Theory of Competing Charge Density Wave, Kekule and Antiferromagnetic ordered Fractional Quantum Hall states in Graphene aligned with Boron Nitride

TL;DR

This paper models and predicts complex phase transitions in fractional quantum Hall states in graphene aligned with boron nitride, revealing new ordered states and transitions driven by magnetic field variations.

Contribution

It extends quantum Hall ferromagnetism theory to fractional states, predicting novel phase transitions and ordered states in graphene on boron nitride.

Findings

Sequence of phase transitions with increasing magnetic field.

Prediction of a canted Kekule density phase (CaKD).

Transitions between Laughlin-like and Halperin-like states.

Abstract

We investigate spin and valley symmetry-broken fractional quantum Hall phases within a formalism that naturally extends the paradigm of quantum Hall ferromagnetism from integer to fractional quantum Hall states, allowing us to construct detailed phase diagrams for a large class of multi-component states. Motivated by recent experiments on Graphene aligned with a Boron Nitride substrate, we predict a sequence of transitions realized by increasing the magnetic field, starting from a sub-lattice polarized state to a valley coherent Kekule charge density wave state and further to an anti-ferromagnetic phase. Moreover for filling fractions such as $\nu=\pm 1/3$, we predict that the system undergoes a transition at low fields, that not only differ by the spin-valley orientation of the fractionally filled flavors but also by their intrinsic fractional quantum Hall nature. This transition is from a Laughlin-like state to a two component Halperin-like state both with a charge density wave order. Moreover for $\nu=\pm 1/3,\pm 2/3$, we predict a "canted Kekule density phase"(CaKD) where the spinors of integer and fractionally occupied components have different orientations in the valley Bloch sphere, in contrast to the Kekule state for the integer quantum Hall state at neutrality where both occupied components have the same orientation in the valley Bloch sphere.