Critical Behavior of a Strongly Interacting 2D Electron System
  With decreasing density $n_s$ the thermopower $S$ of a low-disorder 2D electron system in silicon is found to exhibit a sharp increase by more than an order of magnitude, tending to a divergence at a finite, disorder-independent density $n_t$ consistent with the critical form $(-T/S) \propto (n_s-n_t)^x$ with $x=1.0\pm 0.1$ ($T$ is the temperature). Our results provide clear evidence for an interaction-induced transition to a new phase at low density in a strongly-interacting 2D electron system. 
A. Mokashi, S. Li, Bo Wen, S. V. Kravchenko, A. A. Shashkin, V. T.   Dolgopolov, and M. P. Sarachik
cond-mat.str-el