High-T_c Superconductivity Originating from Interlayer Coulomb Coupling in Gate-Charged Twisted Bilayer Graphene Moiré Superlattices

High-T_C Superconductivity Originating from Interlayer Coulomb Coupling in Gate-Charged Twisted Bilayer Graphene Moiré Superlattices, D. R. Harshman and A. T. Fiory [arXiv]

Unconventional superconductivity in bilayer graphene has been reported for twist angles θ near the first magic angle and charged electrostatically with holes near half filling of the lower flat bands. A maximum superconducting transition temperature T_C ≈ 1.7 K was reported for a device with θ = 1.05° at ambient pressure and a maximum T_C ≈ 3.1 K for a device with θ = 1.27° under 1.33 GPa hydrostatic pressure. A high-T_C model for the superconductivity is proposed herein, where pairing is mediated by Coulomb coupling between charges in the two graphene sheets. The expression derived for the optimal transition temperature, T_C0 = k_B⁻¹Λ(|n_opt − n₀|/2)^1/2e²/ζ, is a function of mean bilayer separation distance ζ, measured gated charge areal densities n_opt and n₀ corresponding to maximum T_C and superconductivity onset, respectively, and the length constant Λ = 0.00747(2) Å. Based on existing experimental carrier densities and theoretical estimates for ζ, T_C0 = 1.94(4) K is calculated for the θ = 1.05° ambient-pressure device and T_C0 = 3.02(3) K for the θ = 1.27° pressurized device. Experimental mean-field transition temperatures T_C^mf = 1.83(5) K and T_C^mf = 2.86(5) K are determined by fitting superconducting fluctuation theory to resistance transition data for the ambient-pressure and pressurized devices, respectively; the theoretical results for T_C0 are in remarkable agreement with these experimental values. Corresponding Berezinskii-Kosterlitz-Thouless temperatures T_BKT of 0.96(3) K and 2.2(2) K are also determined and interpreted.

Dale R. Harshman and Anthony T. Fiory, Journal of Superconductivity and Novel Magnetism 33, 367 (2020).