Induced Superconductivity in Graphene Grown on Rhenium

We report a new way to strongly couple graphene to a superconductor. The graphene monolayer has been grown directly on top of a superconducting Re(0001) thin film and characterized by scanning tunneling microscopy and spectroscopy. We observed a moiré pattern due to the mismatch between Re and graphene lattice parameters that we have simulated with ab initio calculations. The density of states around the Fermi energy appears to be position dependent on this moiré pattern. Tunneling spectroscopy performed at 50 mK shows that the superconducting behavior of graphene on Re is well described by the Bardeen-Cooper-Schrieffer theory and stands for a very good interface between the graphene and its metallic substrate.

Figure 1

Top: STM image showing the moiré pattern due to the lattice paramater mismatch of graphene and Re, measured with a bias voltage $V_{\mathrm{bias}}=224\mathrm{mV}$ and a tunneling current $I=31\mathrm{nA}$. Bottom: STM image exhibiting an atomic periodicity of order six on the hills of the moiré and three in the valleys, measured for $V_{\mathrm{bias}}=10\mathrm{mV}$ and $I=10\mathrm{nA}$.

Figure 2

Ab initio calculation results for the graphene ($8\times 8$)-Re ($7\times 7$) interface: (a) schematic view from top, (b) cross section of the square modulus of the wave function integrated between $E_F$ and $E_F+0.5\mathrm{eV}$. Black and gray diamonds show the unit cell.

Figure 3

Top: Topographic image ($5\times 3{\mathrm{nm}}^2$) measured at $V_{\mathrm{bias}}=800\mathrm{mV}$ and $I=5\mathrm{nA}$. Middle: Conductance map at the Fermi energy. Bottom: Spectra averaged over the hills (red) and valleys (black). Inset: zoom in energy of spectra averaged on individual hills (colors) and the spectrum averaged on the valleys (black).

Figure 4

Normalized differential conductance G measured at 50 mK. Inset: temperature evolution of the superconducting gap.

Figure 5

Closing of the superconducting gap in the vortex core region. Inset: Conductance map of the vortex core ($155\times 135{\mathrm{nm}}^2$) at the Fermi energy.