Spin-polarized electrons in monolayer MoS₂

Coulomb interactions are crucial in determining the ground state of an ideal two-dimensional electron gas (2DEG) in the limit of low electron densities¹. In this regime, Coulomb interactions dominate over single-particle phase-space filling. In silicon and gallium arsenide, electrons are typically localized at these low densities. In contrast, in transition-metal dichalcogenides (TMDs), Coulomb correlations in a 2DEG can be anticipated at experimentally relevant electron densities. Here, we investigate a 2DEG in a gated monolayer of the TMD molybdenum disulfide². We measure the optical susceptibility, a probe of the 2DEG which is local, minimally invasive and spin selective³. In a magnetic field of 9.0 T and at electron concentrations up to n ≃ 5 × 10¹² cm^-2, we present evidence that the ground state is spin-polarized. Out of the four available conduction bands^4,5, only two are occupied. These two bands have the same spin but different valley quantum numbers. Our results suggest that only two bands are occupied even in the absence of a magnetic field. The spin polarization increases with decreasing 2DEG density, suggesting that Coulomb interactions are a key aspect of the symmetry breaking. We propose that exchange couplings align the spins⁶. The Bohr radius is so small⁷ that even electrons located far apart in phase-space interact with each other⁶.