Sharply increasing spin susceptibility near the metal-insulator transition in a two-dimensional electron system

We have measured the effective mass and Lande g factor in very dilute two-dimensional electron systems in silicon and found a sharp increase of the effective mass near the metal-insulator transition, while the g factor remains nearly constant and close to its value in bulk silicon. Using tilted magnetic fields, we have found that the enhanced effective mass is independent of the degree of spin polarization, which points to a spin-independent origin of the mass enhancement and is in contradiction with existing theories