He-atom-beam scattering was used to investigate the reconstruction of the W(001) surface. At low surface temperatures, a sharp (√2 x √2)R45° diffraction pattern was observed. When the surface temperature was increased, the fractional-order beams broadened and shifted towards the specular. At a surface temperature of ≈450K, the time-of-flight spectra revealed 2 different surface phonons along the <110> direction: a normal Rayleigh mode and a strongly anomalous mode with a maximum at a wave vector Q at one-half of the zone boundary QBZ and an energy approaching zero at Q ≈ 0.8QBZ. This mode softened with decreasing temperature, identifying the structural transformation as a continuous displacive transition.

Observation of a Soft Surface-Phonon Mode in the Reconstruction of Clean W(100). H.J.Ernst, E.Hulpke, J.P.Toennies: Physical Review Letters, 1987, 58, 1941-4