Towards the quantum spin Hall regime in (Bi_1−xSb_x)₂Te₃ ultrathin films

Theory predicts that reducing the thickness of 3D topological insulator thin films opens a hybridization

gap at the Dirac point. Depending on film thickness, the nature of this gap oscillates between trivial and

inverted [1, 2]. When the chemical potential of such a thin film lies within an inverted hybridization gap,

the system will be in the quantum spin Hall (QSH) state, with two counterpropagating spin-momentum

locked channels along each edge.

Ultrathin (Bi_1−xSb_x)₂Te₃ is a candidate material, as tuning the Bi/Sb ratio moves the Dirac point into

the bulk band gap [3]. We optimize the morphology of ultrathin (Bi_1−xSb_x)₂Te₃ on Al₂O₃ substrates

by molecular beam epitaxy, by tuning both substrate temperature and vicinal angle. This leads to film

thicknesses expected to lie within the QSH regime.

The films are structured into micrometer-sized devices to perform both local and non-local transport

measurements at cryogenic temperatures. Due to the morphology of the films, we do not expect

a coherent percolation path between source and drain contacts. However, 1D contributions to the

conductivity can be extracted by comparing the scaling of features as a function of device geometry [4].

Based on this, we study the possibility of both 2D and 1D conductance based on electronic transport

data.

References:

[1] PRB 81, 041307 (2010)

[2] PRB 97, 075419 (2018)

[3] Nat Commun 2, 574 (2011)

[4] PRL 123, 047701 (2019)