A journey from fractional quantum Hall liquid to Wigner crystal and back

When two-dimensional (2D) electron systems are subjected to a large perpendicular magnetic field, the resulting suppression of kinetic energy enhances electron-electron interactions. This makes them ideal platforms for studying a wide range of strongly correlated phenomena, including fractional quantum Hall (FQH) states and Wigner crystals (WC). In this talk, we will present unexpected displacement-field (D) induced transitions between the FQH and WC phases at filling factors ν = 1/3 and ν = 7/3. Although the FQH state is typically the ground state at these fillings, we found that at high D, it is suppressed, giving rise to a WC phase. We believe these transitions are driven by Landau-level mixing (LLM) between integer levels. Even more surprising, we observed the emergence of half-filled quantum Hall states near Landau-level crossings at ν = 1/2 and ν = 5/2. These findings are based on temperature-dependent magnetoresistance measurements in hBN-encapsulated bilayer graphene, equipped with dual graphite gates, which provide independent control over both D and ν.