Anomalous lattice dynamics of fcc lanthanum

Abstract

First-principles pseudopotential band-structure calculations were performed for fcc La and revealed a small but sharp nesting feature of the Fermi surface. In order to investigate the relationship between this nesting and possible phonon anomalies, the phonon-dispersion curves were calculated. Along the [\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}] direction a sharp dip in the transverse branch was found at \ensuremath{\xi}=0.42 in agreement with recent low-temperature neutron scattering experiments. Calculations at 14% smaller volume (corresponding to a pressure of 50 kbar) indicate that the frequencies of the phonons along the transverse [\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}] branch decrease with pressure because of the increased magnitude of the electron-phonon matrix elements. This result helps to explain the unusual negative thermal expansion, the dramatic increase of the superconducting transition temperature, and the observed structural transition near 50 kbar.