Interlayer exciton condensates between second Landau level orbitals in double bilayer graphene

TL;DR

This study demonstrates the formation of interlayer exciton condensates in double bilayer graphene, including between second Landau level orbitals, revealing new quantum Hall phenomena influenced by layer polarization and Coulomb interactions.

Contribution

It reports the first observation of interlayer exciton condensates between second Landau level orbitals in double bilayer graphene, expanding understanding of quantum Hall states in layered materials.

Findings

Interlayer exciton condensates observed at N=0 and N=1 Landau levels.

Quantized drag signals indicating interlayer coherence in second Landau levels.

N=1 EC state depends on wavefunction polarization toward hBN interface.

Abstract

We present Coulomb-drag measurements on a heterostructure comprising two Bernal-stacked bilayer graphene (BLG) sheets separated by a 2.5 nm hexagonal boron nitride (hBN) spacer in the quantum Hall (QH) regime. Using top and bottom gate control, together with an interlayer bias, we independently tune the two BLG layers into either the lowest (N = 0) or second (N = 1) Landau level (LL) orbital and probe their interlayer QH states. When both layers occupy the N = 0 orbital, we observe both interlayer exciton condensates (ECs) at integer total filling and interlayer fractional QH states, echoing the results in double monolayer graphene. In contrast to previous studies, however, when both BLG layers occupy the N = 1 orbital, we also observe quantized drag signals, signifying an interlayer exciton condensate formed between the second LLs. By tuning the layer degree of freedom, we find that this N = 1 EC state arises only when the N = 1 wavefunction in each BLG is polarized toward the hBN interface to maximize the interlayer Coulomb interaction.