Anomalous Landau level gaps near magnetic transitions in monolayer WSe$_2$

TL;DR

This study maps the spin phase diagram and Landau level gaps in monolayer WSe$_2$, revealing first-order phase transitions, unexpected constant LL gaps, and the influence of LL filling on exchange energy, indicating re-entrant magnetic phases.

Contribution

It provides the first detailed mapping of spin phase transitions and Landau level behaviors in monolayer WSe$_2$, highlighting unique re-entrant magnetic phenomena.

Findings

First-order phase transitions between spin-polarized Landau levels.

Landau level gaps remain roughly constant over wide magnetic field ranges.

Re-entrant magnetic phase transitions influenced by Landau level filling.

Abstract

First-order phase transitions produce abrupt changes to the character of both ground and excited electronic states. Here we conduct electronic compressibility measurements to map the spin phase diagram and Landau level (LL) energies of monolayer WSe$_2$ in a magnetic field. We resolve a sequence of first-order phase transitions between completely spin-polarized LLs and states with LLs of both spins. Unexpectedly, the LL gaps are roughly constant over a wide range of magnetic fields below the transitions, which we show reflects a preference for opposite spin excitations of the spin-polarized ground state. These transitions also extend into compressible regimes, with a sawtooth boundary between full and partial spin polarization. We link these observations to the important influence of LL filling on the exchange energy beyond a smooth density-dependent contribution. Our results show that WSe$_2$ realizes a unique hierarchy of energy scales where such effects induce re-entrant magnetic phase transitions tuned by density and magnetic field.