title: Quantum-Hall activation gaps in graphene.
abstract: We have measured the quantum-Hall activation gaps in graphene at filling factors nu=2 and nu=6 for magnetic fields up to 32 T and temperatures from 4 to 300 K. The nu=6 gap can be described by thermal excitation to broadened Landau levels with a width of 400 K. In contrast, the gap measured at nu=2 is strongly temperature and field dependent and approaches the expected value for sharp Landau levels for fields B>20 T and temperatures T>100 K. We explain this surprising behavior by a narrowing of the lowest Landau level.
authors.affiliation: High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University Nijmegen, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands. J.Giesbers@science.ru.nl
authors.forename: A J M
authors.initials: AJ
authors.lastname: Giesbers
authors.forename: U
authors.initials: U
authors.lastname: Zeitler
authors.forename: M I
authors.initials: MI
authors.lastname: Katsnelson
authors.forename: L A
authors.initials: LA
authors.lastname: Ponomarenko
authors.forename: T M
authors.initials: TM
authors.lastname: Mohiuddin
authors.forename: J C
authors.initials: JC
authors.lastname: Maan