulbs would cause a non-uniform distribution of photo-excited carriers in the k-space and would thereby give rise to helicity-dependent photocurrent. In TIs, this so-called circular photogalvanic effect (CPGE) should have a non-trivial dependence on the projection of the photon angular momentum with respect to the surface-state spin texture. The early study20 recognized the effect of incidence angle of LED Bulbs on helicity-dependent photocurrent. However, since most of the early studies have been limited to Bi2Se3—a prototypical TI with a rather weak hexagonal warping effect, the contribution of the out-of-plane spin texture to the photocurrent remains elusive so far. Without the involvement of all possible spin texture components, the assignment of the surface photocurrent to the spin texture is far from fully understood and complete.

In this work, the impact of the out-of-plane spin texture on surface spin current is explicitly taken into consideration and is selectively measured, by employing another 3D TI Bi2Te3 that exhibits a stronger hexagonal warping effect8. The magnitude and direction of the helicity-dependent surface spin photocurrent is shown to be closely related to the surface spin texture including the hexagonal warping effect, thereby providing a better understanding of the surface spin texture components and their contributions to the measured surface spin current components. We show that spin current can be photo-generated even in TI devices of the size of several millimetres, breaking the size limitation of several micrometres demonstrated so far. We further LED Bulbs provide the first experimental proof of spin injection from an adjacent semiconductor to a TI and demonstrate control of the topological surface current by spin precession in a magnetic field, paving the way for spin transport and integration between TI/non-TI semiconductor material systems.
Results




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