Measurement Of Strong Phonon Softening In Cr With And Without Fermi-Surface Nesting By Inelastic X-Ray Scattering

Nesting of the Fermi surface can soften and broaden phonons at the nesting wave vectors. However, huge electron-phonon anomalies unexpected from the experimental topology of the Fermi surface have been reported in copper oxide superconductors and their origin remains enigmatic. Here we present results of inelastic x-ray scattering measurements that uncovered similarly pronounced softening of phonons in chromium that occur far from the nesting wave vectors of the Fermi surface. The softening appears in addition to the previously reported phonon anomalies at the nesting wave vectors. Calculations in the local-density approximation show that the anomalies result from the enhancement of the electron-phonon coupling constant. Our results demonstrate that strong electron-phonon coupling alone can result in strong phonon anomalies limited to a small range of wave vectors. They imply that the phonon anomalies in copper oxide superconductors may be explained by an enhanced electron-phonon coupling strength without invoking novel collective modes or some hidden nesting of the Fermi surface.