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Planar graphene Josephson coupling via van der Waals superconducting contacts
이종윤,김민수,Kenji Watanabe,Takashi Taniguchi,이길호,이후종 한국물리학회 2019 Current Applied Physics Vol.19 No.3
We report on the fabrication and transport characteristics of van der Waals (vdW)-contacted planar Josephson junctions. In a device, two pieces of cleaved 2H-NbSe2 superconducting flakes and a monolayer graphene sheet serve as the superconducting electrodes and the normal-conducting spacer, respectively. A stack of NbSe2−graphene−hexagonal-boron-nitride (hBN) heterostructure with clean and flat interfaces was prepared by a dry transfer technique. The outermost hBN layer protected the NbSe2−graphene−NbSe2 Josephson junction from chemical contamination during the fabrication processes. The Josephson coupling was confirmed by a periodic modulation of the junction critical current Ic in a perpendicular magnetic field. The temperature dependence of Ic showed long and diffusive Josephson coupling characteristics. The temperature dependence of the superconducting gap, obtained from the multiple Andreev reflection features, followed the Bardeen−Cooper−Schrieffer (BCS) prediction.
Interface States in Bilayer Graphene Encapsulated by Hexagonal Boron Nitride
Lee, Kayoung,Liu, En-Shao,Watanabe, Kenji,Taniguchi, Takashi,Nah, Junghyo American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.48
<P>The threshold voltages at the onset of conduction for electron and hole branches can provide information on band gap values or interface states in a gap. We measured conductivity of bilayer graphene encapsulated by hexagonal boron nitride as a function of back and top gates, where another bilayer graphene is used as a top gate. From the measured conductivity the transport gap values were extracted assuming zero interface trap states, and they are close to the theoretically expected gap values. From a little discrepancy an average density of interface states per energy within a band gap (<I>D</I><SUB>it</SUB>) is also estimated. The data clearly show that <I>D</I><SUB>it</SUB> decreases as a bilayer graphene band gap increases rather than being constant. Despite the decreasing trend of <I>D</I><SUB>it</SUB>, interestingly the total interface states within a gap increases linearly as a band gap increases. This is because of ∼2 × 10<SUP>10</SUP> cm<SUP>-2</SUP> interface states localized at band edges even without a band gap, and other gap states are equally spread over the gap.</P> [FIG OMISSION]</BR>