Low carrier concentration crystals of the topological insulator Bi_2−xSb_xTe_3−ySe_y: a magnetotransport study

Abstract

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 et al 2011 Phys. Rev. B 84 165311). Here we present a study to further refine the bulk-insulating properties of BSTS. We have synthesized BSTS 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 μm. The magnetoconductance of exfoliated BSTS nanoflakes shows 2D weak antilocalization with α ≃ −1 as expected for transport dominated by topological surface states.

Quantum oscillations of the topological surface states in low carrier concentration crystals of Bi2−xSbxTe3Pan1, Никитин2, Wu3 et al. 2016Solid State Communications7100View full textAdd to dashboardCite

“…For a spheroidal Fermi surface with an aspect ratio of 2 these values of k F would result in a bulk carrier density n 3D = (1/2) × (2k F ) 3 /(3π 2 ) of ∼ 10 19 cm −3 assuming no spin degeneracy. This value exceeds the bulk carrier concentration calculated from the Hall data [22] by a factor of 1000.…”

“…For a sample with a typical thickness of 100 µm, the ratio of the surface conductance over the total conductance is about 27 %. Upon further reducing the sample thickness, this ratio increases and it can be as large as 97 % for a 1 µm thick sample [22]. The magnetoconductance of BSTS nanoflakes, prepared around the optimum composition, showed 2D weak antilocalization with an amplitude α ≃ −1, as expected for transport dominated by topological surface states [22].…”

“…For details of the single crystal growth procedure and characterization of the crystals by magnetotransport we refer to Ref. [22]. Flat rectangular samples were cut from the single-crystalline boule using a scalpel blade.…”

“…1(a) we show the longitudinal resistance R xx of BSTS crystals with (x, y) = (0.50, 1.3) and (x, y) = (0.54, 1.3) as a function of the magnetic field at T = 4.2 K and θ = 0 • . After the initial sharp rise connected to the suppression of the weak antilocalization (WAL) in low fields [22], the magnetoresistance increases in a quasi-linear manner without saturation. Selected crystals showed a clear SdH effect.…”

“…Recently we reported an extensive magnetotransport study that aimed at the further investigation of the bulk-insulating properties of BSTS [22]. Single crystals with composition Bi 1.46 Sb 0.54 Te 1.7 Se 1.3 produced the highest resistivity (12.6 Ωcm) and lowest bulk carrier density (0.2 ×10 16 cm −3 ) at low temperatures.…”