ah ha! Surface mechanics!
“Extrinsic surface states are usually defined as states not originating from a clean and well ordered surface. Surfaces that fit into the category extrinsic are:
Surfaces with defects, where the translational symmetry of the surface is broken.
Surfaces with adsorbates
Interfaces between two materials, such as a semiconductor-oxide or semiconductor-metal interface
Interfaces between solid and liquid phases.
Generally, extrinsic surface states cannot easily be characterized in terms of their chemical, physical or structural properties.”
https://www.sciencedirect.com/science/article/pii/S2542435119305823