Coulomb drag refers to the phenomenon in which a charge current in one electronic circuit induces a responsive current in a neighbouring circuit solely through Coulomb interactions. For conventional interactions between electrons, the induced drag current in the passive layer is orders of magnitude weaker than the active current due to the strong dielectric screening effect between them. Here we show a Coulomb drag effect between an active normal conductor and a passive superconductor of Josephson junction arrays, where the passive current is of the same order as the active one. The drag force originates from the interactions between the substantially enhanced dynamical quantum fluctuations of the superconducting phases in the passive layer and normal electrons in the active layer. We demonstrate this effect in devices composed of monolayer graphene and LaAlO3/SrTiO3 heterointerface. The estimated passive-to-active ratio can reach about 0.3 at the optimal gate voltage and the temperature dependence follows that of the typical Josephson energy between superconducting puddles. From an engineering perspective, our device may work as a current or voltage transformer, and the drag mechanism lays the foundation for synchronizing Josephson-junction-array-based terahertz radiators.
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We thank F. Wu for helpful discussions. R.T., L.L., X.F., L.G., L.Z., Y.Y., Z.Z. and C.Z. were supported by the National Natural Science Foundation of China (grant nos. 92165201, 11804326, 11974324), the Innovation Program for Quantum Science and Technology (grant no. 2021ZD0302800), the CAS Project for Young Scientists in Basic Research (grant no. YSBR-046), the Strategic Priority Research Program of Chinese Academy of Sciences (grant no. XDC07010000), the Anhui Initiative in Quantum Information Technologies (grant no. AHY170000), Hefei Science Center CAS (grant no. 2020HSC-UE014) and the Fundamental Research Funds for the Central Universities (grant no. WK3510000013). L.L. was also supported by USTC Tang Scholar. H.-Y.X. was supported by the National Natural Science Foundation of China (grant no. 12074039). Part of this work was carried out at the Center for Micro- and Nanoscale Research and Fabrication, University of Science and Technology of China.
The authors declare no competing interests.
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Extended Data Fig. 1 Berezinskii-Kosterlitz-Thouless transition of LAO/STO interface in the final G-LAO/STO Devices.
a,d, The V-I curves of the LAO/STO interface plotted on a semi-log scale for Devices #1 and #3, respectively. In a, the black lines are V ∝ Iα fits of the data in the transition region, and the gray line corresponds to the V ∝ I3 dependence. In d, the black and gray lines correspond respectively to V ∝ I and V ∝ I3 dependencies. b,e, T-dependence of the exponent α, as deduced from the fits shown in a and d, respectively. c,f, R-T curves of the LAO/STO interface plotted on a [dln(R)/dT]-2/3 scale for Devices #1 and #3, respectively.
Extended Data Fig. 2 Negligible impact of the back-gate voltage on the doping level of graphene.
a, Schematic illustration of the back-gate voltage (VBG) tuning. b,c, RLAO/STO and Rgraphene as functions of VBG measured at 400 mK and 100 mK, respectively.
Extended Data Fig. 3 Negligible impact of the interlayer-gate voltage on the electronic performance of LAO/STO interface.
a, Schematic illustration of the interlayer-gate voltage (Vint) tuning. b, RLAO/STO as a function of Vint measured at 100 mK and 400 mK. In contrast to graphene (Fig. 1c in the main text), the impact of Vint on the electronic performance of LAO/STO interface is negligible. This is due to the fact that the typical carrier density of the LAO/STO interface is orders of magnitude higher than that of the graphene layer near the Dirac point.
Extended Data Fig. 4 Exclusion of the impact of interlayer leakage/tunneling on the observed drag signal.
a,b, Schematic diagrams of the drag set-ups where the grounded point at the LAO/STO interface is switched. c, Comparison of Vdrag-Idrive curves for two different set-ups measured at T = 200 mK and VBG = 0 V.
Extended Data Fig. 5 Negligible interlayer leakage.
Interlayer leakage current as a function of Vint for different VBG.
Extended Data Fig. 6 Drag resistances in different G-LAO-STO devices.
a,b, Rdrag and RLAO/STO as functions of T for Devices #2 and #3, respectively. Insets: enlarged-views of the RLAO/STO vs T curves. For Device #3, non-zero Rdrag is obtained even at the lowest measuring temperature, which is consistent with the fact that the LAO/STO interface acting as the drag layer does not enter into the fully SC state.
Extended Data Fig. 7 Evolution of drag effect under magnetic field applied perpendicular to the interfacial plane of LAO/STO.
From top to bottom: Rdrag and RLAO/STO as functions of T measured under 0 T, 0.05 T and 0.5 T magnetic field, respectively.
Extended Data Fig. 8 Counter-current measurements.
a, Schematic illustration of the set-up for the counter-current measurement. b, VLAO/STO as a function of ILAO/STO measured with varying Igraphene (Idrive). The measurements were conducted at T = 200 mK and VBG = 0 V in Device #1. c, VLAO/STO as a function of ILAO/STO and Igraphene (Idrive).
Extended Data Fig. 9 Evolution of drag resistance with the doping level of graphene.
a, Schematic illustration for the drag set-up, where the carrier density/polarity of graphene can be readily tuned using the interlayer-gate voltage (Vint). b, Rdrag and Rgraphene as functions of Vint. The measurements were conducted at T = 40 mK and VBG = 0 V in a G-LAO/STO device with 14-unit-cell LAO (Device #4).
Extended Data Fig. 10 Fits of the drag coefficient of PAR (r = -Rdrag/RLAO/STO) for Device #3.
r as a function of T and the corresponding fitting curves obtained using equations r(T) = r0[1-(T/T*)2] (orange dashed line) and r(T) = r0[1-(T/TP)2]exp(-T/T0) (black dashed line). Inset: The corresponding semi-log plots. The experiments were conducted at VBG = 0 V. Via the fitting, the ratio r at the zero-temperature limit is estimated to be around 0.8×104.
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Tao, R., Li, L., Xie, HY. et al. Josephson–Coulomb drag effect between graphene and a LaAlO3/SrTiO3 superconductor. Nat. Phys. 19, 372–378 (2023). https://doi.org/10.1038/s41567-022-01902-7