Severe Dirac Mass Gap Suppression in Sb₂Te₃-Based Quantum Anomalous Hall Materials

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Phenomena

Background:

• The quantum anomalous Hall (QAH) effect arises in ferromagnetic topological insulators (FMTIs) due to a Dirac mass gap in surface states.
• Despite a significant mean mass gap, the QAH effect is often observed only below 1 K.

Purpose of the Study:

• To investigate the reasons behind the low-temperature limitation of the QAH effect in FMTIs.
• To compare the electronic structure of a Cr-doped FMTI with its nonmagnetic parent material.

Main Methods:

• Atomic-resolution Landau level spectroscopic imaging.
• Comparative analysis of Cr_0.08(Bi_0.1Sb_0.9)_1.92Te₃ and (Bi_0.1Sb_0.9)₂Te₃ electronic structures.

Main Results:

• Spatially random variations in Dirac energy were observed in the nonmagnetic parent material.
• Similar Dirac energy variations and uncorrelated mass gap disorder were found in the FMTI.
• These electronic disorders collectively reduce the minimum mass gap to below 100 μeV in nanoscale regions.

Conclusions:

• Electronic disorder in surface states is the primary cause for the low-temperature detection of the QAH effect.
• The combined effects of Dirac energy variations and mass gap disorder fundamentally limit the QAH effect in Sb₂Te₃-based FMTIs.