Аннотация:In this report, we show that the photoelectromagnetic effect may be an effective tool for detection of topological surface states in topological insulators which is demonstrated on an example of
(Bi1-хInх)2Se3 solid solutions.
Observation of the Dirac-like surface electron states in the topological insulator (TI) Bi2Se3 by means of the angle resolved and spin resolved photoemission spectroscopy techniques initiated extensive studies of electron transport properties of the topological surface layer. The photoelectromagnetic (PEM) effect exited by terahertz laser radiation was demonstrated to be an effective tool for detection of surface electron states since it is not sensitive to the bulk free carrier concentration [1], and it may provide a way to discriminate the topological states from the other ones at the sample surface. For establishing a plausible correspondence between the PEM effect features and the Dirac-like electron contribution, it is expedient to compare data obtained for the TI and trivial phases. In this work, we present experimental results obtained for the solid solutions (Bi1-xInx)2Se3 which reveal a transition from the TI phase (x < 0.05) characterized by the inverted energy band structure to the trivial semiconductor (х > 0.06) with the direct energy spectrum [2,3].
(Bi1-хInх)2Se3 (0 ≤ x ≤ 0.18) single crystals were grown by the Bridgman method. The alloy composition x was determined by the XRD, X-ray fluorescence analysis and EDX. According to the photoluminescence spectroscopy data, the gapless state corresponds to the alloy composition x ~ 0.06. All samples are highly degenerate semiconductors with the free electron concentration ~ 1019 cm-3. The PEM effect was studied at temperatures 4.2 K – 20 K in magnetic fields up to 3 T using laser pulses at the wavelengths of 90, 148 and 280 μm. The peak power of the incident radiation was up to 10 kW and varied by calibrated attenuators.
The PEM effect was observed in all the samples at T < 14 K, at all laser wavelengths employed, and it was odd in magnetic field. The PEM effect sign corresponds to the net electron flux directed from the sample surface to its bulk, which is an evidence of enhanced mobility values of the surface electrons both in the topological and trivial phases, since there is no generation of extra carriers. However, it is shown that for the TI phase, the PEM effect amplitude is defined by the number of incident radiation quanta, whereas for the trivial phase, it depends on the power of a laser pulse irrespective of its wavelength. The possible reasons for such a difference are discussed, taking into account the strong damping of electron-electron interaction in the near-surface topological layer.