Exchange Energy of the Ferromagnetic Electronic Ground State in a Monolayer Semiconductor

Mobile electrons in the semiconductor monolayer MoS2 form a ferromagnetic state at low temperature. The Fermi sea consists of two circles: one at the K point, the other at the K˜ point, both with the same spin. Here, we present an optical experiment on gated MoS2 at low electron density in which excitons are injected with known spin and valley quantum numbers. The resulting trions are identified using a model which accounts for the injection process, the formation of antisymmetrized trion states, electron-hole scattering from one valley to the other, and recombination. The results are consistent with a complete spin polarization. From the splittings between different trion states, we measure the exchange energy Σ, the energy required to flip a single spin within the ferromagnetic state, as well as the intervalley Coulomb exchange energy J. We determine Σ=11.2 meV and J=5 meV at n=1.5×1012 cm−2 and find that J depends strongly on the electron density n. Published by the American Physical Society 2024