# Anomalous Drag in Double Bilayer Graphene Quantum-Hall Superfluids

## TL;DR

This paper investigates the unique superfluid properties of double bilayer graphene in the quantum Hall regime, revealing anomalous drag phenomena linked to orbital degeneracy.

## Contribution

It demonstrates that superfluid phase stiffness vanishes at the $n=0/1$ orbital degeneracy in double bilayer graphene, explaining recent anomalous drag observations.

## Key findings

- Superfluid phase stiffness vanishes at $n=0/1$ degeneracy.
- Charged excitation gap remains large despite stiffness vanishing.
- Anomalous drag observed near degeneracy point.

## Abstract

Semiconductor double-layers in the quantum Hall regime tend to have superfluid exciton condensate ground states when the total filling factor is an odd integer, provided that the Landau orbitals at the Fermi level in the two layers have the same orbital character. Since the $N=0$ Landau level of bilayer graphene contains states with both $n=0$ and $n=1$ orbital character, the physics of double bilayers falls outside previously studied cases. We show that the superfluid phase stiffness vanishes in double bilayer graphene when $n=0$ and $n=1$ orbitals states are degenerate in one of the layers, even though the gap for charged excitations remain large, and speculate that this property is behind the recent discovery of strong anomalous drag near a $n=0/1$ degeneracy point.