Phonon spectrum in the spin-Peierls phase of CuGeO$_3$

TL;DR

This study combines advanced DFT calculations and neutron spectroscopy to comprehensively analyze phonon excitations in the low-temperature dimerized phase of CuGeO₃, elucidating complex phonon behaviors and potential spin-phonon interactions.

Contribution

It provides the first systematic comparison of phonon spectra in CuGeO₃'s spin-Peierls phase using both theoretical and experimental methods, revealing detailed phonon features and their relation to magnetic interactions.

Findings

Excellent agreement between DFT and neutron spectroscopy results

Identification of steeply dispersive phonon modes and anticrossings

Discussion of possible spin-phonon hybridization effects

Abstract

CuGeO$_3$ has long been studied as a prototypical example of the spin-Peierls transition in a $S = 1/2$ Heisenberg chain. Despite intensive investigation of this quasi-one-dimensional material, systematic measurements and calculations of the phonon excitations in the dimerized phase have not to date been possible, leaving certain aspects of the spin-Peierls phenomenon unresolved. We perform state-of-the-art density functional theory (DFT) calculations to compute the electronic structure and phonon dynamics in the low-temperature dimerized phase. We also perform high-resolution neutron spectroscopy to measure the full phonon spectrum over multiple Brillouin zones. We find excellent agreement between our numerical and experimental results that extend to all measurement temperatures. Notable features of our phonon spectra include a number of steeply dispersive modes, nonmonotonic dispersion features, and specific phonon anticrossings, which we relate to the mode eigenvectors. By calculating the magnetic interactions within DFT and studying the effects of different phonon modes on the superexchange paths, we discuss the possibility of observing spin-phonon hybridization effects in experiments performed both in and out of equilibrium.