We have investigated superconductor induced proximity effect in diffusive multiwalled carbon nanotubes. Using current bias, we find discontinuous IV curves at temperatures < 100 mK, which suggest gate
tunable supercurrents up to 1.5 nA. The superconducting state is not a true zero resistance state but our sample displays a resistance which scales approximatively as 1√I_{c}). The critical current is
reduced as a function of temperature above T = 100 mK as expected for a diffusive SNS system;. For the critical magnetic field we find about 12 mT, which reflects the destruction of the energy gap Δ
~ 1 K in the Ti/Al contacts of thickness 10nm/70 nm. Using voltage bias, we find strong peaks in the differential conductance that we assign to disorder enhancement of multiple Andreev reflections.

Current I as a function of bias voltage V for different gate voltages. The solid straight line displays the normal state IV curve measured in a magnetic field of B = 0.2 T at V_{g} = 3.202 V. The arrows A and B illustrate the determination of the maximum supercurrent I_{cm} in the nonhysteretic and hysteretic cases. The inset on the lower right illustrates a magnification of the IV curve at V_{g} = 3.214 V, where clear hysteresis is visible and the critical current I_{cm} = 0.15 nA. The inset on the upper left displays an AFM image of our sample (scale bar: 1 μm). The data were measured in a two lead configuration on the middle section at T = 65 mK.

I_{cm} vs. zero bias resistance R_{0} and normal state resistance R_{n} measured at T = 80 mK. The open and filled circles displays R_{0} and R_{n} at V_{g} = 3.21... 3.33 V, respectively. The filled triangles denote R_{0} at V_{g} = 5.98... 6.02 V. The solid curve is a power law fit with I_{cm} ∝ R_{0}^{-1.35} while the dashed line represents I_{cm} ∝ R_{n}^{-4.93}. The dotted curve displays the phase-diffusion relation I_{cm} ∝ R_{0}^{-1} valid at E_{J} « k_{B}T. The inset displays the normal state conductance G_{n} = 1/R_{n} vs V_{g}. The range of the superconducting IV curves in Fig. 1 (V_{g} = 3.214–3.244 V) is indicated by the gray shading.

Temperature dependence of I_{cm} (plain circles) and R_{0} (empty circles) measured at _{g} =_{g} = 3.489 V. The solid curve is a fit obtained from Eqs. taken from A. D. Zaikin and G. F. Zharkov, Sov. J. Low Temp. Phys. 7, 184 (1981) and P. Dubos et al., Phys. Rev. B 63, 064502 (2001) using D = 2.2 x 10^{-3} m^{2}/s, L = 0.4 μm, and R_{n} = 17 kΩ. The dashed line displays R0 calculated from Eq. (1) using Z_{env} = 400 Ω and E_{J}(T) from the above T-dependence fit.

Conductance G vs bias voltage V. Gate voltage V_{g} has been stepped over V_{g} = 3.323... 3.355 V in steps of 2 mV. The dashed vertical lines indicate locations for the first Andreev reflection process if governed by unrenormalized Δ_{lead}/e = 139 μeV.

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Gate-controlled superconductivity in a diffusive multiwalled carbon nanotube