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

Document Type

Article

Publication Date

2-1-2020

Abstract

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 TC ≈ 1.7 K was reported for a device with θ = 1.05° at ambient pressure and a maximum TC ≈ 3.1 K for a device with θ = 1.27° under 1.33 GPa hydrostatic pressure. A high-TC 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, TC0 = kB −1Λ(|nopt − n0|/2)1/2e2/ζ, is a function of mean bilayer separation distance ζ, measured gated charge areal densities nopt and n0 corresponding to maximum TC and superconductivity onset, respectively, and the length constant Λ = 0.00747(2) Å. Based on existing experimental carrier densities and theoretical estimates for ζ, TC0 = 1.94(4) K is calculated for the θ = 1.05° ambient-pressure device and TC0 = 3.02(3) K for the θ = 1.27° pressurized device. Experimental mean-field transition temperatures TC mf = 1.83(5) K and TC 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 TC0 are in remarkable agreement with these experimental values. Corresponding Berezinskii-Kosterlitz-Thouless temperatures TBKT of 0.96(3) K and 2.2(2) K are also determined and interpreted.

Identifier

85070071634 (Scopus)

Publication Title

Journal of Superconductivity and Novel Magnetism

External Full Text Location

https://doi.org/10.1007/s10948-019-05183-9

e-ISSN

15571947

ISSN

15571939

First Page

367

Last Page

378

Issue

2

Volume

33

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