Scaling of the magnetoconductivity of silicon MOSFET's: evidence for a   quantum phase transition in two dimensions
  For a broad range of electron densities $n$ and temperatures $T$, the in-plane magnetoconductivity of the two-dimensional system of electrons in silicon MOSFET's can be scaled onto a universal curve with a single parameter $H_{\sigma}(n,T)$, where $H_{\sigma}$ obeys the empirical relation $H_{\sigma}=A (n) [\Delta(n)^2 +T^2]^{1/2}$. The characteristic energy $k_B \Delta$ associated with the magnetic field dependence of the conductivity decreases with decreasing density, and extrapolates to 0 at a critical density $n_0$, signaling the approach to a zero-temperature quantum phase transition. We show that $H_{\sigma}=AT$ for densities near $n_0$. 
S. A. Vitkalov, Hairong Zheng, K. M. Mertes, M. P. Sarachik, and T. M.   Klapwijk