Fractional quantum Hall valley ferromagnetism in the extreme quantum limit

TL;DR

This study demonstrates that fractional quantum Hall states in an AlAs two-dimensional electron system exhibit robust ferromagnetism and strong interactions, with tunable valley polarization revealing a rich phase diagram at high magnetic fields.

Contribution

It introduces a method to continuously tune valley polarization via in-situ strain and reveals persistent fractional quantum Hall states during valley polarization transitions.

Findings

Fractional quantum Hall states remain strong during valley polarization transitions.

Valley polarization can be continuously tuned using in-situ strain.

The system exhibits robust ferromagnetism and strong interactions among composite fermions.

Abstract

Electrons' multiple quantum degrees of freedom can lead to rich physics, including a competition between various exotic ground states, as well as novel applications such as spintronics and valleytronics. Here we report magneto-transport experiments demonstrating how the valley degree of freedom impacts the fractional quantum states (FQHSs), and the related magnetic-flux-electron composite fermions (CFs), at very high magnetic fields in the extreme quantum limit when only the lowest Landau level is occupied. Unlike in other multivalley two-dimensional electron systems such as Si or monolayer graphene and transition-metal dichalcogenides, in our AlAs sample we can continuously tune the valley polarization via the application of in-situ strain. We find that the FQHSs remain exceptionally strong even as they make valley polarization transitions, revealing a surprisingly robust ferromagnetism of the FQHSs and the underlying CFs. Our observation implies that the CFs are strongly interacting in our system. We are also able to obtain a phase diagram for the FQHS and CF valley polarization in the extreme quantum limit as we monitor transitions of the FHQSs with different valley polarizations.