## A combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_22H_14    [PDF]

F. Capitani, M. Hoeppner, B. Joseph, L. Malavasi, G. A. Artioli, L. Baldassarre, A. Perucchi, M. Piccinini, S. Lupi, P. Dore, L. Boeri, P. Postorino
We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C$_{22}$H$_{14}$) under pressure up to 8~GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the phonon peaks experimentally observed . Through a detailed analysis of the phonon eigenvectors, We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a \sim 20 % compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure. The Grueneisen parameter of the 1380~cm^{-1} a_1 Raman peak ($\gamma_p=0.1$) is much lower than the effective value ($\gamma_d=0.8$) due to K doping. This is an indication that the phonon softening in K doped samples is mainly due to charge transfer and electron-phonon coupling.
View original: http://arxiv.org/abs/1306.1030