Spontaneous Valley Polarization of Interacting Carriers in a Monolayer Semiconductor

We report magneto-absorption spectroscopy of gated WSe_2 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.