Mesoscopic transport

- Electronic transport properties of graphene/metal dots hybrids

We investigate the effect of the electron correlations generated by an electronic order (superconductivity or magnetism) which can be transferred into the graphene by means of proximity effect. Graphene has indeed been shown to effectively preserve either superconducting or spin-polarized currents injected from contacting electrodes.
The relative inertness of its exposed surface makes the superconducting proximity effect very efficient as the correlated electrons of the superconductor can effectively couple to the π electron cloud of graphene and generate superconducting correlations at distances exceeding several hundreds of nm.
To maintain electron coherence over larger distances (up to the entire graphene layer, which can reach the meter scale) while retaining the unique 2D properties of the graphene sheet, another strategy can be adopted : a large array of metal islands is placed in a non-percolating network on top of the graphene sheet (see figure, top right). The system does not behave as a "metal-graphene-metal" one-dimensional junction anymore but rather as a 2D metal/graphene hybrid material. For that purpose, the array of metal nanodots needs to be deposited with a submicrometer pitch. Such a network can be achieved by self-assembly using the spontaneous dewetting of evaporated metallic thin films (figure, top left).


Top left : atomic force micrograph (scan size 1 mm) showing dewetted In islands on graphene. Top right : schematic diagram of a typical graphene/hybrid transistor device, in which the supporting substrate is used as a backgate. Each cluster can be the source of correlated electrons in the surrounding graphene area (symbolized by a red halo). The electronic coupling between metal clusters islands can be tuned by varying the electrostatic voltage applied to a back gate represented by the tuning knob. Bottom left & right : resistance of a graphene transistor, obtained by exfoliation of graphite (left) and by chemical vapor deposition (right) decorated with Sn clusters as a function of the backgate voltage and temperature.

In a superconductor-decorated graphene transistor (figure, top right), the transition towards a global superconducting state results from the percolation of local superconductivity induced by the assembly of dots.
Depending on the electronic disorder within the graphene layer, the superconductivity induced in the whole device exhibits different characteristics. In the case of low disorder (exfoliated) graphene, the device shows a transition towards a superconducting state at all gate voltages (see figure, bottom left), typical of a 2D superconductor. If one uses a graphene layer which exhibits significant lattice disorder, the 2D superconducting state cannot be preserved at low charge carrier density (near the charge neutrality point), a regime in which an insulating state sets in (figure, bottom right).
By sweeping the gate voltage, a continuous transition from a superconducting to a truly insulating state can be induced. An intermediate metallic regime is also present at the transition showing sheet resistance of the order of the universal resistance quantum. Such a hybrid system provides the first experimental proof of an electrostatically controlled superconducting-to-insulating transition based on the proximity effect.


- Edge contact van der Waals heterostructures



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