  We report experimental observation of an unexpectedly large thermopower in
mesoscopic two-dimensional (2D) electron systems on GaAs/AlGaAs
heterostructures at sub-Kelvin temperatures and zero magnetic field. Unlike
conventional non-magnetic high-mobility 2D systems, the thermopower in our
devices increases with decreasing temperature below 0.3 K, reaching values in
excess of 100 $\mu$V/K, thus exceeding the free electron estimate by more than
two orders of magnitude. With support from a parallel independent study of the
local density of states, we suggest such a phenomenon to be linked to intrinsic
localized states and many-body spin correlations in the system.
