Effects of interatomic interaction range on the charge-density-wave and spin-Peierls instabilities

Effects of an exponential long-range screening of interatomic interactions on the Peierls and spin-Peierls instabilities are investigated within the framework of the Random Phase Approximation. It is found that a long-range screening of the Kac-Baker type lowers the characteristic energy of phonons with increasing interatomic interaction range, thermal fluctuations thus becoming dominant causing an enhancement of the classical character of the two structural instabilities. From a qualitative standpoint, the Peierls temperature increases exponentially for relatively strong electron-phonon interactions, but linearly in the weak electron-phonon coupling regime when the interatomic interaction range increases. However, the spin-Peierls temperature always increases linearly with an increase of the interatomic interaction range.