High-T-C Superconductivity Originating from Interlayer Coulomb Coupling in Gate-Charged Twisted Bilayer Graphene Moire Superlattices

摘要

Unconventional superconductivity in bilayer graphene has been reported for twist angles theta 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 approximate to 1.7 K was reported for a device with theta = 1.05 degrees at ambient pressure and a maximum T-C approximate to 3.1 K for a device with theta = 1.27 degrees 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)(-1)?(|n(opt) - n(0)|/2)(1/2)e(2)/zeta, is a function of mean bilayer separation distance zeta, measured gated charge areal densities n(opt) and n(0) corresponding to maximum T-C and superconductivity onset, respectively, and the length constant ? = 0.00747(2) angstrom. Based on existing experimental carrier densities and theoretical estimates for zeta, T-C0 = 1.94(4) K is calculated for the theta = 1.05 degrees ambient-pressure device and T-C0 = 3.02(3) K for the theta = 1.27 degrees 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.