Quantum oscillations in a dipolar excitonic insulator

Quantum oscillations in magnetization or resistivity are a defining feature of metals subject to an external magnetic field. The phenomenon is generally not expected in insulators without a Fermi surface. The observations of quantum oscillations in Kondo insulating materials have provided a rare counterexample and attracted much theoretical interest. However, the magnetic oscillations in correlated insulators remain poorly understood. Here we report the observations of resistivity quantum oscillations in an excitonic insulator realized in Coulomb-coupled electron-hole double layers with gate-tunability that allows the phenomenon to be explored in a more controllable fashion than in bulk materials. When the cyclotron energy of the electrons or holes is tuned to be comparable to or larger than the exciton binding energy, recurring transitions between excitonic insulators and electron-hole decoupled quantum Hall states are observed. Compressibility measurements show an oscillatory exciton binding energy as a function of magnetic field and electron-hole pair density. Coulomb drag measurements further reveal the formation of excitons with finite angular momentum. Our results are qualitatively captured by mean-field theory calculations. The study demonstrates a new platform for studying quantum oscillations in correlated insulators.