Gate-tuned Differentiation of Surface-conducting States in Bi1.5Sb0.5Te1.7Se1.3 Topological-insulator Thin Crystals

TL;DR

This study uses gate-tuning and magnetic field measurements to distinguish topological surface conduction from bulk and trivial surface conduction in Bi1.5Sb0.5Te1.7Se1.3 topological insulator crystals, advancing understanding of their surface states.

Contribution

It introduces a method combining field-angle, temperature, and gate-voltage dependence to reliably separate topological surface conduction from other conduction types in TI crystals.

Findings

Successfully minimized bulk conduction in crystals.

Fermi level tuned to the topological bottom surface.

Weak anti-localization confirmed topological surface states.

Abstract

Using field-angle, temperature, and back-gate-voltage dependence of the weak anti-localization (WAL) and universal conductance fluctuations of thin Bi1.5Sb0.5Te1.7Se1.3 topological-insulator single crystals, in combination with gate-tuned Hall resistivity measurements, we reliably separated the surface conduction of the topological nature from both the bulk conduction and topologically trivial surface conduction. We minimized the bulk conduction in the crystals and back-gate tuned the Fermi level to the topological bottom-surface band while keeping the top surface insensitive to back-gating with the optimal crystal thickness of ~?100 nm. We argue that the WAL effect occurring by the coherent diffusive motion of carriers in relatively low magnetic fields is more essential than other transport tools such as the Shubnikov-de Hass oscillations for confirming the conduction by the topologically protected surface state. Our approach provides a highly coherent picture of the surface transport properties of TIs and a reliable means of investigating the fundamental topological nature of surface conduction and possible quantum-device applications related to momentum-locked spin polarization in surface states.