Tuning the quantum oscillations of surface Dirac electrons in the topological insulator Bi$_2$Te$_2$Se by liquid gating

TL;DR

This study demonstrates that ionic liquid gating can significantly tune the quantum oscillations of surface Dirac electrons in Bi$_2$Te$_2$Se, enabling precise measurement of the Dirac spectrum and revealing surface-specific mobility enhancements.

Contribution

It shows that liquid gating can increase quantum oscillation periods and surface mobility, and provides detailed analysis of the gating effects on surface and bulk electronic properties.

Findings

Quantum oscillation period increased 6-fold by gating

Fermi energy reaches N=1 Landau level at high gate voltages

Surface mobility is strongly enhanced on one surface

Abstract

In Bi$_2$Te$_2$Se, the period of quantum oscillations arising from surface Dirac fermions can be increased 6-fold using ionic liquid gating. At large gate voltages, the Fermi energy reaches the $N$ = 1 Landau level in a 14-Tesla field. This enables the $\frac12$-shift predicted for the Dirac spectrum to be measured accurately. A surprising result is that liquid gating strongly enhances the surface mobility. By analyzing the Hall conductivity, we show that the enhancement occurs on only one surface. We present evidence that the gating process is fully reversible (hence consistent with band-bending by the $E$-field from the anion layer accumulated). In addition to the surface carriers, the experiment yields the mobility and density of the bulk carriers in the impurity band. By analyzing the charge accumulation vs. gate voltage, we also obtain estimates of the depletion width and the areal depletion capacitance $C_d/A$. The value of $C_d/A$ implies an enhanced electronic polarizability in the depletion region.