  Surface Acoustic Waves (SAW) have been utilized to investigate the properties
of a two-dimensional electron system, subjected to a perpendicular magnetic
field and monochromatic microwave radiation, in the regime where the so-called
microwave-induced zero-resistance states form. Contrary to conventional
magneto-transport in Hall bar and van der Pauw geometries, the collimated SAW
beam probes only the bulk of the electronic system exposed to this wave. Clear
signatures appear in the SAW propagation velocity corroborating that neither
contacts, nor sample edges are a root source for their emergence. By virtue of
the directional nature of this probing method and with the assistance of
theoretical modelling, we were also able to demonstrate that the SAW response
depends on the angle between its propagation vector and the orientation of
domains which spontaneously form when zero-resistance is observed in transport.
This confirms in unprecedented manner the formation of an inhomogeneous phase
under these non-equilibrium conditions.
