  This paper describes the mechanism of defect-mediated relaxation in a
dodecagonal square-triangle random tiling phase exhibited by a simulated binary
mixture of soft discs in 2D. We examine the internal transitions within the
elementary mobile defect (christened the 'zipper') that allow it to move, as
well as the mechanisms by which the zipper is created and annihilated. The
structural relaxation of the random tiling phase is quantified and we show that
this relaxation is well described by a model based on the distribution of
waiting times for each atom to be visited by the diffusing zipper. This system,
representing one of the few instances where a well defined mobile defect is
capable of structural relaxation, can provide a valuable test case for general
theories of relaxation in complex and disordered materials.
