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

TL;DR

This paper introduces a new parameter linking out-of-plane strain to in-plane phonon frequency shifts in graphite, revealing the significant role of out-of-plane strain and c-electron compression under high pressure.

Contribution

It proposes a novel parameter to quantify out-of-plane strain effects on in-plane vibrational modes, enhancing understanding of pressure-induced phonon shifts in graphite.

Findings

Out-of-plane strain significantly influences in-plane phonon frequencies.

The compression of c-electrons affects vibrational mode shifts.

A new parameter effectively captures out-of-plane strain contributions.

Abstract

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