Crossover of Three-Dimensional Topological Insulator of Bi2Se3 to the Two-Dimensional Limit

TL;DR

This study investigates how ultrathin Bi2Se3 films transition from three-dimensional topological insulators to two-dimensional systems, revealing a thickness-dependent energy gap and controllable spin-splitting in surface states.

Contribution

It provides the first experimental observation of a gap opening in topologically protected surface states of Bi2Se3 below six quintuple layers, and demonstrates control of spin-splitting via substrate-induced potential differences.

Findings

Energy gap appears below six quintuple layers.

Surface states exhibit Rashba-type spin-orbit splitting.

Spin-splitting is tunable by substrate potential.

Abstract

Bi2Se3 is theoretically predicted1 2and experimentally observed2,3 to be a three dimensional topological insulator. For possible applications, it is important to understand the electronic structure of the planar device. In this work, thickness dependent band structure of molecular beam epitaxy grown ultrathin films of Bi2Se3 is investigated by in situ angle-resolved photoemission spectroscopy. An energy gap is observed for the first time in the topologically protected metallic surface states of bulk Bi2Se3 below the thickness of six quintuple layers, due to the coupling between the surface states from two opposite surfaces of the Bi2Se3 film. The gapped surface states exhibit sizable Rashba-type spin-orbit splitting, due to breaking of structural inversion symmetry induced by SiC substrate. The spin-splitting can be controlled by tuning the potential difference between the two surfaces.