  Viscous fluid in an ultrahigh-mobility two-dimensional electron gas (2DEG) in
GaAs/AlGaAs quantum wells is systematically studied through measurements of
negative magnetoresistance (NMR) and photoresistance under microwave radiation,
and the data are analyzed according to recent theoretical work by e.g.,
Alekseev, Physical Review Letters 117,166601 (2016). Size-dependent and
temperature dependent NMR are found to conform to the theoretical predictions.
In particular, transport of 2DEG with relatively weak Coulomb interaction
(interparticle interaction parameter r_s<1) manifests a crossover between
viscous liquid and viscous gas. The size dependence of microwave induced
resistance oscillations and that of the '2nd harmonic' peak indicate that 2DEG
in a moderate magnetic field should be regarded as viscous fluid as well. Our
results suggest that the hydrodynamic effects must be considered in order to
understand semiclassical electronic transport in a clean 2DEG.
