HgTe quantum wells for QHE metrology under soft cryomagnetic conditions: permanent magnets and liquid ${^4He}$ temperatures

TL;DR

This study demonstrates that HgTe quantum wells can exhibit quantum Hall effect plateaus at relatively low magnetic fields and temperatures, making them suitable for resistance standards under relaxed cryomagnetic conditions.

Contribution

The paper provides experimental evidence that HgTe quantum wells can operate as QHE resistance standards using permanent magnets and liquid helium temperatures, due to their unique band structure.

Findings

Observation of wide QHE plateaus at low magnetic fields

Critical magnetic field B_c depends on quantum well width

Band structure analysis explains the phenomena qualitatively

Abstract

HgTe quantum wells with a thickness of ${\sim}$7 nm may have a graphene-like band structure and have been recently proposed to be potential candidates for quantum Hall effect (QHE) resistance standards under the condition of operation in the fields above certain critical field $B_c$, above which the topological phase (with parasitic edge conduction) disappears. We present experimental studies of the magnetoresistance of different of HgTe quantum wells as a function temperature and magnetic field, determining the critical magnetic field $B_c$. We demonstrate that for QWs of specific width $B_c$ becomes low enough to grant observation of remarkably wide QHE plateaus at the filling factor ${v=-1}$ (holes) in relaxed cryomagnetic conditions: while using commercial 0.82 T Neodymium permanent magnets and temperature of a few Kelvin provided by ${^4He}$ liquid system only. Band structure calculations allow us to explain qualitatively observed phenomena due to the interplay between light holes and heavy holes energy sub-bands (side maxima of the valence band). Our work clearly shows that the peculiar band structure properties of HgTe QWs with massless Dirac fermions make them an ideal platform for developing metrological devices with relaxed cryomagnetic conditions.