The spin susceptibility in Si-MOSFETs.

We have performed for the first time DMC simulations of a symmetric two-valley electron gas with variable spin polarization, both in the strictly 2D limit and with a thickness appropriate to Si-MOSFETs. We find that valley degeneracy substantially reduces the spin susceptibility enhancement $\alpha $, with respect to the conventional 2DEG. The farther reduction of $\alpha $, caused by thickness, brings our prediction in excellent agreement with the available experimental data on Si-MOSFETs[1] up to r $_{s} \quad \approx $ 5. The agreement extends to the full experimental range (r $_{s}\le $8) when weak disorder is kept into account within a response function formalism, generalizing to the two valley system the approach previously employed for the conventional 2DEG [2]. [1] See, e.g, S.V. Kravchenko and M.P.Sarachik, Rep. Prog. Phys. \textbf{67}, 1 (2004) and references therein. [2] S. De Palo et. al., Phys. Rev. Lett. \textbf{94}, 226405 (2005).\textbf{ }