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 approximately 25>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 approximately 0.35, and partially polarized for approximately 0.35<t<2, depending on the density. At r(s) approximately 25-30, and t approximately 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 approximately 25-26 and establish them for finite T as well.