title: Observation of low energy Raman modes in twisted bilayer graphene.
abstract: Two new Raman modes below 100 cm(-1) are observed in twisted bilayer graphene grown by chemical vapor deposition. The two modes are observed in a small range of twisting angle at which the intensity of the G Raman peak is strongly enhanced, indicating that these low energy modes and the G Raman mode share the same resonance enhancement mechanism, as a function of twisting angle. The ~94 cm(-1) mode (measured with a 532 nm laser excitation) is assigned to the fundamental layer breathing vibration (ZO' mode) mediated by the twisted bilayer graphene lattice, which lacks long-range translational symmetry. The dependence of this mode's frequency and line width on the rotational angle can be explained by the double resonance Raman process that is different from the previously identified Raman processes activated by twisted bilayer graphene superlattice. The dependence also reveals the strong impact of electronic-band overlaps of the two graphene layers. Another new mode at ~52 cm(-1), not observed previously in the bilayer graphene system, is tentatively attributed to a torsion mode in which the bottom and top graphene layers rotate out-of-phase in the plane.
authors.affiliation: Department of Physics, University of Northern Iowa, Cedar Falls, Iowa 50614, United States. rui.he@uni.edu
authors.forename: Rui
authors.initials: R
authors.lastname: He
authors.forename: Ting-Fung
authors.initials: TF
authors.lastname: Chung
authors.forename: Conor
authors.initials: C
authors.lastname: Delaney
authors.forename: Courtney
authors.initials: C
authors.lastname: Keiser
authors.forename: Luis A
authors.initials: LA
authors.lastname: Jauregui
authors.forename: Paul M
authors.initials: PM
authors.lastname: Shand
authors.forename: C C
authors.initials: CC
authors.lastname: Chancey
authors.forename: Yanan
authors.initials: Y
authors.lastname: Wang
authors.forename: Jiming
authors.initials: J
authors.lastname: Bao
authors.forename: Yong P
authors.initials: YP
authors.lastname: Chen