Spin Susceptibility of an Ultra-Low-Density Two-Dimensional Electron System

We determine the spin susceptibility in a two-dimensional electron system in GaAs/AlGaAs over a wide range of low densities from 2×10⁹ cm^-2 to 4×10¹⁰ cm^-2. Our data can be fitted to an equation that describes the density dependence as well as the polarization dependence of the spin susceptibility. It can account for the anomalous g factors reported recently in GaAs electron and hole systems. The paramagnetic spin susceptibility increases with decreasing density as expected from theoretical calculations.