Spontaneous valley polarization of interacting carriers in a monolayer semiconductor

TL;DR

This study uses high magnetic field spectroscopy to reveal how interactions cause spontaneous valley polarization in a monolayer semiconductor, showing tunable valley Zeeman energy and instability against valley polarization.

Contribution

It demonstrates interaction-driven spontaneous valley polarization in monolayer WSe2, emphasizing the role of exchange interactions beyond single-particle models.

Findings

Observation of well-resolved Landau levels in both valleys.

Tunable valley Zeeman energy with hole density.

Spontaneous valley polarization due to LL alignment.

Abstract

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 $\sigma^\pm$ 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.