text: The simplest configuration considered in Ref. 1 consists of an interface between an isotropic material with permittivity  and a uniaxial crystal with permittivities  and  for the ordinary and the extraordinary waves respectively. The crystal C axis is parallel to the interface. For this configuration, the DSW can propagate along the interface within certain angular intervals with respect to the C axis, provided that the condition of   is satisfied. Thus DSW are supported by interfaces with positive birefringent crystals only ().  The angular interval is defined by the parameter 
  The angular intervals for the DSW phase and group velocities ( and ) are different. The phase velocity interval is proportional to  and even for the most strongly birefringent natural crystals is very narrow  (rutile) and  (calomel). However the physically more important group velocity interval is substantially larger (proportional to  ). Calculations give  for rutile, and  for calomel.