Weak localization of bulk channels in topological insulator thin films

Abstract

Weak antilocalization (WAL) is expected whenever strong spin-orbit coupling or scattering comes into play. Spin-orbit coupling in the bulk states of a topological insulator is very strong, enough to result in the topological phase transition. However, the recently observed WAL in topological insulators seems to have an ambiguous origin from the bulk states. Starting from the effective model for three-dimensional topological insulators, we find that the lowest two-dimensional (2D) bulk subbands of a topological insulator thin film can be described by the modified massive Dirac model. We derive the magnetoconductivity formula for both the 2D bulk subbands and surface bands. Because with relatively large gap, the 2D bulk subbands may lie in the regimes where the unitary behavior or even weak localization (WL) is also expected, instead of always WAL. As a result, the bulk states may contribute small magnetoconductivity or even compensate the WAL from the surface states. Inflection in magnetoconductivity curves may appear when the bulk WL channels outnumber the surface WAL channels, providing a signature of the weak localization from the bulk states.

Reversible Fermi Level Tuning of a Sb2Te3 Topological Insulator by Structural Deformation

“…Therefore, the MR change in the bulk state is proportional to B 2 , which is different from a WAL effect, as shown in Supporting Information Figure S6. 35 Finally, we concluded that only the top surface state contributes to the WAL effect and bottom surface state did not contribute because of the rough surface of the polyimide substrate..…”

Robust topological surface states in Sb2Te3layers as seen from the weak antilocalization effect

“…It is very unlikely that the contributions from the surface state and the bulk state become identical.) Lu and Shen 35 have shown theoretically that the bulk states may give rise to a weak localization (WL) effect despite the strong spin-orbit coupling. They argued that the universal experimental observation of |α| < 1 that can be even α ∼ −0.3 is indication of the bulk contribution.…”

Topological Insulator Thin Films and Heterostructures: Epitaxial Growth, Transport, and Magnetism

“…In practice, while this equation is for 2D states with parabolic -not Dirac -dispersion, it usually fits actual data quite well, but often deviates from strict multiples of 1/2. Therefore, in analyzing transport data, there is always the question of whether the surfaces can be considered as two independent channels or whether they are coupled through the bulk and what the contribution of the bulk is [89]. The value of also typically changes as samples are gated through the Dirac point, sometimes showing a clean crossover from = 1∕2 to = 1 [31,90].…”