Gate-Tunable Superconductor-Insulator Transition in Bilayer-Graphene Josephson Junctions

Bilayer graphene shows opening of electric-field-induced band gap, the size of which is proportional to the intensity of the electric field. We report electronic transport measurements on superconducting proximity effect in planar dual-gated bilayer-graphene Josephson junction with Pb$_{0.93}$In$_{0.07}$ (PbIn) electrodes ($\Delta_{PbIn}$ $\sim$ 1.1meV, $T_c$ = 7.0 K). The junction resistance along the charge-neutral point (CNP) increases as we modulate top- and back-gate voltages away from the zero-gap CNP. The resistive state near the CNP shows a variable-range-hopping-type insulating behavior in $R$-$T$ curve with lowering temperature crossing the superconducting transition of PbIn electrodes. However, a highly doped regime shows metallic $R$-$T$ behavior and junction becomes superconducting below $T_c$. Moreover, magnetic-field-induced Fraunhofer supercurrent modulation, microwave-induced Shapiro steps, and multiple Andreev reflection (MAR) are observed, which indicate the formation of genuine Josephson coupling across the planar junctions below $T_c$ with sufficiently transparent superconductor$-$bilayer graphene interface. The separatrix of the superconductor-insulator transition corresponds to the square junction conductance of $G_{sq}\sim$ 6$-$8$e^{2}/h$.