Origin of the low critical observing temperature of the quantum anomalous Hall effect in V-doped (Bi, Sb)2Te3 film

TL;DR

This paper investigates why the quantum anomalous Hall effect in V-doped (Bi, Sb)2Te3 films occurs at much lower temperatures than the ferromagnetic transition, revealing that bulk valence band localization impedes higher temperature quantization.

Contribution

It provides a systematic ARPES study showing the bulk valence band maximum is above the surface state Dirac point, explaining the low critical temperature.

Findings

Bulk valence band maximum is higher than the Dirac point.

Localization of bulk valence band carriers affects the QAH effect.

The temperature discrepancy is linked to bulk band structure.

Abstract

The experimental realization of the quantum anomalous Hall (QAH) effect in magnetically-doped (Bi, Sb)2Te3 films stands out as a landmark of modern condensed matter physics. However, ultra-low temperatures down to few tens of mK are needed to reach the quantization of Hall resistance, which is two orders of magnitude lower than the ferromagnetic phase transition temperature of the films. Here, we systematically study the band structure of V-doped (Bi, Sb)2Te3 thin films by angle-resolved photoemission spectroscopy (ARPES) and show unambiguously that the bulk valence band (BVB) maximum lies higher in energy than the surface state Dirac point. Our results demonstrate clear evidence that localization of BVB carriers plays an active role and can account for the temperature discrepancy.