Alteration of superconductivity and radial breathing modes in suspended ropes of carbon nanotubes by organic polymer coatings

We have altered the superconductivity of a suspended rope of single walled carbon nanotubes, by coating it with organic polymers. Upon coating, the normal state resistance of the rope changes by less than 20%. But superconductivity, which on the bare rope shows up as a substantial resistance decrease below $300\mathrm{mK}$, is gradually suppressed. We correlate this to the suppression of radial breathing modes, measured with raman spectroscopy on suspended single and double-walled carbon nanotubes. This points to the breathing phonon modes as being responsible for superconductivity in ropes of single walled carbon nanotubes.

Figure 1

(Color online) Temperature dependence of the resistance of the $7\mathrm{nm}$ wide, $1\mathrm{\mu }\mathrm{m}$-long rope containing 40 SWNT. (a) Before coating. (b) After benzene-dithiol deposition. (c) After additional coating with PMMA. (d) After completely covering the rope and the slit with PMMA. Lines are guides to the eyes. Curves with full symbols are constructed by extracting the differential conductance at zero current from a $dV∕dI\left(I\right)$ curve, at different temperatures. This ensures a stable temperature of the sample, which, for reasons which may have to do with the low heat conduction of small superconducting samples, was slow in reaching thermal equilibrium. Curves with open symbols are the recorded differential resistance at zero current as the sample is slowly (a point per minute) heated. The two open symbol curves at stage $a$ correspond to two consecutive cool downs of the uncoated sample, and show the reproducibility of the experiment. Inset: Comparison at higher temperature. The upturn in resistance around $10\mathrm{K}$ is the signature of twiston modes characteristic of well ordered carbon nanotube ropes. Both curves, before and after complete coating are similar, except for the unstable resistance in stage $d$ above $10\mathrm{K}$.

Figure 2

(Color online) Differential resistance of the nanotube rope at stages $a$ through $d$. Curve $a$ was taken at $28\mathrm{mK}$ with an ac excitation of $1\mathrm{nA}$, curve $b$ at $14\mathrm{mK}$ with $i_{ac}=0.1\mathrm{nA}$, curve $c$ at $15\mathrm{mK}$ with $i_{ac}=0.1\mathrm{nA}$, and curve $d$ at $15\mathrm{mK}$ with $i_{ac}=1\mathrm{nA}$. Both the up (dashed) and down current sweep directions are plotted for curve $b$, illustrating the asymmetry of these current-biased curves.

Figure 3

(Color online) Magnetoresistance at the different coating stages. A large positive magnetoresistance is seen at low temperature for stages $a$, $b$, and $c$. At stage $d$ the magnetoresistance is still positive, but small (0.1%), identical to the magnetoresistance of stage $c$ at $1\mathrm{K}$ (inset). Left resistance scale: Stages $a$, $b$, and $c$ at low temperature. Right resistance scale is relative to stage $d$ that shows a very small positive magnetoresistance. The line is a guide to the eye.

Figure 4

(Color online) Influence of PMMA on the Raman spectra of suspended SWNTs. Raman spectra obtained at room temperature with a $\mathrm{He}\text{−}\mathrm{Ne}$ laser of probe area roughly $1\mathrm{\mu }{\mathrm{m}}^2$. Raman spectra before deposition (a), after deposition (b), and after removal of PMMA in acetone (c). The spectra are of different parts of the sample, chosen because of their similar frequencies (see text). RBM, $D$, and $T$, respectively, stand for radial breathing mode, disordered graphite mode, tangential modes. Inset: field emission scanning electron micrograph of a typical sample of CVD-grown tubes.