Exploration of the effect of inter-orbital hybridization on lattice instabilities in one-dimensional chains*

Fermi surface nesting effects have largely been acclaimed as the mechanism resulting in lattice instabilities and the formation of charge and spin density waves as temperature is decreased. These concepts are mainly based on the discontinuities in the charge susceptibility represented by the bands crossing the Fermi energy. In 1D, this results in the familiar Peierls instability for a wave vector equal to 2kf. According to Peierls' work, the ground state of one-dimensional chains of hydrogen atoms at half electron filling is dimerized. In its original form, this argument should apply to other species such as lithium, but ab initio density functional calculations predict that equally spaced chains of this species are not susceptible to lattice instabilities. This is in spite of there being only one band crossing the Fermi energy which as in H is a half filled band in one electron theory. We show that the root cause of this apparent failing of the Peierls model is due to its disregard of the form of the charge carrier wavefunction, notably of the presence of orbital hybridisation. Calculating the charge density susceptibility for these systems while fully taking inter-orbital hybridization into account through a tight binding model leads to an elimination of the divergence usually indicative of a lattice instability in lithium chains.