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Register a new userFaraday rotation due to surface states in the topological insulator (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$
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Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)$_2$Te$_3$ thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron- and hole-type Dirac fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.
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We show Shubnikov-de Haas oscillations in topological insulator (Bi$_{x}$Sb$_{1-x}$)$_{2}$Te$_{3}$ films whose carrier type is p-type (x = 0.29, 0.34) and n-type (x = 0.42). The physical properties such as the Berry phase, mobility, and the scattering time are significantly changed by tuning the Fermi-level position with the concentration x. The Landau-level fan diagram in the sample with x = 0.42 showed the $pi$ Berry phase and its mobility was as high as 17,000 cm$^{2}$/V/s, whereas the others had the 2$pi$ Berry phase and much lower mobility. This suggests that because the bulk band of the sample with x = 0.42 does not cross the Fermi level, it becomes bulk insulating, resulting in the topological surface-state dominating transport. Thus, we can switch sample properties from degenerate to bulk insulating by tuning the concentration x, which is consistent with results of angle-resolved photoemission spectroscopy.
We report on the observation of a terahertz radiation induced photon drag effect in epitaxially grown $n$- and $p$-type (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ three dimensional topological insulators with different antimony concentrations $x$ varying from 0 to 1. We demonstrate that the excitation with polarized terahertz radiation results in a $dc$ electric photocurrent. While at normal incidence a current arises due to the photogalvanic effect in the surface states, at oblique incidence it is outweighed by the trigonal photon drag effect. The developed microscopic model and theory show that the photon drag photocurrent is due to the dynamical momentum alignment by time and space dependent radiation electric field and implies the radiation induced asymmetric scattering in the electron momentum space.
We report a high-field magnetotransport study on selected low-carrier crystals of the topological insulator Bi$_{2-x}$Sb${_x}$Te$_{3-y}$Se$_{y}$. Monochromatic Shubnikov - de Haas (SdH) oscillations are observed at 4.2~K and their two-dimensional nature is confirmed by tilting the magnetic field with respect to the sample surface. With help of Lifshitz-Kosevich theory, important transport parameters of the surface states are obtained, including the carrier density, cyclotron mass and mobility. For $(x,y)=(0.50,1.3)$ the Landau level plot is analyzed in terms of a model based on a topological surface state in the presence of a non-ideal linear dispersion relation and a Zeeman term with $g_s = 70$ or $-54$. Input parameters were taken from the electronic dispersion relation measured directly by angle resolved photoemission spectroscopy on crystals from the same batch. The Hall resistivity of the same crystal (thickness of 40~$mu$m) is analyzed in a two-band model, from which we conclude that the ratio of the surface conductance to the total conductance amounts to 32~%.
In 3D topological insulators achieving a genuine bulk-insulating state is an important research topic. Recently, the material system (Bi,Sb)$_{2}$(Te,Se)$_{3}$ (BSTS) has been proposed as a topological insulator with high resistivity and a low carrier concentration (Ren textit{et al.} cite{Ren2011}). Here we present a study to further refine the bulk-insulating properties of BSTS. We have synthesized Bi$_{2-x}$Sb${_x}$Te$_{3-y}$Se$_{y}$ single crystals with compositions around $x = 0.5$ and $y = 1.3$. Resistance and Hall effect measurements show high resistivity and record low bulk carrier density for the composition Bi$_{1.46}$Sb$_{0.54}$Te$_{1.7}$Se$_{1.3}$. The analysis of the resistance measured for crystals with different thicknesses within a parallel resistor model shows that the surface contribution to the electrical transport amounts to 97% when the sample thickness is reduced to $1 mu$m. The magnetoconductance of exfoliated BSTS nanoflakes shows 2D weak antilocalization with $alpha simeq -1$ as expected for transport dominated by topological surface states.
The interplay between magnetism and non-trivial topology in magnetic topological insulators (MTI) is expected to give rise to a variety of exotic topological quantum phenomena, such as the quantum anomalous Hall (QAH) effect and the topological axion states. A key to assessing these novel properties is to tune the Fermi level in the exchange gap of the Dirac surface band. MnBi$_2$Te$_4$ possesses non-trivial band topology with intrinsic antiferromagnetic (AFM) state that can enable all of these quantum states, however, highly electron-doped nature of the MnBi$_2$Te$_4$ crystals obstructs the exhibition of the gapped topological surface states. Here, we tailor the material through Sb-substitution to reveal the gapped surface states in MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ (MBST). By shifting the Fermi level into the bulk band gap of MBST, we access the surface states and show a band gap of 50 meV at the Dirac point from quasi-particle interference (QPI) measured by scanning tunneling microscopy/spectroscopy (STM/STS). Surface-dominant conduction is confirmed below the Neel temperature through transport spectroscopy measured by multiprobe STM. The surface band gap is robust against out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. The realization of bulk-insulating MTI with the large exchange gap offers a promising platform for exploring emergent topological phenomena.
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