THE ROLE OF NONADIABATICITY AND 2-DIMENSIONAL FERMI SURFACES IN THE RECONSTRUCTION OF MO(001) AND W(001)

The driving mechanisms for the incommensurate reconstruction of Mo(001) and the commensurate reconstruction of W(001) are the subject of great debate, particularly concerning the role played by collective electronic phenomena. In this regard, the structure of the two-dimensional Fermi surfaces is the key physical parameter. We present here angle-resolved photoemission measurements of the two-dimensional Fermi surfaces of both Mo(001) and W(001). The Fermi surfaces obtained display significant nesting in the surface Brillouin zone, and strongly support a collective electronic origin to the reconstructions. The data is shown to be consistent with recent theoretical work and with measurements using He scattering of soft phonon modes on both surfaces. We also show that non-adiabatic effects should be large on these surfaces and discuss how this leads to structural instability.