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The effective mass and g-factor of the strongly correlated 2-D electron fluid. Evidence for a coupled-valley condensate in the Si system

The effective mass m*, and the Lande g-factor of the uniform 2-D electron fluid (2DEF) are calculated as a function of the spin polarization zeta, and the density parameter r_s, using a non-perturbative analytic approach. Our theory is in good accord with the m*g* data of Zhu et al. for zeta=0 for the GaAs-2DEF, and striking agreement with the data of Shashkin et al for the Si-2DEF. While g* is enhanced in GaAs, m* is enhanced in Si. The latter arises from singlet-pair excitations in the two valleys forming a coupled-valley state occurring at the critical density of ~1.10^{11}$ e/cm^2.