  Phase-change materials are ubiquitous in technology because of their ability
to transition between amorphous and crystalline phases fast and reversibly,
upon shining light or passing a current. Here we argue that to fully understand
their electronic properties it is necessary to define a novel electronic phase:
the amorphous obstructed insulator. It differs from an obstructed insulator
crystal in that it presents localized edge or surface states irrespective of
the sample termination. Consequently, we show that obstructed amorphous
insulators in three-dimensions host a surface two-dimensional flat band,
detectable using ARPES. Our work establishes basic models for materials where
topological and obstructed properties can be switched on and off externally,
including two-dimensional surface flatbands.
