The free energy of domain wall fluctuations in xenon on graphite

The problem of soliton degrees of freedom in physisorbed adlayers, as distinct from thermal degrees of freedom is considered. The authors propose making the separation between the two by applying a separate fine- and coarse-grained scaling for the phonon and domain wall motion respectively. This distinguishes the two different length scales in the temperature regime where both modes are present. The contribution to the free energy of domain wall fluctuations about the saddle point is calculated to lowest order in powers of the displacement from the static configuration. The wall fluctuation spectrum is found to have a small gap, justifying earlier theories which treated the walls as fixed. They also indicate how the saddle-point Hamiltonian is used to determine the properties of the incommensurate-commensurate transition.