Helium-atom-scattering study of the structure and phonon dynamics of the W(001) surface between 200 and 1900 K

Abstract

High-resolution helium-atom-scattering measurements of angular distributions and time-of-flight spectra are reported for W(001) in the temperature range from 200 to 1900 K. Angular variations in the total intensity show a reconstruction related superstructure peak whose angular position and intensity are temperature dependent in the range from 400 to 220 K. At 220 K the peak intensity is maximum and its position corresponds to that expected for a reconstructed (&2X&2) 845' low-temperature phase. Time-of-flight spectra show that the behavior of the superstructure peak at higher temperatures is attributable to the elastic component and must therefore be due to a surface structural feature. Of the measured phonon inelastic peaks in the time-of-flight spectra, the one assigned to a longitudinal mode shows the typical strong temperature dependence of a soft mode in the temperature range 450~T~220 K. For T~220 K two new phonon modes appear, associated with the Brillouin-zone boundary corresponding to the c(2X2) phase. Both observations suggest that the overall phase transition on W(001) is of the "displacive" and not the "order-disorder" type, as inferred from several recent structural studies.All experimental findings are consistent with a charge-density-wave mechanism driving the reconstruction in agreement with the recent theory of Wang and Weber [Phys. Rev. Lett. 58, 1452.

Surface Peierls transition on Cu(001) covered with heavier p-block metals

“…Direct observation of the phonon softening at T > T c was done by means of inelastic He scattering [58][59][60][61]. The longitudinal surface phonon mode showed well-defined dispersion at T > 450 K, but exhibited considerable softening at ∼ 0.8M upon approaching T c , in qualitative agreement with the CDW scenario.…”

“…It, however, is difficult to verify experimentally whether the surfaces transforms to undisplaced (1×1) structure at higher temperatures. The only experiment up to 600 K by high-energy ion scattering was interpreted to indicate displaced character at 600 K [54], which was later claimed to be consistent with undisplaced structure [58].…”

Order-disorder character of the(3×3)to(3×3Petaccia1, Floreano2, Goldoni3 et al. 2001Phys. Rev. B29280View full textAdd to dashboardCite

“…To our knowledge no data have been published at temperatures higher than 300 K, while it is well known that the displacive phase transitions are always accompanied by an intermediate disordered stage around the critical temperature, [18][19][20] manifesting the change in surface corrugation only at a temperature substantially higher than T c . It is therefore of utmost importance to have a clear indication of the order-disorder character of the transition around T c and to know if the two components observed in the Sn 4d core level spectra at LT and RT are also present far from T c , up to the highest temperature where this phase is stable (∼ 550 K).…”

Surface Diffusion Anomaly near a Substrate Phase Transition: H onW(100)

“…At present, there are no reliable estimates for the H͞W͑100͒ system regarding the relative dominance of the electronic and vibrational contributions to the friction. However, near the reconstructive phase transition of W͑100͒, the electronic excitations basically do not change because they are determined by the short-range order, while the vibrational excitations are known to develop anomalous temperature dependence due to critical fluctuations of long wavelengths [19,20]. In particular, the modes near the critical wave vector q 0 ͑p͞a, p͞a͒ soften and become overdamped as T approaches T C .…”