Unexpected edge conduction in HgTe quantum wells under broken time reversal symmetry

TL;DR

This study reveals unexpected persistent edge conduction in HgTe quantum wells under broken time reversal symmetry, challenging existing QSH theories and suggesting new physics involving material properties and interactions.

Contribution

The paper provides a rigorous framework for magnetic field effects on QSH devices and reports novel persistent edge conduction under broken TRS in HgTe quantum wells.

Findings

Edge conduction persists up to 9 T despite broken TRS.

Edge conduction observed at zero magnetic field in 7.5 nm HgTe QW.

Contrasts with trivial band structure control device.

Abstract

The realization of quantum spin Hall (QSH) effect in HgTe quantum wells (QWs) is considered a milestone in the discovery of topological insulators. The QSH edge states are predicted to allow current to flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction of QSH theory that remains to be experimentally verified is the breakdown of the edge conduction under broken time reversal symmetry (TRS). Here we first establish a rigorous framework for understanding the magnetic field dependence of electrostatically gated QSH devices. We then report unexpected edge conduction under broken TRS, using a unique cryogenic microwave impedance microscopy (MIM), on a 7.5 nm HgTe QW device with an inverted band structure. At zero magnetic field and low carrier densities, clear edge conduction is observed in the local conductivity profile of this device but not in the 5.5 nm control device whose band structure is trivial. Surprisingly, the edge conduction in the 7.5 nm device persists up to 9 T with little effect from the magnetic field. This indicates physics beyond simple QSH models, possibly associated with material- specific properties, other symmetry protection and/or electron-electron interactions.