Abstract
Three-dimensional topological insulators have been demonstrated in recent years, which possess intriguing gapless, spin-polarized Dirac states with linear dispersion only on the surface. The spin polarization of the topological surface states is also locked to its momentum, which allows controlling motion of electrons using optical helicity, i.e., circularly polarized light. The electrical and thermal transport can also be significantly tuned by the helicity-control of surface state electrons. Here, we report studies of photo-thermoelectric effect of the topological surface states in Bi2Te2Se thin films with large tunability using varied gate voltages and optical helicity. The Seebeck coefficient can be altered by more than five times compared to the case without spin injection. This deep tuning is originated from the optical helicity-induced photocurrent which is shown to be enhanced, reduced, turned off, and even inverted due to the change of the accessed band structures by electrical gating. The helicity-selected topological surface state thus has a large effect on thermoelectric transport, demonstrating great opportunities for realizing helicity control of optoelectronic and thermal devices.
Highlights
Three-dimensional topological insulators have been demonstrated in recent years, which possess intriguing gapless, spin-polarized Dirac states with linear dispersion only on the surface
We studied the photo-thermoelectric effect of topological surface states (TSS) in 3D topological insulator Bi2Te2Se thin films
The decrease of the sheet resistance versus back-gate voltage indicates the n-type carrier, and the monotonic decrease indicates that the Fermi level is always above the Dirac point at all back gate voltages used in the experiment as illustrated in the band structure shown in the inset of Fig. 1d15,23. (Absolute value of the Seebeck coefficient is used in the discussion below since the sample is always n-type) Our measurements confirm Ohmic contact, since no signature of photodiode due to Schottky barriers is observed in the photocurrent scanning[41]
Summary
Three-dimensional topological insulators have been demonstrated in recent years, which possess intriguing gapless, spin-polarized Dirac states with linear dispersion only on the surface. The thermoelectric properties can be tuned by electrical field-effect gating[15,19,30,37] and temperature[38] via effectively modulating the carrier densities of both TSS and bulk states, and the polarization-dependent photocurrent can be tuned[22,37,39].
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