ELECTRONIC SUSCEPTIBILITY AND THE MO (001) SURFACE PHASE-TRANSITION

The electronic susceptibility of the Mo (001) surface is calculated, in order to study the phase transition in which the displacement of the surface atoms couples surface states at the Fermi energy. It is found that there is only a small Kohn anomaly in the response to atomic displacements with the wavevector coupling the surface states-so the phase transition is not due to the temperature dependence of the Fermi distribution as in charge-density-wave systems. Instead it is suggested that the Mo (001) surface is unstable for arbitrary atomic displacements, with the surface states marginally favouring the displacement which couples them together. The surface atoms then move in an anharmonic potential and the phonon frequencies are temperature dependent, the mode with the coupling wavevector going soft first and giving the observed phase transition. The possibility of surface magnetism is also considered.