Cascade of multi-electron bubble phases in monolayer graphene at high Landau level filling

TL;DR

This study maps the phase diagram of electron solid states in high Landau levels of monolayer graphene, revealing a cascade of phase transitions between bubble phases with varying electron numbers per bubble, and examines their temperature-dependent melting behavior.

Contribution

It provides the first detailed experimental mapping of electron bubble phases in high Landau levels of monolayer graphene, highlighting a cascade of density-tuned phase transitions.

Findings

Identification of multiple electron bubble phases in N=2, 3, 4 Landau levels.

Observation of a cascade of phase transitions between different bubble states.

Finite temperature data indicating melting of electron solids around 1K.

Abstract

The phase diagram of an interacting two-dimensional electron system in a high magnetic field is enriched by the varying form of the effective Coulomb interaction, which depends strongly on the Landau level index. While the fractional quantum Hall states that dominate in the lower energy Landau levels have been explored experimentally in a variety of two-dimensional systems, much less work has been done to explore electron solids owing to their subtle transport signatures and extreme sensitivity to disorder. Here we use chemical potential measurements to map the phase diagram of electron solid states in $N=2$, $N=3$, and $N=4$ Landau levels in monolayer graphene. Direct comparison between our data and theoretical calculations reveals a cascade of density-tuned phase transitions between electron bubble phases up to two, three or four electrons per bubble in the N=2, 3 and 4 Landau levels respectively. Finite temperature measurements are consistent with melting of the solids for T$\approx$1K.