Atomic-scale visualization of the interlayer Rydberg exciton complex in moiré heterostructures

Abstract Exciton complex, namely bound aggregate of multiple charged particles, represents an archetypical exemplification of correlated few-body physics. The system of long-lived excitonic states, facilitated by the negative band gap, optical pumping, electrostatic gating or magnetic field, sustains composite bosons or fermions with fascinating physics such as moiré-dominated correlated phenomena and exciton superfluidity. Although a broad spectrum of intriguing excitonic phases has been revealed via global measurements, the atomic-scale accessibility towards the intra-exciton few-body information and inter-exciton nanoscale order has yet to be established. Here, without the aforementioned approaches to maintain excitons, we instead realize the ground-state interlayer exciton complexes through the intrinsic charge transfer in monolayer YbCl 3 /highly oriented pyrolytic graphite (HOPG) heterostructure. Combining scanning tunneling microscope (STM) and first-principle calculations, a comprehensive understanding of the electronic structure of rare-earth insulator YbCl 3 is established, and excitonic in-gap states with Rydberg-like orbitals are directly visualized. It is found that the out-of-plane excitonic charge clouds exhibit oscillating nodal structure of Rydberg type, while their in-plane arrangements are particularly determined by moiré periodicity. Exploiting the tunneling probe to reflect the shape of charge clouds, we reveal the principal quantum number hierarchy of Rydberg series, which points to an excitonic energy-level configuration with unusually large binding energy on the magnitude of electron volts. The direct imaging of ground-state exciton complexes at the lattice scale demonstrates the feasibility of mapping out the charge clouds of excitons microscopically and paves a brand-new way to directly investigate the nanoscale order of exotic correlated phases in the moiré system.