The optical susceptibility is a local, minimally-invasive and spin-selective probe of the ground state of a two-dimensional electron gas. We apply this probe to a gated monolayer of MoS2. We demonstrate that the electrons are spin polarized. Of the four available bands, only two are occupied. These two bands have the same spin but different valley quantum numbers. We argue that strong Coulomb interactions are a key aspect of this spontaneous symmetry breaking. The Bohr radius is so small that even electrons located far apart in phase space interact, facilitating exchange couplings to align the spins.
@article{arxiv.1807.06636,
title = {Spin-Polarized Electrons in Monolayer MoS$_2$},
author = {Jonas G. Roch and Guillaume Froehlicher and Nadine Leisgang and Peter Makk and Kenji Watanabe and Takashi Taniguchi and Richard J. Warburton},
journal= {arXiv preprint arXiv:1807.06636},
year = {2019}
}