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由绝缘体间隔的两个导体间的层间拖拽效应是研究准粒子耦合和探索层间关联物态的重要途径, 而具有丰富可调物性的二维层状材料为研究拖拽效应提供了更多可能. 本文制备了石墨烯-氮化硼-二硒化铌这一电双层结构, 系统研究了二维半金属和二维超导体之间的拖拽效应. 当石墨烯作为驱动层, 二硒化铌作为被动层时, 可以在二硒化铌超导转变温度区间发现显著的拖拽响应, 表现出明显的超流拖拽特征; 而当二硒化铌处于正常金属态时, 拖拽信号消失. 磁场、栅压调制下的测试进一步表明, 拖拽响应与二硒化铌的超导转变直接相关, 且其符号不依赖石墨烯载流子类型. 更为重要的是, 通过对比不同器件发现, 这类超流拖拽响应仅出现在空气中解理的薄层二硒化铌中, 因此二硒化铌中超导的不均匀性是产生该效应的关键. 其机制可能源自非均匀超导体中量子涨落与二维半金属电荷密度波动之间的库仑耦合.In an electronic double-layer system composed of two spatially adjacent but electrically insulated conductors, when current flows through one conductor (drive layer), its charge carriers transfer energy/momentum to the charge carriers in the other conductor (drag layer) through interlayer Coulomb coupling, thus generating a measurable voltage or current in the drag layer, This phenomenon is known as interlayer drag effect. This effect provides a critical approach for studying quasiparticle interactions and investigating interlayer-correlated quantum states. Two-dimensional layered materials with highly tunable properties provide new opportunities for exploring the drag effect. In this study, we fabricate an electronic double-layer structure consisting of graphene and NbSe2 to systematically investigate the drag effect between a two-dimensional semimetal and a two-dimensional superconductor, wherein a thin hBN layer serves as the insulating spacer. When graphene acts as the drive layer and NbSe2 acts as the drag layer, a significant positive drag response is observed within the superconducting transition temperature range of NbSe2. In contrast, the drag signal vanishes when NbSe2 is in its normal metallic state. The measurements of magnetic field dependence reveal that the drag response disappears under high fields where the superconductivity of NbSe2 is suppressed, further confirming its direct correlation with the superconducting transition. The gate-voltage modulation experiments reveal that the drag response peaks when adjusting the Fermi level of graphene across the Dirac point. This is attributed to the reduced screening of interlayer interactions due to the ultra-low carrier concentration at this point. Notably, the sign of the supercurrent drag does not depend on the carrier type in graphene, ruling out the traditional momentum-transfer drag mechanism. Our results collectively demonstrate the realization of supercurrent drag effect, which has been attributed to Coulomb coupling between the quantum fluctuations of the superconducting phases in a superconductor and the charge densities in a normal conductor in previous study. Notably, by comparing different devices, it is found that this type of supercurrent drag responses occurs only in the thin NbSe2 layers cleaved in air. No significant signals are detected in thick NbSe2 layers or thin layers cleaved under the protection of argon. These results establish the importance of superconducting inhomogeneity in NbSe2 for generating supercurrent drag effect, indicating that drag measurements can also serve as a novel probe for investigating superconducting properties. Further investigation into the polarity and intensity of supercurrent drag signals may advance our understanding of inhomogeneous superconductivity, as well as interactions between normal carriers and Cooper pairs.
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Keywords:
- two-dimensional materials /
- supercurrent drag effect /
- superconducting inhomogeneity /
- quantum fluctuations
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图 1 (a)石墨烯-NbSe2拖拽器件示意图; (b)石墨烯-NbSe2拖拽器件的光学显微图像, 单层石墨烯和双层NbSe2的轮廓分别用黑白虚线标示, 插图为转移前NbSe2层的光学显微图像
Fig. 1. (a) Schematic diagram graphene-NbSe2 drag device; (b) optical image of graphene-NbSe2 drag device, the outlines of monolayer graphene and NbSe2 flakes are marked with black and white dashed lines, respectively. Inset is the optical image of the NbSe2 layer before transfer.
图 2 (a)拖拽测试示意图; (b)器件#1中层间拖拽电阻(Rdrag)和NbSe2层内电阻($ {\text{R}}_{{\text{NbSe}}_{2}} $)随温度的变化曲线
Fig. 2. (a) Schematic of the setup for drag measurements; (b) interlayer drag resistance (Rdrag) and intralayer resistance of NbSe2 ($ {\text{R}}_{{\text{NbSe}}_{2}} $) as functions of temperature for Device #1.
图 3 (a)器件#1中Rdrag和$ {\text{R}}_{{\text{NbSe}}_{2}} $随磁场强度的变化曲线, 测试温度为1.6 K; (b)Rdrag和(c) $ {\text{R}}_{{\text{NbSe}}_{2}} $随温度和磁场强度的变化
Fig. 3. (a) Rdrag and $ {\text{R}}_{{\text{NbSe}}_{2}} $ as functions of magnetic field for Device #1, measured at 1.6 K; (b) Rdrag and (c) $ {\text{R}}_{{\text{NbSe}}_{2}} $ as functions of temperature and magnetic field.
图 4 (a)器件#1中Rdrag和石墨烯层内电阻Rgraphene随层间栅压的变化曲线, 测试温度为1.6 K; (b)Rdrag和(c)Rgraphene随温度和层间栅压的变化
Fig. 4. (a) Rdrag and intralayer resistance of graphene (Rgraphene) as functions of interlayer gate voltage (Vint) for Device #1, measured at 1.6 K; (b) Rdrag and (c) Rgraphene as functions of temperature and Vint.
图 5 (a)器件#2中Rdrag和$ {\text{R}}_{{\text{NbSe}}_{2}} $随温度的变化曲线, 插图为该器件中所使用的NbSe2层的光学显微图像, 由薄层区域(~3层)和厚层区域(~5层)两部分构成; (b)拖拽耦合比r和$ {\text{R}}_{{\text{NbSe}}_{2}} $随温度的变化曲线
Fig. 5. (a) Rdrag and $ {\text{R}}_{{\text{NbSe}}_{2}} $ as functions of temperature for Device #2, inset is optical image of the NbSe2 layer in this device, which is composed of a thin region Ⅰ (~3 layers) and a thick region Ⅱ (~5 layers); (b) passive-to-active ratio r (drag current/drive current) and $ {\text{R}}_{{\text{NbSe}}_{2}} $ as functions of temperature.
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