Scanning tunneling spectroscopy of proximity superconductivity in epitaxial multilayer graphene

Fabian D. Natterer, Jeonghoon Ha, Hongwoo Baek, Duming Zhang, William G. Cullen, Nikolai B. Zhitenev, Young Kuk, and Joseph A. Stroscio

Phys. Rev. B 93, 045406 – Published 7 January 2016

Abstract

We report on spatial measurements of the superconducting proximity effect in epitaxial graphene induced by a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial multilayer graphene on SiC. The aluminum films were discontinuous, with networks of trenches in the film morphology reaching down to exposed graphene terraces. Scanning tunneling spectra measured on the graphene terraces show a clear decay of the superconducting energy gap with increasing separation from the graphene-aluminum edges. The spectra were well described by BCS theory. The decay length for the superconducting energy gap in graphene was determined to be greater than 400 nm. Deviations in the exponentially decaying energy gap were also observed on a much smaller length scale of tens of nanometers.

Received 9 October 2015

DOI:https://doi.org/10.1103/PhysRevB.93.045406

Authors & Affiliations

Fabian D. Natterer^1,*, Jeonghoon Ha^1,2, Hongwoo Baek^1,3, Duming Zhang^1,2, William G. Cullen^1,2, Nikolai B. Zhitenev¹, Young Kuk³, and Joseph A. Stroscio^1,†

¹Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, U.S.A.
²Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, U.S.A.
³Department of Physics and Astronomy, Seoul National University, Seoul, 151-747, Korea

^*Present address: IBM Research Center Almaden, 650 Harry Road, San Jose, CA 95120, U.S.A.
^†Author to whom correspondence should be addressed: joseph.stroscio@nist.gov

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Vol. 93, Iss. 4 — 15 January 2016

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Images

Figure 1
Growth of superconducting aluminum films on epitaxial graphene on a SiC substrate. (a) Large AFM scan of the aluminum topography showing pits between the aluminum terraces which reach down to the top graphene layer. The height scale covers a range of 198 nm from dark to bright. (b) STM image of a trench region between aluminum islands. The white arrows indicate a monolayer graphene step, and the dashed line traces the position of the spectroscopy measurements in Figs. 2 and 3. (c) High-resolution STM image of the graphene lattice obtained at the location marked by the red square in (b). The larger wavelength modulations are due to a moiré pattern with period of $\approx 2.8 nm$ due to rotational misalignment of $\approx 5^{\circ}$ between the top and second graphene layers.
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Figure 2
Graphene superconducting tunneling spectroscopy. Differential tunneling spectra (symbols) measured at several lateral positions from an aluminum-graphene edge at $Δ x = 0$ (see Fig. 3). The solid lines are nonlinear fits using the modified BCS theory by Maki [19, 20], with gap energies indicated on the right of the graph [21]. The effective temperature of $T_{eff} = 232 mK$ , representing the residual electrical noise in the system, was determined from the best fit to the aluminum spectrum at $x = 0$ and subsequently held fixed to extract the distance-dependent gap width on graphene. The error in the gap energy was determined from the chi-square minimization in nonlinear least-square fits to the Maki theory.
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Figure 3
Proximity-induced superconductivity in epitaxial graphene. (a) $d I / d V$ vs $V_{b}$ tunneling spectra, measured on the graphene terrace starting at an aluminum-graphene edge along the dashed lines shown in (c) and Fig. 1. The spectra are displayed in a color scale, where the brown color indicates a superconducting gap induced by proximity to the nearby aluminum islands. (b) The superconducting gap [21] determined by fitting the spectra in (a) to the modified BCS theory of Maki [19, 20]. The dashed line is a guide to the eye to show the abrupt change in gap energy near the graphene-aluminum interface. The gap energies for $(325 nm \geq x > 0 nm)$ are fit to an exponential decay (solid line), yielding a graphene coherence length of $ξ = (429 \pm 9) nm$ [21]. The error estimates in the gap energy and coherence length were determined from the chi-square minimization of the nonlinear fits. (c) The graphene topographic height and STM image along the path of the spectral measurements in (a) and (b).
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