We report magneto-absorption spectroscopy of gated WSe2 monolayers in high magnetic fields up to 60~T. When doped with a 2D Fermi sea of mobile holes, well-resolved sequences of optical transitions are observed in both σ± circular polarizations, which unambiguously and separately indicate the number of filled Landau levels (LLs) in both K and K′ valleys. This reveals the interaction-enhanced valley Zeeman energy, which is found to be highly tunable with hole density p. We exploit this tunability to align the LLs in K and K′, and find that the 2D hole gas becomes unstable against small changes in LL filling and can spontaneously valley-polarize. These results cannot be understood within a single-particle picture, highlighting the importance of exchange interactions in determining the ground state of 2D carriers in monolayer semiconductors.
@article{arxiv.2008.02374,
title = {Spontaneous valley polarization of interacting carriers in a monolayer semiconductor},
author = {Jing Li and Mateusz Goryca and Nathan P. Wilson and Andreas V. Stier and Xiaodong Xu and Scott A. Crooker},
journal= {arXiv preprint arXiv:2008.02374},
year = {2020}
}