ABSTRACT

We present detailed magnetic properties of Ni magnetic nano-particles embedded in multiwall carbon nanotubes (MWCNTs). The measured room-temperature saturation magnetization for the nickel nanoparticles is about three times larger than the expected value from the nickel concentration determined independently from inductively coupled plasma mass spectrometer and high energy synchrotron x-ray diffractometer. What is more intriguing is that the Curie-Weiss constant above the Curie temperature of nickel is enhanced by a factor of 12.2. We show that the moment enhancement factor is about two orders of magnitude larger than that predicted from a magnetic-proximity effect. Alternatively, the giant moment enhancement can be naturally explained if MWCNTs are ultra-high temperature superconductors. There is also independent evidence

of ultrahigh temperature superconductivity in MWCNTs. The measured room-temperature diamagnetic susceptibility of pure MWCNTs for the magnetic field parallel to the tube-axis direction agrees quantitatively with the expected diamagnetic Meissner effect. Because of a finite number of transverse conduction channels in ultra-thin superconducting tubes, quantum phase slips are significant and the on-tube resistance is not expected to be zero below the mean-field superconducting transition temperature. Nonetheless, the room-temperature on-tube resistivity has been found to be indistinguishable from zero for many individual MWCNTs. We further show that the temperature dependencies of the resistivity in individual single-wall carbon nanotubes (SWCNTs) are inconsistent with ballistic electrical transport mechanism but can be quantitatively explained in terms of quantum phase slips in quasi-one-dimensional superconductors. 10.1 IntroductionGraphene is a sheet of carbon atoms distributed in a honeycomb lattice and is the building block for carbon-based materials such as graphite and carbon nanotubes (CNTs). The massless relativistic Dirac fermions in graphene are a result of its unique electronic structure, characterized by conical valence and conduction bands that meet at a single point in momentum space. The integer quantum Hall effect recently observed in graphene [Novoselov et al. (2005); Zhang et al. (2005)] is related to the Dirac fermions. More intriguingly, there were reports of ultrahigh-temperature superconducting behaviors in carbon films [Antonowicz (1974); Lebedev (2004)], CNTs [Zhao and Wang (2001); Zhao (2004, 2006)], graphite [Kopelevich et al. (2000)], and graphite-sulfur composites [Da Silva et al. (2001); Moehlecke et al. (2004)]. Highly oriented pyrolithic graphite (HOPG) was shown to display either a partial superconducting or a ferromagnetic-like response to an applied magnetic field even at temperatures well above room temperature [Kopelevich et al. (2000)]. In addition to the observation of unusual high-temperature ferromagnetism in the carbon-based materials [Mombru et al. (2005); Esquinazi et al. (2003); Cervenka et al. (2009)], there was a report of extra magnetic moment induced in graphite due to a large magnetic proximity effect between graphite

and magnetic nanoparticles [Coey et al. (2002)]. Similarly, high-temperature magnetic data of multiwall carbon nanotube (MWCNT) mat samples embedded with Fe and/or Fe3O4 nanoparticles [Zhao and Beeli (2008)] indicated that the room-temperature saturation magnetizations of the magnetic nanoparticles embedded in the MWCNTs are enhanced by a factor of about 2-3 as compared with what they would be expected to have for free magnetic nanoparticles. Recently, the study has been extended to nickel nanoparticles embedded in MWCNTs [Wang et al. (2010)]. Highenergy synchrotron x-ray diffraction spectrum shows that the sample contains about 0.43% nickel that has face-centered-cubic structure and is ferromagnetic. Magnetic measurements show that the room-temperature saturation magnetization of the nickel nanoparticles embedded in the MWCNTs is enhanced by a factor of about 3.4 as compared with that expected for free (unembedded) nanoparticles [Wang et al. (2010)]. What is more intriguing is that the Curie-Weiss constant above the Curie temperature is enhanced by a factor of 12.2. The observed enhancement in the saturation magnetization is about two orders of magnitude larger than that predicted from the magnetic proximity effect [Wang et al. (2010)]. The colossal enhancement in the Curie-Weiss constant is even more difficult to explain in terms of the magnetic proximity effect [Wang et al. (2010)]. Alternatively, it is possible that these moment enhancements might arise from the interplay between ferromagnetism of magnetic nanoparticles and ultrahigh temperature superconductivity in MWCNTs [Wang et al. (2010)]. The existence of ultrahigh-temperature superconductivity in the carbon-based materials is not accidental. The unique electronic structures of the carbon-based materials make them ideal for high-temperature superconductivity. Several theoretical models based on different types of interactions predict high-temperature superconductivity in quasi-one-dimensional (quasi-1D) and/or quasi-two-dimensional (quasi-2D) electronic systems. Alexandrov and Mott [Alexandrov and Mott (1995)] demonstrated that strong electron-phonon coupling can lead to the formation of intersite bipolarons and that the Bose-Einstein condensation of the bipolarons can explain high-temperature superconductivity in cuprates. Little [Little (1964)] proposed that high-temperature or room-temperature superconductivity could be realized by exchanging

high-energy excitons in quasi-1D systems. Lee and Mendoza showed that superconductivity as high as 500 K can be achieved through a pairing interaction mediated by undamped acoustic plasmon modes in quasi-1D systems [Lee and Mendoza (1989)]. High-temperature superconductivity can also occur in a multi-layer electronic system due to an attraction of charge carriers in the same conducting layer via exchange of virtual plasmons in neighboring layers [Cui and Tsai (1991)]. If the plasmon-mediated pairing mechanisms are relevant, one should be able to find high-temperature superconductivity in quasi-one-dimensional and/or multi-layer systems such as cuprates, CNTs, and graphite. In contrast to the mechanisms based on the attractive interactions between electrons by virtually exchanging phonons, excitons, and/or plasmons, an exotic model based on resonating-valence-bond (RVB) theory originally proposed by Anderson [Anderson (1987)] even predicts ultrahigh temperature d-wave superconductivity in heavily doped graphene [Black-Schaffer and Doniach (2007)]. In order to confirm the existence of superconductivity, it is essential to provide two important signatures: the diamagnetic Meissner effect and the sharp resistive transition to the zero resistance state. However, these two essential signatures are less obvious in a quasi-one-dimensional superconducting wire that has a finite number of transverse conduction channels. Due to large superconducting fluctuations, the resistive transition in quasi-1D superconductors could be very broad. Because of the finite number of transverse channels, the four-probe resistance never goes to zero even though the on-wire resistance approaches zero. Because the magnetic penetration depth may be far larger than the transverse dimension, the diamagnetic Meissner effect might be negligible, which makes it difficult to distinguish between the diamagnetic Meissner effect and orbital diamagnetism. Therefore, it is not straightforward to unambiguously identify quasi-1D superconductivity in ultra-thin wires such as CNTs because their magnetic and electrical properties are not typical of bulk superconductivity. In this chapter, we will present the detailed magnetic properties of MWCNTs embedded with Ni nanoparticles [Wang et al. (2010)]. Magnetic measurements were carried out using a quantum design vibrating sample magnetometer (VSM). Inductively coupled plasma mass spectrometer and high-energy synchrotron x-ray

diffractometer were used to accurately determine the impurity concentration. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to characterize MWCNTs and magnetic nanoparticles embedded. In section 10.2, we will present the experimental results for MWCNTs embedded with Ni nanoparticles. In section 10.3, we will identify the diamagnetic Meissner effect in the magnetic field parallel to the tube axis up to room temperature for aligned MWCNTs that are physically separated and have negligibly small magnetic impurities. The magnitude of the Meissner effect is in quantitative agreement with the predicted penetration depth expected from the measured carrier density. In section 10.4, we will provide the evidence of negligible on-tube resistances at room temperature in individual MWCNTs, and rule out the possibility of ballistic transport as a mechanism for the negligible on-tube resistances at room temperature. Alternatively, the temperature dependencies of the resistivity of individual single-wall carbon nanotubes (SWCNTs) can be quantitatively explained in terms of quantum phase slips in quasi-one-dimensional superconductors. In section 10.5, we will give concluding remarks and discuss possible microscopic mechanisms for high-temperature superconductivity in CNTs. 10.2 Magnetic Properties of Nickel

Purified MWCNT mat samples (Lot No. TS0636) from SES Research of Houston were synthesized by chemical vapor deposition under catalyzation of nickel nanoparticles. Some of nickel nanoparticles got into the nanotubes during the nanotube growth. After purification of as-grown MWCNT mat samples, most nickel nanoparticles were removed except for those embedded inside the innermost shells. The morphology of the mat sample can be seen from scanning electron microscopy image shown in Figure 10.1a. The SEM image was taken by a field emission scanning electron microcopy (FE-SEM, Hitachi S-4800) using an accelerating voltage of 3 kV. One can see that the outer diameters of these MWCNTs are in the range of 3050 nm and centered around 40 nm. The mean inner diameter of the MWCNTs is about 10 nm, as seen from the transmission electron

microscopy images (Figure 10.1b,c) recorded by FEI Tecnai F20 with an accelerating voltage of 200 kV. The nickel nanoparticles sit inside the innermost shells near the ends of the tubes, as labeled by A, B, C, and D in Figure 10.1c. Some nickel nanoparticles are connected to form a continuous chain (see a location labeled by D in Figure 10.1c).