Structure of the low-temperature phase of molybdenum (001) investigated by helium-atom scattering

Using helium-atom scattering, we provide evidence that the low-temperature phase of the clean Mo(001) surface is a high-order commensurate structure. The reciprocal lattice vector ${\mathbf{Q}}_{\mathit{s}}$ of the superstructure is determined to be ${\mathbf{Q}}_{\mathit{s}}$=3/7${\mathbf{G}}_{11}$, in accord with the Q value at which a longitudinal surface phonon becomes soft in the high-temperature phase giving rise to a periodic lattice distortion. Upon further cooling, extra satellites appear in the diffraction pattern, which cannot be observed with the in situ low-energy electron diffraction system. These extra features observed in He diffraction can be explained by a periodic lattice distortion involving normal as well as lateral displacements of the surface atoms.