Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3

By Dohun Kim, Sungjae Cho, N. Butch, P. Syers, K. Kirshenbaum, S. Adam, J. Paglione, M. Fuhrer. Nature Physics, 2011

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Despite their name, the bulk electrical conductivity of most topological insulators is relatively high, masking many of the important characteristics of its protected, surface conducting states. Counter-doping reduces the bulk conductivity of Bi2Se3 significantly, allowing these surface states and their properties to be clearly identified. The newly discovered three-dimensional strong topological insulators (STIs) exhibit topologically protected Dirac surface states1,2. Although the STI surface state has been studied spectroscopically, for example, by photoemission3,4,5 and scanned probes6,7,8,9,10, transport experiments11,12,13,14,15,16,17 have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (∼ 10 nm), low-doped Bi2Se3 (≈1017 cm−3) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region18,19,20. A theory of charge disorder in a Dirac band19,20,21 explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e2/h per surface) and the residual (puddle) carrier density (0.4×1012 to 4×1012 cm−2). From the measured carrier mobilities 320–1,500 cm2 V−1 s−1, the charged impurity densities 0.5×1013 to 2.3×1013 cm−2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1×1013 to 3×1013 cm−2), identifying dopants as the charged impurities.

Subfields: Atomic and Molecular Physics, and Optics, Materials Chemistry

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