  Transport measurement, which applies an electric field and studies the
migration of charged particles, i.e. the current, is the most widely used
technique in condensed matter studies. It is generally assumed that the quantum
phase remains unchanged when it hosts a sufficiently small probing current,
which is, surprisingly, rarely examined experimentally. In this work, we study
the ultra-high mobility two-dimensional electron system using a propagating
surface acoustic wave, whose traveling speed is affected by the electrons'
compressibility. The acoustic power used in our study is several orders of
magnitude lower than previous reports, and its induced perturbation to the
system is smaller than the transport current. Therefore we are able to observe
the quantum phases become more incompressible when hosting a perturbative
current.
