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

Abstract

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 percent. But superconductivity, which on the bare rope shows up as a substantial resistance decrease below 300 mK, is gradualy suppressed. We correlate this to the suppression of radial breathing modes, measured with Raman Spectroscopy on suspended Single and Doublewalled carbon nanotubes. This points to the breathing phonon modes as being responsible for superconductivity in carbon nanotubes. PACS numbers:Carbon nanotubes have been heralded as model systems to explore one dimensionnal (1D) conductors, in which electron-electron interactions lead to a non conventional ground state, the Luttinger liquid [1]. In particular, the single particle density of states is depressed at low energy, with a power law whose exponent depends on the interaction strength. The experimental observation of the power law suppression of the tunnel conductance at low energy was therefore interpreted as a proof of this ground state and of the strength of repulsive electron electron interactions in Single Walled Carbon Nanotubes (SWNT). The discovery of superconductivity in suspended individual ropes of SWNT therefore came as a big surprise [2,3]. As for the case of metallofullerene molecules [6], the question of which phonon modes give rise to the attractive interaction leading to superconductivity is still unsettled. Whereas Sédéki et al.[10] and Gonzalez [11,12] consider the coupling to optical phonon modes, De Martino and Egger [13,14] conjecture that attractive interactions can be mediated by low energy phonon modes, and specifically the radial breathing modes (RBM), which are the compression and dilatation modes of carbon nanotubes. They find that the attractive interaction may be strong enough to overcome the repulsive interactions in SWNT, especially in ropes of SWNT where the Coulomb interaction can be screened because the SWNT are packed so closely together. Their theory [14] also could reproduce the temperature dependence of the superconducting transition observed in ropes.To experimentally test the role played by the phonon modes on superconductivity, we have gradually coated a suspended rope of SWNT with organic material. We show that superconductivity is gradually destroyed. Parallel Raman experiments show that the radial breathing modes are affected by coating, thereby hinting to these modes as playing a major role in the superconductivity of carbon nanotubes. The experiments described hereafter also help define the criteria required to observe a superconducting transition in carbon naotubes.The sample whose superconductivity we have altered is a rope containing roughly 40 SWNT, as determined from its diameter (10 nm) measured in a Transmission Electron Microscope. This rope is produced by the arc discharge method followed by purification [15,16]. The rope is suspended across a 1 µm wide slit etched in a suspended sili...

Radial breathing mode in silicon nanowires: Anab initiostudy

“…15,16 Such conclusions apply however to pristine tubes since, e.g., coating of tubes by weakly bound ͑physisorbed͒ molecules was shown to quench the RBM mode. 17 In the present case of SiNWs, we can expect our results to be significantly altered upon functionalization by large molecules or coating by an all-around insulating gate.…”

Superconducting properties of carbon nanotubes

“…To understand the link between the weakening of superconductivity and the effect of coating on the phonon modes of these ropes, we have investigated the effect of PMMA deposition on the Raman response of other suspended nanotubes [56]. We compared the relative intensity of the RBM with respect to the tangential modes (TM).…”

Disorder-induced superconductivity in ropes of carbon nanotubes

“…[2][3][4] The nanotubes, which are nearly of the same diameter, can have different kind of helicities but in general 1 3 of them are metallic. 5,6 The transport properties of the rope is found to be strongly dependent on the amount of disorder within the tubes. 7,8 It has been reported that the intertube electronic transfer is enhanced in the presence of disorder, leading to a charge-carrier delocalization.…”

“…10,11 Later on, intrinsic superconductivity has been reported in ropes of CNT with a transition temperature T c = 0.55 K. 6,12,13 Ferrier et al 6 studied the dependence of the superconducting transition temperature on the number of the metallic tubes included in the rope and on the amount of disorder. They found that superconductivity arises only in ropes with more than 100 tubes.…”

“…SWNT can also be assembled to form ropes of ordered parallel tubes arranged in a triangular lattice [2][3][4]. The nanotubes, which are nearly of the same diameter, can have different kind of helicities, but in general 1 3 of them are metallic [5,6]. The transport properties of the rope is found to be strongly dependent on the amount of disorder within the tubes [7,8].…”