Electronic effects in the Mo(001) surface reconstruction: Two-dimensional Fermi surfaces and nonadiabaticity

The origins of the incommensurate reconstruction that Mo(001) undergoes when cooled below 220 K have been the subject of significant debate since its discovery. Much of the debate has centered on the relative importance of collective electronic phenomena in driving the reconstruction. We present here the two-dimensional Fermi surface for Mo(001), as measured using high-resolution angle-resolved photoemission spectroscopy. This Fermi surface consists of two structures: well-defined hole pockets about the M¯ points in the surface Brillouin zone, and triangular-shaped regions midway along the Σ¯ line where an electron state came very close (<0.2 eV) to ${\mathit{E}}_{\mathit{F}}$. These regions can be considered as Fermi-surface crossings for the purposes of coupling to surface vibrations, and thus for playing a role in driving the reconstruction. The Fermi surface is shown to support a modified version of the charge-density-wave model for the reconstruction, with the incommensurate reconstruction being pinned by nested Fermi vectors along Σ¯. We discuss the role of nonadiabatic phenomena in this reconstruction, show that the electronic structure predicts that such phenomena should occur, and provide an experimental basis for the inclusion of strong electron-phonon coupling in the charge-density-wave model of the reconstruction. Finally, our results for Mo(001) are compared with our earlier results for the two-dimensional Fermi surface of W(001), and we conclude that for both surfaces a combination of Fermi-surface instabilities and strong nonadiabatic effects is the most probable driving mechanism for the surface reconstructions.