Spin-polarized stable phases of the 2D electron fluid at finite temperatures

The Helmholtz free energy F of the 2D electron fluid is calculated using a mapping to a classical Coulomb fluid [Phys. Rev. Lett. 87, 206404 (2001)]. For density parameters r(s) such that similar to25 > r(s), the fluid is unpolarized at all temperatures t = T/E-F, where E-F is the Fermi energy. For higher r(s), the system is fully spin polarized for t smaller than similar to0.35, and partially polarized for similar to0.35 < t < 2, depending on the density. At r(s) similar to 25-30, and t similar to 0.35, an "ambispin" phase where F is almost independent of the spin polarization is found. These results support claims for a stable, fully polarized fluid phase at T = 0 for r(s) larger than similar to25-26 and establish them for finite T as well.