Counterflow Response of Bilayer Excitons Across a Gate-Defined Junction

In quantum Hall graphene bilayers, excitons form through the pairing of electrons and holes across opposite layers, providing a model platform for exploring collective bosonic ground states. Transport measurements have long established that a Bose–Einstein condensate of such excitons exhibits the hallmarks of a superfluid phase, characterized by dissipationless counterflowing currents [1]. Here, we explore the transport response of this phase in the presence of a gate-defined junction. By tuning the junction bias, we continuously evolve the system from a transparent regime supporting superfluid transport to a decoupled, resistive state. Furthermore, when the bilayer regions outside the junction are tuned to the dilute regime hosting an exciton insulator [2], we observe signatures of hysteretic melting. This gate-tunable junction thus provides an important first step toward detecting Josephson tunneling of exciton condensates [3] and opens new opportunities for exploring the intriguing possibility of a crystalline phase of bilayer excitons.

[1] Eisenstein, J. P. Exciton Condensation in Bilayer Quantum Hall Systems. Annu. Rev. Condens. Matter Phys. 5, 159–181 (2014).

[2] Zeng, Y. et al. Evidence for a Superfluid-to-solid Transition of Bilayer Excitons. https://doi.org/10.48550/ARXIV.2306.16995 (2023)

[3] Wang, T., Fan, R., Dai, Z. & Zaletel, M. P. Designing an exciton-condensate Josephson junction in quantum Hall heterostructures. Phys. Rev. B 112, L041120 (2025).