Sharply increasing effective mass: a precursor of the spontaneous spin polarization in a dilute two-dimensional electron system

TL;DR

This study measures the effective mass and g-factor in dilute 2D silicon electron systems, revealing a sharp mass increase as density decreases, indicating a potential precursor to spontaneous spin polarization beyond traditional theories.

Contribution

It demonstrates that the effective mass, not the g-factor, drives spin susceptibility enhancement, challenging existing models of electron interactions in dilute 2D systems.

Findings

Effective mass sharply increases with decreasing electron density.

g-factor remains nearly constant near bulk silicon value.

Mass increase is independent of spin polarization degree.

Abstract

We have measured the effective mass, m, and Lande g-factor in very dilute two-dimensional electron systems in silicon. Two independent methods have been used: (i) measurements of the magnetic field required to fully polarize the electrons' spins and (ii) analysis of the Shubnikov-de Haas oscillations. We have observed a sharp increase of the effective mass with decreasing electron density while the g-factor remains nearly constant and close to its value in bulk silicon. The corresponding strong rise of the spin susceptibility may be a precursor of a spontaneous spin polarization; unlike in the Stoner scenario, it originates from the enhancement of the effective mass rather than the increase of g-factor. Furthermore, using tilted magnetic fields, we have found that the enhanced effective mass is independent of the degree of spin polarization and, therefore, its increase is not related to spin exchange effects, in contradiction with existing theories. Our results show that the dilute 2D electron system in silicon behaves well beyond a weakly interacting Fermi liquid.