Composite fermions and broken symmetries in graphene

• Published in: Nature communications 2015-01, Vol.6 (1), p.5838-5838, Article 5838
• Main Authors: Amet, F., Bestwick, A. J., Williams, J. R., Balicas, L., Watanabe, K., Taniguchi, T., Goldhaber-Gordon, D.
• Format: Article
• Language: English
• Subjects: 639/301/119/2794; 639/301/357/918; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms6838

The electronic properties of graphene are described by a Dirac Hamiltonian with a four-fold symmetry of spin and valley. This symmetry may yield novel fractional quantum Hall (FQH) states at high magnetic field depending on the relative strength of symmetry-breaking interactions. However, observing such states in transport remains challenging in graphene, as they are easily destroyed by disorder. In this work, we observe in the first two Landau levels the two-flux composite-fermion sequences of FQH states between each integer filling factor. In particular, the odd-numerator fractions appear between filling factors 1 and 2, suggesting a broken-valley symmetry, consistent with our observation of a gap at charge neutrality and zero field. Contrary to our expectations, the evolution of gaps in a parallel magnetic field suggests that states in the first Landau level are not spin-polarized even up to very large out-of-plane fields. The collective behaviour of electrons trapped in a two-dimensional plane gives rise to fractional quantum Hall states. Amet et al . now observe previously unseen states in boron-nitride-graphene heterostructures in a strong magnetic field that indicate the formation of composite fermions.