title: Ultrafast dynamics of massive dirac fermions in bilayer graphene.
abstract: Bilayer graphene is a highly promising material for electronic and optoelectronic applications since it is supporting massive Dirac fermions with a tunable band gap. However, no consistent picture of the gap's effect on the optical and transport behavior has emerged so far, and it has been proposed that the insulating nature of the gap could be compromised by unavoidable structural defects, by topological in-gap states, or that the electronic structure could be altogether changed by many-body effects. Here, we directly follow the excited carriers in bilayer graphene on a femtosecond time scale, using ultrafast time- and angle-resolved photoemission. We find a behavior consistent with a single-particle band gap. Compared to monolayer graphene, the existence of this band gap leads to an increased carrier lifetime in the minimum of the lowest conduction band. This is in sharp contrast to the second substate of the conduction band, in which the excited electrons decay through fast, phonon-assisted interband transitions.
authors.affiliation: Department of Physics and Astronomy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark.
authors.forename: Søren
authors.initials: S
authors.lastname: Ulstrup
authors.affiliation: Institute of Condensed Matter Physics, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
authors.forename: Jens Christian
authors.initials: JC
authors.lastname: Johannsen
authors.affiliation: Sincrotrone Trieste, 34149 Trieste, Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Cilento
authors.affiliation: Department of Physics and Astronomy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark.
authors.forename: Jill A
authors.initials: JA
authors.lastname: Miwa
authors.affiliation: Sincrotrone Trieste, 34149 Trieste, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Crepaldi
authors.affiliation: IOM-CNR Laboratorio TASC, Area Science Park, 34012 Trieste, Italy.
authors.forename: Michele
authors.initials: M
authors.lastname: Zacchigna
authors.affiliation: Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom.
authors.forename: Cephise
authors.initials: C
authors.lastname: Cacho
authors.affiliation: Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom.
authors.forename: Richard
authors.initials: R
authors.lastname: Chapman
authors.affiliation: Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom.
authors.forename: Emma
authors.initials: E
authors.lastname: Springate
authors.affiliation: Institut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany.
authors.forename: Samir
authors.initials: S
authors.lastname: Mammadov
authors.affiliation: Institut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany.
authors.forename: Felix
authors.initials: F
authors.lastname: Fromm
authors.affiliation: Institut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Raidel
authors.affiliation: Institut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Seyller
authors.affiliation: Sincrotrone Trieste, 34149 Trieste, Italy and Department of Physics, University of Trieste, 34127 Trieste, Italy.
authors.forename: Fulvio
authors.initials: F
authors.lastname: Parmigiani
authors.affiliation: Institute of Condensed Matter Physics, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
authors.forename: Marco
authors.initials: M
authors.lastname: Grioni
authors.affiliation: SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews, Fife KY16 9SS, United Kingdom.
authors.forename: Phil D C
authors.initials: PD
authors.lastname: King
authors.affiliation: Department of Physics and Astronomy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark.
authors.forename: Philip
authors.initials: P
authors.lastname: Hofmann