Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si quantum wells we determine the carrier-density-dependence of the magnetic susceptibility. Assuming weak interaction we evaluate the density of states at the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11 cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the mobility from q_TF yields good agreement with experimental values justifying the approach. The decrease in D(E_F) is explained by potential fluctuations which lead to tail states that make screening less efficient and - in a positive feedback - cause an increase of the potential fluctuations. Even in our high mobility samples the fluctuations exceed the electron-electron interaction leading to the formation of puddles of mobile carriers with at least 1 micrometer diameter.
@article{arxiv.cond-mat/0010077,
title = {Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells},
author = {Z. Wilamowski and N. Sandersfeld and W. Jantsch and D. Toebben and F. Schaeffler},
journal= {arXiv preprint arXiv:cond-mat/0010077},
year = {2009}
}