Electrical control of the superconducting-to-insulating transition in graphene–metal hybrids

Journal name:
Nature Materials
Volume:
11,
Pages:
590–594
Year published:
DOI:
doi:10.1038/nmat3335
Received
Accepted
Published online

Graphene1 is a sturdy and chemically inert material exhibiting an exposed two-dimensional electron gas of high mobility. These combined properties enable the design of graphene composites, based either on covalent2 or non-covalent3 coupling of adsorbates, or on stacked and multilayered heterostructures4. These systems have shown tunable electronic properties such as bandgap engineering3, reversible metal–insulating transition2, 4 or supramolecular spintronics5. Tunable superconductivity is expected as well6, but experimental realization is lacking. Here, we show experiments based on metal–graphene hybrid composites, enabling the tunable proximity coupling of an array of superconducting nanoparticles of tin onto a macroscopic graphene sheet. This material allows full electrical control of the superconductivity down to a strongly insulating state at low temperature. The observed gate control of superconductivity results from the combination of a proximity-induced superconductivity generated by the metallic nanoparticle array with the two-dimensional and tunable metallicity of graphene. The resulting hybrid material behaves, as a whole, like a granular superconductor showing universal transition threshold and localization of Cooper pairs in the insulating phase. This experiment sheds light on the emergence of superconductivity in inhomogeneous superconductors, and more generally, it demonstrates the potential of graphene as a versatile building block for the realization of superconducting materials.

At a glance

Figures

  1. Self-assembled graphene–tin nanohybrids.
    Figure 1: Self-assembled graphene–tin nanohybrids.

    a, Atomic force micrograph of a 1 μm2 area of the device, obtained by dewetting of an evaporated tin film of nominal thickness 10 nm. b, Transmission electron micrograph of the decorated sample showing the self-organized network of tin nanoparticles (scale bar, 200 nm) separated by clean graphene. c, Transmission electron micrograph of the sample transferred on a membrane and observed at a grazing angle (the dashed line corresponds to the graphene surface). The negative wetting angle of tin nanoparticles on graphene (black arrow) can be clearly seen. d, Scanning electron micrograph of the Sn nanoparticles network on graphene (scale bar, 300 nm). e,f, Sketches of the device. Changing the gate voltage modulates the extension of phase-coherent domains in graphene. S, superconductor. g, Photograph of the studied device. The dark region between the four electrodes is the decorated graphene sheet. The blue cast is due to the presence of tin nanoparticles on the whole surface. The enhanced contrast of the graphene sheet with respect to the silica sides comes from the difference of grain sizes and gaps between nanoparticles on graphene and on SiO2.

  2. Sheet resistance as a function of temperature for different gate voltages.
    Figure 2: Sheet resistance as a function of temperature for different gate voltages.

    a, Sheet resistance as a function of sample temperature for different gate voltages, plotted on a log scale. From top to bottom, voltage offsets from the charge neutrality point (V D = −36 V, see Fig. 3) are ΔV g = V gV D = 0, 3, 6, 8, 10, 12, 14, 16, 17, 18, 19, 20, 21, 22, 26, 31, 36, 41, 46, 56, 66, 76, 86, 96 V. b, The same data (lower part) plotted on a linear scale to emphasize the behaviour between 1 K and 4 K. The black dashed line indicates the critical temperature of tin. The red dashed line is a guide to the eye showing the minimum resistivity. c, Higher magnification of the critical region. d, Phase diagram of the SIT transition. log(R/T) versus gate voltage and temperature. I, insulating; S, superconducting. We call the region where R is below the noise floor (RS = 0.5 Ω) superconducting. The black line is the iso-value of the normal-state resistivity obtained at B = 1 T (see Supplementary Fig. S2) that appeared the closest to the border between the two states.

  3. Sheet resistance as a function of gate voltage for different temperatures.
    Figure 3: Sheet resistance as a function of gate voltage for different temperatures.

    Gate-voltage dependence of sheet resistance for the lowest temperatures. The vertical dotted line indicates the charge neutrality point. The horizontal one indicates the quantum of resistance for Cooper pairs (top axis: carrier density calculated using the gate capacitance per unit area for 285 nm of SiO2 Cbg = 121 μF m−2).

  4. Localization of Cooper pairs under a magnetic field.
    Figure 4: Localization of Cooper pairs under a magnetic field.

    a, Sheet resistance as a function of magnetic field, measured at T = 300 mK. The black curve shows the magnetoresistance at the charge neutrality point. The red curve has been measured deep in the superconducting region. b, Temperature dependence of the sheet resistance at three different magnetic fields (indicated in the upper panel). The black lines are fits to the Efros–Schklovsky law, giving the following activation temperatures: T1 = 7.8 K, T1′ = 32.6 K and T1′′ = 2.5 K for B = 0 T, 0.15 T and 1 T, respectively.

  5. Universal scaling of the transition.
    Figure 5: Universal scaling of the transition.

    Finite-size scaling analysis of the quantum phase transition, using data from Fig. 3. The best collapse was found using νz = 1.05±0.05. Inset, subset of the data used for the scaling: 0.6 K < T < 1.3 K and a gate voltage interval of ±6 V around the critical point V gc = −20 V.

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Author information

Affiliations

  1. Institut Néel, CNRS-UJF-INP, BP 166, 38042 Grenoble cedex 9, France

    • Adrien Allain,
    • Zheng Han &
    • Vincent Bouchiat

Contributions

V.B. and A.A. conceived the experiments, Z.H. grew the graphene, A.A. and Z.H. fabricated the samples and carried out the measurements, A.A. and V.B. analysed the data and wrote the paper.

Competing financial interests

The authors declare no competing financial interests.

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