Excitonic superfluid phase in Double Bilayer Graphene

TL;DR

This paper reports the experimental observation of an exciton condensate in double bilayer graphene, demonstrating superfluidity and layer correlation in the quantum Hall regime, and highlighting the tunability of the phase at temperatures above 4K.

Contribution

First observation of exciton superfluid phase in double bilayer graphene within the quantum Hall regime, expanding the understanding of strongly interacting Bosonic systems in solid-state materials.

Findings

Correlation between layers confirmed by quantized Hall drag.

Dissipationless counterflow indicates superfluidity.

Condensate stabilized at temperatures over 4K.

Abstract

Spatially indirect excitons can be created when an electron and a hole, confined to separate layers of a double quantum well system, bind to form a composite Boson. Because there is no recombination pathway such excitons are long lived making them accessible to transport studies. Moreover, the ability to independently tune both the intralayer charge density and interlayer electron-hole separation provides the capability to reach the low-density, strongly interacting regime where a BEC-like phase transition into a superfluid ground state is anticipated. To date, transport signatures of the superfluid condensate phase have been seen only in quantum Hall bilayers composed of double well GaAs heterostructures. Here we report observation of the exciton condensate in the quantum Hall effect regime of double layer structures of bilayer graphene. Correlation between the layers is identified by quantized Hall drag appearing at matched layer densities, and the dissipationless nature of the phase is confirmed in the counterflow geometry. Independent tuning of the layer densities and interlayer bias reveals a selection rule involving both the orbital and valley quantum number between the symmetry-broken states of bilayer graphene and the condensate phase, while tuning the layer imbalance stabilizes the condensate to temperatures in excess of 4K. Our results establish bilayer graphene quantum wells as an ideal system in which to study the rich phase diagram of strongly interacting Bosonic particles in the solid state.