Symmetry-breaking of carbon nanotubes vibrational modes induced by transversal deformations

In this work we combine an atomic force microscope (AFM) with a setup to perform confocal Raman spectroscopy to follow, \textit{in situ}, the evolution of the G-band feature of a Single Wall Carbon Nanotube (SWNT) with transversal pressure applied to the nanotube via the AFM probe. We observe a previously elusive and fundamental symmetry-breaking effect of the totally symmetric tangential optical TO modes in the G-band feature which exhibits two distinct Raman active modes with an anomalous frequency behavior with increasing applied pressure, while the totally symmetric longitudinal optical LO component remains unaltered. We propose a simple analytical model based on a mass-spring ring system, which satisfactory explains the main observed effects and shows that the pressure effects change with tube flattening.