Graphite under uniaxial compression along c-axis: a new parameter relates out-of-plane strain to in-plane phonon frequency

摘要

Stacking graphene sheets forms graphite. Two in-plane vibrational modes ofgraphite, E1u and E2g(2), are derived from graphene E2g mode, the shifts ofwhich under compression are all considered as results of in-plane bondshortening. Values of Gruneisen parameter have been reported to quantify suchrelation. However, the reason why the shift rates of these three modes withpressure differ is unclear. In this work, we introduce a new parameter toquantify the contribution of out-of-plane strain to the in-plane vibrationalfrequencies, suggesting that the compression of \pi-electrons plays anon-negligible part in both graphite and graphene under high pressure.