We uncover the interlayer shear mode of multi-layer graphene samples, ranging from bilayergraphene (BLG) to bulk graphite, and show that the corresponding Raman peak measures the interlayer coupling. This peak scales from∼43cm −1 in bulk graphite to∼31cm −1 in BLG. Its low energy makes it a probe of near-Dirac point quasi-particles, with a Breit-Wigner-Fano lineshape due to resonance with electronic transitions. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions.Single Layer Graphene (SLG) has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability [1,2]. As the knowledge of the basic properties of SLG increases, an ever growing effort is being devoted to a deeper understanding of Few Layer Graphene (FLG) samples [3][4][5], and to their application in useful devices. For example, since SLG absorbs 2.3% of the incident light [6], FLG can be used to beat the transmittance of Indium Tin Oxide(∼90%) [2], and to engineer near-market transparent conductors [7], exploiting the lower sheet resistance afforded by combining more than one SLG [2,7]. Bilayer graphene (BLG) is a tunable band gap semiconductor [8], tri-layer graphene (TLG) has a unique electronic structure consisting, in the simplest approximation, of massless SLG and massive BLG subbands [9][10][11]. FLG with less than 10 layers do each show a distinctive band structure [11]. The layers can be stacked as in graphite, or have any orientation. This gives rise to a wealth of electronic properties, such as the appearance of a Dirac spectrum even in FLG [12].There is thus an increasing interest in the physics and applications of FLG. Optical microscopy can count the number of layers [13,14], but does not offer the insights of Raman spectroscopy, being this sensitive to quasiparticle interactions [15]. Raman spectroscopy is one of the most useful and versatile tools to probe graphene samples [15,16]. The measurement of the SLG, BLG, and FLG Raman spectra[15] triggered a huge effort to understand phonons, electron-phonon, magneto-phonon and electron-electron interactions, and the influence on the Raman process of number and orientation of layers, electric or magnetic fields, strain, doping, disorder, edges, and functional groups [16].The SLG phonon dispersions comprise three acoustic and three optical branches. A necessary, but not sufficient, condition for a phonon mode to be Raman active is to satisfy the Raman fundamental selection rule, i.e. to be at the Brillouin Zone centre, Γ, with wavevector q ≈ 0 [17]. SLG has six normal modes at Γ: [18]. There are two degenerate in-plane optical modes, E 2g , and one out-of-plane optical mode B 2g [18]. E 2g modes are Raman active, while B 2g is neither Raman nor IR active [18]. In the case of graphite there are 4 atoms per unit cell, and only half of them have fourth neighbors that either lie directly above or below in adjacent layers. Therefore the two atoms of the unit cell in each layer are now inequivalent. ...
