Entropy of strongly correlated electrons in a partially filled Landau level

Abstract: We use high-resolution chemical potential measurements to extract the entropy of monolayer and bilayer graphene in the quantum Hall regime via the Maxwell relation $\left.\frac{d\mu}{dT}\right|_N = -\left.\frac{dS}{dN}\right|_T$. Measuring the entropy from $T=300$K down to $T=200$mK, we identify the sequential emergence of quantum Hall ferromagnetism, fractional quantum Hall states (FQH), and various charge orders by comparing the measured entropy in different temperature regimes with theoretical models. At the lowest temperature of $T \approx 200$mK we perform a detailed study of the entropy near even-denominator fractional quantum Hall states in bilayer graphene, and comment on the possible topological origin of the observed excess entropy.