Proximity induced superconducting multiwalled carbon nanotubes

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√Ic). 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 Vg = 3.202 V. The arrows A and B illustrate the determination of the maximum supercurrent Icm in the nonhysteretic and hysteretic cases. The inset on the lower right illustrates a magnification of the IV curve at Vg = 3.214 V, where clear hysteresis is visible and the critical current Icm = 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.
Icm vs. zero bias resistance R0 and normal state resistance Rn measured at T = 80 mK. The open and filled circles displays R0 and Rn at Vg = 3.21... 3.33 V, respectively. The filled triangles denote R0 at Vg = 5.98... 6.02 V. The solid curve is a power law fit with Icm ∝ R0-1.35 while the dashed line represents Icm ∝ Rn-4.93. The dotted curve displays the phase-diffusion relation Icm ∝ R0-1 valid at EJ « kBT. The inset displays the normal state conductance Gn = 1/Rn vs Vg. The range of the superconducting IV curves in Fig. 1 (Vg = 3.214–3.244 V) is indicated by the gray shading.
Temperature dependence of Icm (plain circles) and R0 (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 m2/s, L = 0.4 μm, and Rn = 17 kΩ. The dashed line displays R0 calculated from Eq. (1) using Zenv = 400 Ω and EJ(T) from the above T-dependence fit.
Conductance G vs bias voltage V. Gate voltage Vg has been stepped over Vg = 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 Δlead/e = 139 μeV.

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Phys. Rev. Lett. 98, 087002 (2007)