Softening of the Bond-Stretching Phonon of Ba1-xKxBiO3 at the Metal to Insulator Transition

TL;DR

This study investigates phonon dispersion in Ba1-xKxBiO3 across the metal-insulator transition, revealing phonon softening linked to the emergence of the Fermi surface in the metallic phase.

Contribution

It provides detailed phonon dispersion data across the transition and shows phonon softening correlates with metallicity, offering insights into electron-phonon interactions.

Findings

Phonon energies near b1 were consistent with optical data.

Strong phonon softening occurs at q=0.25 in the metallic phase.

No softening observed in the insulating phase.

Abstract

The dispersion of phonons in Ba1-xKxBiO3 along the (3+q, 0, 0) direction in reciprocal space was determined for x=0, 0.30, 0.37, 0.52 using high-resolution inelastic X-ray scattering. The observed phonon energies near \Gamma were in good agreement with published optical data. It was found that two high-energy modes strongly soften near q=0.25 when the system becomes metallic (x >0.35). There was no softening in the insulating phase, even at x=0.30, very near the metal-insulator transition. This suggests that the phonon softening is linked to the appearance of the Fermi-surface.