Dirac-Screening Stabilized Surface-State Transport in a Topological Insulator

Abstract

We report magnetotransport studies on a gated strained HgTe device. This material is a threedimensional topological insulator and exclusively shows surface-state transport. Remarkably, the Landaulevel dispersion and the accuracy of the Hall quantization remain unchanged over a wide density range (3 × 10 11 cm −2 < n < 2 × 10 12 cm −2 ). These observations imply that even at large carrier densities, the transport is surface-state dominated, where bulk transport would have been expected to coexist already. Moreover, the density dependence of the Dirac-type quantum Hall effect allows us to identify the contributions from the individual surfaces. A k · p model can describe the experiments but only when assuming a steep band bending across the regions where the topological surface states are contained. This steep potential originates from the specific screening properties of Dirac systems and causes the gate voltage to influence the position of the Dirac points rather than that of the Fermi level.

Probing Quantum Capacitance in a 3D Topological Insulator

“…While unstrained HgTe is a zero gap semiconductor with inverted band structure [8,9], the degenerate Γ8 states split and a gap opens at the Fermi energy E F if strained. This system is a strong topological insulator [10], explored by transport [6,7,11], angleresolved photoemission spectroscopy [12], photoconductivity, and magneto-optical experiments [13][14][15][16]; also, the proximity effect has been investigated [17]. Since these two-dimensional electron states (2DES) have high electron mobilities of several 10 5 cm 2 =V s, pronounced Shubnikov-de Haas (SdH) oscillations of the resistivity and quantized Hall plateaus commence in quantizing magnetic fields [6,7,11], stemming from both top and bottom 2DES.…”

Double valley Dirac fermions for 3D and 2D Hg 1− x Cd x Te with strong asymmetry

“…[1] the existence of 3D Topological Insulators (TI) in the HgTe 75-80 nm wide strained layers has been predicted theoretically, and it was later confirmed experimentally in the work [2]. Since then many papers devoted to the the topological surface states (TSS) existing at the surfaces of such structures and which can be described by the pseudo-Dirac fermions [2][3][4][5][6] were published. The Dirac-like dispersion is observed due the uniaxial tensile strain along (001) axis which lifts the degeneracy of the Γ 8 band by breaking the cubic symmetry at the Γ point and opens the insulating gap.…”

“…The authors of these papers restricted their consideration only to different potentials originated from the different dielectric constancts ǫ on both surfaces of the structure in question, or to the external gate voltage (see, for instance, Ref. [3]). …”

“…In this case the external gate voltage is necessary p-2 for two reasons: first, to decrease the concentration, which is especially important for low carrier densities when the broadening is stronger due to reduced screening [3]; and second, to split the TSS in order to observe two filling factors originating from two different independent 2DEG at the surfaces corresponding to two Dirac-cones under magnetic field according to the relation ν = (N t + N b + 1). Similar approach was used in case of pure 3D strained HgTe structures with the Nb as superconductor [37].…”

“…The discussion carried on in most of the papers devoted to 3D HgCdTe TI [2,3] as well as to 2D HgTe TI [8] were based on two complementary models: 1. the kp-model (which allows to get the Dirac point for 6.4 nm wide QW and the TSS for 3D TI against strained gap); 2. simple one/two Dirac cones model for QW/3D TI structures respectively. These allow to explain the magneto transport experiments [2,8,35,36].…”