J. Phys. IV France 112 (2003) 123
DOI: 10.1051/jp4:2003848

Phonon anomalies in displacive phase transitions by surface X-ray scattering

G. Landmesser¹, T.R. Finlayson², R.L. Johnson³, M. Aspelmeyer¹, A. Plech¹, J. Peisl¹ and U. Klemradt<sup>1

1</sup>  Sektion Physik, Ludwig-Maximilians-Universität-München, Geschwister-Scholl-Platz 1, 80539 München, Germany
²  School of Physics and Materials Engineering, Monash University, P.O. Box 27, Clayton, Victoria 3800, Australia
³  II Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany

Abstract
Phonon anomalies in displacive phase transitions have long been suspected to aid the nucleation of the new phase. In materials with strong but incomplete phonon softening, the crystal surface might additionally play an important role, since the enhanced softening of associated surface phonons effectively lowers the nucleation barrier. X-ray scattering under grazing angles is discussed with regard to experimental studies of such phase transitions between 2 and 3 dimensions, as in this geometry all x-ray techniques provide information from a controlled depth below the surface adjustable from nanometers to microns. However, the sensitivity to premonitory phonon softening varies greatly with the scattering technique employed. It is argued that soft modes confined to a narrow interval in q need not show up in the Debye-Waller factor while giving a strong signal in thermal diffuse scattering. Synchrotron radiation data from Ni ₂MnGa provide evidence for this behavior and confirm also the feasibility of depth-resolved studies of the [ 0] TA ₂ phonon softening.

© EDP Sciences 2003