Onset of the Quantum Hall Effect at 5 mT in Double-Layer Graphene

<title>Abstract</title> For several decades, research has centered on electronic systems where an electron’s kinetic energy rivals its interaction energy. Although many systems have been proposed, none enable continuous tuning of key parameters, such as doping or work function. By contrast, graphene permits precise carrier density control via gating, but sample inhomogeneities hinder access to the low-density regime where electron-electron interactions dominate. Improving graphene’s carrier mobility remains essential for fundamental studies and device applications. Here, we describe a way to reduce the influence of external inhomogeneity by at least an order of magnitude in graphene, which immediately opens access to the numerous novel effects of this electronic system. This work uses a standard dry-transfer technique to fabricate a double-layer graphene (DLG) device, separated by an ultra-thin hexagonal boron nitride (hBN). The upper and lower graphene layers lie in opposite half-spaces, providing mutual screening that sufficiently reduces scattering from random Coulomb potentials. This architecture yields a residual charge density on the order of 2×10⁸ cm^-2, previously achievable only in ultraclean suspended graphene, alongside unprecedented quantum mobility, reaching 5×10⁶ cm²V^-1s^-1. Shubnikov–de Haas oscillations appear at magnetic fields as low as 2 mT, and well-defined integer quantum Hall features develop by 5 mT, highlighting exceptional carrier transport properties. Notably, a fractional Quantum Hall plateau at a filling factor ν_tot= −8/3 emerges at 3 T, underscoring the device’s suitability for probing strongly correlated electronic phases in graphene-based heterostructures.