Terahertz conductivity of topological surface states in Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂ - PubMed (original) (raw)

Terahertz conductivity of topological surface states in Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂

Chi Sin Tang et al. Sci Rep. 2013.

Abstract

Topological insulators are electronic materials with an insulating bulk and conducting surface. However, due to free carriers in the bulk, the properties of the metallic surface are difficult to detect and characterize in most topological insulator materials. Recently, a new topological insulator Bi₁.₅Sb₀.₅Te₁.₇Se₁.₃ (BSTS) was found, showing high bulk resistivities of 1-10 Ω.cm and greater contrast between the bulk and surface resistivities compared to other Bi-based topological insulators. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we present complex conductivity of BSTS single crystals, disentangling the surface and bulk contributions. We find that the Drude spectral weight is 1-2 orders of magnitude smaller than in other Bi-based topological insulators, and similar to that of Bi₂Se₃ thin films, suggesting a significant contribution of the topological surface states to the conductivity of the BSTS sample. Moreover, an impurity band is present about 30 meV below the Fermi level, and the surface and bulk carrier densities agree with those obtained from transport data. Furthermore, from the surface Drude contribution, we obtain a ~98% transmission through one surface layer--this is consistent with the transmission through single-layer or bilayer graphene, which shares a common Dirac-cone feature in the band structure.

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Figures

Figure 1

Figure 1. In-plane DC resistivity.

Inset = ARPES data of BSTS single crystal. Notice the band dispersion near the Fermi level.

Figure 2

Figure 2. Amplitude of complex transmittance, , at various temperatures.

Figure 3

Figure 3

(a) Real conductance _G_1(ω) at 30 K. Inset shows the imaginary conductance _G_1(ω). (O) = data. Solid lines = Drude (green) and Lorentz (blue) contributions, and the fit to data (red). (b) Low-frequency real conductivity σ1 versus temperature. (O) = data. Solid line = fit to Eq. (3).

Figure 4

Figure 4. Temperature dependence of the (a) Drude plasma frequency ωpD, (b) Lorentz plasma frequency ωpL, (c) Drude scattering rate γD, and (d) Lorentz scattering rate γL.

Solid line = fit to Eq. (3).

Figure 5

Figure 5. Schematic of band diagram of BSTS, with the (donor) impurity band lying just below the Fermi level.

The relative positions of the valence band, Dirac point, Fermi level and conduction band were taken from Ref. for a Bi1.5Sb0.5Te1.7Se1.3 sample. Δ = activation gap.

Figure 6

Figure 6. Spectral weight of various Bi-based topological insulators at 5 K calculated using Eq. 4.

Dashed line represents the contribution from topological surface carriers of Bi2Se3 thin film. Adapted with permission from Fig. 4 of Ref. . Copyright 2012 by the American Physical Society.

Figure 7

Figure 7. Real conductivity [_σ_1(ω)] of a 90-μm-thick BSTS sample at 10 K, 50 K, 90 K and 150 K. Experimental data were taken in the sequence: 10 K → 50 K → 90 K → 150 K → 10 K → 50 K → 90 K → 150 K.

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