Condensed-Matter Physics & Materials Science Seminar

Phonon anomalies can be defined as deviations of phonon dispersions from predictions of balls-and-springs models of lattice dynamics. Most these anomalies result from electron-phonon coupling and can be understood using band theory. Phonon anomalies in Chromium, cuprates, and iron pnictides recently observed in these systems by inelastic X-ray scattering (IXS) are very different in that they are unexpected from band theory. In chromium there is a well-known softening of the transverse acoustic phonon that is believed to be associated with the nesting of the Fermi surface. However, we found that this anomaly extends very far into the Brillouin zone, to wavevectors that do not satisfy the nesting condition. In the cuprates, phonon anomalies at some wavevectors that do not appear in band-theory-based calculations have been reported for some time. I will review these results but will focus primarily on new IXS results for the Hg-based system. Iron pnictides show a completely different behavior: Dispersions of some branches agree well with band-theory calculations whereas other complete branches are softened by about 20%. One theme that ties together these diverse effects is strong coupling between magnetic and charge degrees of freedom.