Odd integer quantum Hall effect in graphene

TL;DR

This paper proposes that in graphene under uniform real and pseudo-magnetic fields, odd integer quantum Hall plateaus can occur due to valley degeneracy lifting, with electron interactions potentially leading to a full integer quantization of Hall conductivity.

Contribution

It introduces a theoretical framework for observing odd integer quantum Hall effects in graphene through uniform pseudo-magnetic fields and analyzes interaction effects on the Landau levels.

Findings

Valley degeneracy can be lifted by pseudo-magnetic fields.

Interactions can induce a gapped insulating phase with ferrimagnetic order.

Quantized Hall plateaus at all integer values are possible with interactions.

Abstract

A possible realization of Hall conductivity, quantized at odd integer factors of $e^2/h$ for graphene's honeycomb lattice is proposed. I argue that, in the presence of \emph{uniform} real and pseudo-magnetic fields, the valley degeneracy from the higher Landau levels can be removed. A pseudo-magnetic field may arise from bulging or stretching of the graphene flake. This may lead to observation of plateaus in the Hall conductivity at quantized values $f e^2/h$, with $f=\pm 3, \pm 5$ etc, which have not been observed in measurement of Hall conductivity. However, in a collection of noninteracting Dirac fermions living in the honeycomb lattice subject to real and pseudo field, the zeroth Landau level still enjoys the valley and the spin degeneracy. Upon including the Zeeman coupling, the spin degeneracy is removed from all the Landau levels. The effects of short ranged electron-electron interactions are also considered, particularly, the onsite Hubbard repulsion (U) and the nearest-neighbor Coulomb repulsion (V). Within the framework of the extended Hubbard model with only those two components of finite ranged Coulomb repulsion, it is shown that infinitesimally weak interactions can place the system in a gapped insulating phase by developing a \emph{ferrimegnatic} order, if $U>>V$. Therefore, one may expect to see the plateaus in the Hall conductivity at all the integer values, $f=0,\pm 1,\pm 2, \pm3,...$. Scaling behavior of interaction induced gap at $f=1$ in presence of finite pseudo flux is also addressed. Qualitative discussion on finite size effects and behavior of the interaction induced gap when the restriction on uniformity of the fields are relaxed, is presented as well. Possible experimental set up that can test relevance of our theory has been proposed.