Superconductivity In Carbon Nanotubes Research Articles

Topological superconductivity in carbon nanotubes with a small magnetic flux

We show that a one-dimensional topological superconductor can be realized in carbon nanotubes, using a relatively small magnetic field. Our analysis relies on the intrinsic curvature-enhanced spin-orbit coupling of the nanotubes, as well as on the orbital effect of a magnetic flux threaded through the nanotube. Tuning experimental parameters, we show that a half-metallic state may be induced in the nanotube. Coupling the system to an Ising superconductor, with an appreciable spin-triplet component, can then drive the nanotube into a topological superconducting phase. The proposed scheme is investigated by means of real-space tight-binding simulations, accompanied by an effective continuum low-energy theory, which allows us to gain some insight on the roles of different terms in the Hamiltonian. We calculate the topological phase diagram and ascertain the existence of localized Majorana zero modes near the edges. Moreover, we find that in the absence of a magnetic field, a regime exists where sufficiently strong interactions drive the system into a time-reversal-invariant topological superconducting phase.

Few-Wall Carbon Nanotube Coils.

While various electronic components based on carbon nanotubes (CNTs) have already been demonstrated, the realization of miniature electromagnetic coils based on CNTs remains a challenge. Coils made of single-wall CNTs with accessible ends for contacting have been recently demonstrated but were found unsuitable to act as electromagnetic coils because of electrical shorting between their turns. Coils made of a few-wall CNT could in principle allow an insulated flow of current and thus be potential candidates for realizing CNT-based electromagnetic coils. However, no such CNT structure has been produced so far. Here, we demonstrate the formation of few-wall CNT coils and characterize their structural, optical, vibrational, and electrical properties using experimental and computational tools. The coils are made of CNTs with 2, 3, or 4 walls. They have accessible ends for electrical contacts and low defect densities. The coil diameters are on the order of one micron, like those of single-wall CNT coils, despite the higher rigidity of few-wall CNTs. Coils with as many as 163 turns were found, with their turns organized in a rippled raft configuration. These coils are promising candidates for a variety of miniature devices based on electromagnetic coils, such as electromagnets, inductors, transformers, and motors. Being chirally and enantiomerically pure few-wall CNT bundles, they are also ideal for fundamental studies of interwall coupling and superconductivity in CNTs.

Two-Gap Superconductivity in Niobium Carbide-Coated Single-Walled Carbon Nanotubes: A First-Principles Study

Superconductivity in carbon nanotubes is attracting worldwide attention because of the reported high superconducting transition temperature in small-diameter single-walled carbon nanotubes (SWCNTs). However, it is well known that superconductivity in low-dimensional (quasi-1D) systems is not so common due to low density of states (DOS), strong quantum fluctuations and other phenomena in such systems. In this paper, we present theoretical investigations of the proximity effect of superconducting niobium carbide on single-walled carbon nanotube using density functional theory (DFT). The relaxed structure shows that Nb atoms are held around the SWCNT, forming a layer through weak van der Waals’ forces. The stability of the structure has been confirmed by Hirshfeld analysis and Mullikan population analysis as well. The study of the electronic band structure of the pristine and modified SWCNT shows a fascinating condensation of electronic states and a striking shift in the Fermi level. Further, two additional band gaps have appeared below the valence band suggesting some kind of pairing mechanism being operational. This indicates the possibility of superconducting behaviour in SWCNT in proximity of niobium carbide. The relaxed structure thus envisions the feasibility and stability of NbC-coated SWCNTs which will have superconducting properties as well as the remarkable mechanical and optical properties of SWCNTs. This prediction seeks interest of the researchers to try and develop such a novel nanomaterial which, if realized, will prove to be highly significant for many technological applications.

Ferrimagnetic Ordering and Room Temperature Superconductivity in Carbon Nanotubes

The phenomenon of the formation of uncompensated antiferromagnetic ordering coexisting with the superconductivity at the room temperature in carbon nanotubes, produced by high energy ion beam modification of diamond single crystals in ⟨100⟩ direction is argued.

Topological superconducting state and Majorana fermions in carbon nanotubes

We show that carbon nanotubes (CNT) are good candidates for realizing one-dimensional topological superconductivity with Majorana fermions localized near the end points. The physics behind topological superconductivity in CNT is novel and is mediated by a recently reported curvature-induced spin-orbit coupling which itself has a topological origin. In addition to the spin-orbit coupling, an important new requirement for a robust topological state is broken chirality symmetry about the nanotube axis. We use topological arguments to show that, for recently realized strengths of spin-orbit coupling and broken chirality symmetry, a robust topological gap of around 500 mK is achievable in carbon nanotubes.

Ferrimagnetic Spin-Wave Resonance and Superconductivity in Carbon Nanotubes Incorporated in Diamond Matrix

The phenomenon of ferrimagnetic spin-wave resonance [uncompensated antiferromagnetic spin-wave resonance] has been detected for the first time. It has been observed in carbon nanotubes, produced by high-energy ion beam modification of diamond single crystals in 〈100〉 direction. Peculiarities of spin-wave resonance observed allow to insist on the formation in given nanotubes of s +-superconductivity at room temperature, coexisting with uncompensated antiferromagnetic ordering.

Induced and intrinsic superconductivity in carbon nanotubes

Metallic single wall carbon nanotubes have attracted considerable interest as 1D quantum wires combining a low carrier density and a high mobility. It was believed for a long time that low temperature transport was exclusively dominated by the existence of unscreened Coulomb interactions leading to insulating behaviour at low temperature. However, experiments have also shown evidence of superconductivity in carbon nanotubes. We distinguish two fundamentally different physical situations. When carbon nanotubes are connected to superconducting electrodes, they exhibit proximity-induced superconductivity strongly dependent on the transmission of the electrodes. On the other hand, intrinsic superconductivity was also observed in suspended ropes of carbon nanotubes, in doped or very small diameter individual tubes. These experiments indicate the presence of attractive interactions in carbon nanotubes which overcome Coulomb repulsion at low temperature and enable investigation of superconductivity in a 1D limit never explored before.

Superconducting properties of carbon nanotubes

Abstract Metallic single wall carbon nanotubes have attracted much interest as 1D quantum wires combining a low carrier density and a high mobility. It was believed for a long time that low temperature transport was exclusively dominated by the existence of unscreened Coulomb interactions leading to an insulating behavior at low temperature. However experiments have also shown evidence of superconductivity in carbon nanotubes. We distinguish two fundamentally different physical situations. When carbon nanotubes are connected to superconducting electrodes, they exhibit proximity induced superconductivity with supercurrents which strongly depend on the transmission of the electrodes. On the other hand intrinsic superconductivity was also observed in suspended ropes of carbon nanotubes and recently in doped individual tubes. These experiments indicate the presence of attractive interactions in carbon nanotubes which overcome Coulomb repulsion at low temperature, and enables investigation of superconductivity in a 1D limit never explored before. To cite this article: M. Ferrier et al., C. R. Physique 10 (2009).

Meissner effect in films of ropes of boron-doped single-walled carbon nanotubes; Correlation with applied pressure and boron-doped multi-walled nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. Recently, we have reported successful boron doping into single-walled CNTs (SWNTs) and also revealed its correlation with superconductivity. In the present study, we report results of pressure-applied magnetization measurements in films consisting of ropes of boron-doped SWNTs. It reveals that Tc and magnitude for Meissner effect is mostly independent of applied pressure, while magnitude of graphite diamagnetism drastically increases as pressure increases. We also report result of resistance measurements in the samples and also correlation of boron doped SWNTs with multi-walled CNTs, in which we reported superconductivity previously.

Pressure dependence of Meissner effect in films of ropes of boron-doped carbon nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. Recently, we have reported successful boron doping into single-walled CNTs (SWNTs) and also revealed its correlation with superconductivity. In the present study, we report results of high-pressure magnetization measurements in films consisting of ropes of boron-doped SWNTs. It reveals that T c and magnitude for Meissner effect are mostly independent of applied pressure, while the magnitude of graphite diamagnetism drastically increases as pressure increases. We also report the observation of resistance drop and correlation of boron doping with multi-walled CNTs, in which we reported superconductivity previously.

Superconductivity in Thin Films of Boron-Doped Carbon Nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. However, its correlation with carrier doping has not been reported. We report on the Meissner effect found in thin films consisting of assembled boron (B)-doped single-walled CNTs (B-SWNTs). We find that only B-SWNT films consisting of low boron concentration leads to evident Meissner effect with Tc=12 K and also that a highly homogeneous ensemble of the B-SWNTs is crucial. The first-principles electronic-structure study of the B-SWNTs strongly supports these results.

Bardeen–Cooper–Schrieffer formalism of superconductivity in carbon nanotubes

We develop the Bardeen–Cooper–Schrieffer (BCS) formalism for the superconductivity of carbon nanotubes. It is found that the superconducting transition temperature T c of single-wall carbon nanotubes decreases exponentially with the increase of the tube diameter because the density of states near the Fermi energy is inversely proportional to the tube diameter. For the multi-wall carbon nanotubes, the Cooper paring hopping between layers enhances the superconducting correlation and increases the superconducting transition temperature, which is consistent with the experimental observation.

The influence of dimensionality on superconductivity in carbon nanotubes

We investigate the electronic instabilities in carbon nanotubes (CNs), looking for thebreakdown of the one-dimensional Luttinger liquid regime due to strong screening of thelong-range part of the Coulomb repulsion. We show that such a breakdown is realized in bothultra-small single walled CNs and multi-walled CNs, while a purely electronic mechanismcould explain the superconductivity (SC) observed recently in ultra-small (diameter∼0.4 nm) single-walledCNs (Tc∼15 K) and entirely end-bonded multi-walled ones(Tc∼12 K). We show that both the doping and the screening of the long-range part of theelectron–electron repulsion, needed to allow the SC phase, are related to the intrinsically3D nature of the environment where the CNs operate.

Theoretical approach to electronic screening and correlated superconductivity in carbon nanotubes

A theoretical analysis of the superconductivity observed recently in Carbon nanotubes is proposed. We argue that ultra-small (diameter $ \sim 0.4 nm$) single wall carbon nanotubes (with transition temperature $T_c\sim 15 ^{o}K$) and entirely end-bonded multi-walled ones ($T_c\sim 12 ^{o}K$) can superconduct by an electronic mechanism, basically the same in both cases. By a Luttinger liquid -like approach, one finds enhanced superconducting correlations due to the strong screening of the long-range part of the Coulomb repulsion. Based on this finding, we perform a detailed analysis on the resulting Hubbard-like model, and calculate transition temperatures of the same order of magnitude as the measured ones.

Plasmon exchange model for superconductivity in carbon nanotubes and metallic nanowires

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15 K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T c less than 1 K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T c in a single-wall nanotube. Our theoretical results can explain the observed T c in a single-wall nanotube. In particular, we find that T c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T c has some sort of oscillatory behaviour as a function of the surface number density of electrons.

Microscopic model of superconductivity in carbon nanotubes.

We propose the model of a manifold of one-dimensional interacting electron systems to account for the superconductivity observed in ropes of nanotubes. We rely on the strong suppression of single-particle hopping between neighboring nanotubes in a disordered rope and conclude that the tunneling takes place in pairs of electrons, which are formed within each nanotube due to the existence of large superconducting correlations. Our estimate of the transition temperature is consistent with the values that have been measured experimentally in ropes with about 100 metallic nanotubes.

Proximity-induced superconductivity in carbon nanotubes

Single wall carbon nanotubes (SWNT) are model systems for the study of electronic transport in one-dimensional conductors. They are expected to exhibit strong electronic correlations and non-Fermi liquid behavior as suggested by recent experiments. The possibility to induce supercurrents through such molecular wires is a challenging question both for experimentalists and theoreticians. In this paper we show experimental evidence of induced superconductivity in a SWNT. This proximity effect is observed in a single 1 nm diameter SWNT, in individual cristalline ropes containing about 100 nanotubes and also on multiwalled tubes. These samples are suspended as strings between two superconducting electrodes (double layer Au-Re, Au-Ta or Sn film) at a distance varying between 100 and 2 000 nm. This allows their structural study in a transmission electron microscope. When their resistance is low enough, SWNT become superconducting with surprisingly high critical currents (in the micro-Ampere range for a single tube of normal state resistance 25 kΩ). This critical current, extensively studied as function of temperature and magnetic field, exhibits unusual features which are not observed in conventional Superconducting-Normal-Superconducting junctions and can be related to the strong 1D character of these samples. We also show evidence of a huge sensitivity of dc transport properties of the tubes to electromagnetic radiation in the radio-frequency range.