Analysis of 13C-NMR spectra in C60 superconductors : Hyperfine coupling constants, electronic correlation effect, and magnetic penetration depth

A 13C-NMR anisotropic hyperfine coupling tensor was determined as 2π(-1.68, -1.68, 3.37)×106 rad/sec for C603- in A3C60 superconductors, where A is an alkali metal, by analyzing 13C-NMR spectra below 85 K. Combined with an isotropic coupling constant of (2π×0.69)×106 rad/sec, the 2s and 2p characters of the electronic wave functions at the Fermi level were deduced. The results were compatible with local-density-approximation band calculations. From a simulation of 13C-NMR spectra at superconducting state, the traceless chemical (orbital) shift tensor and isotropic chemical shift were determined as (67, 34, -101) ppm and ∼150 ppm, respectively. An estimated magnetic penetration depth is larger than 570 nm in K3C60. Furthermore, the modified Korringa relation, T1TK2∼βS (with Knight shift K, spin-lattice relaxation time T1, and Korringa constant S), clearly showed the existence of weak but substantial antiferromagnetic spin fluctuation in A3C60; β=0.40–0.58 with an error of ±20%.0 The Stoner enhancement factor was also determined as 1–1.5 from a comparison between spin susceptibility obtained from NMR and band-calculation results

Carbon superatom thin films

Assembling clusters on surfaces has emerged as a novel way to grow thin films with targeted properties. In particular, it has been proposed from experimental findings that fullerenes deposited on surfaces could give rise to thin films retaining the bonding properties of the incident clusters. However the microscopic structure of such films is still unclear. By performing quantum molecular dynamics simulations, we show that C_28 fullerenes can be deposited on a surface to form a thin film of nearly defect free molecules, which act as carbon superatoms. Our findings help clarify the structure of disordered small fullerene films and also support the recently proposed hyperdiamond model for solid C_28.Comment: 13 pages, RevTeX, 2 figures available as black and white PostScript files; color PostScript and/or gif files available upon reques

13C NMR and static magnetic susceptibility in C60 superconductors : Possible influence of Kondo impurity

The static spin susceptibility, χsSQ and χsNMR, in C60 superconductors K3C60 and Rb3C60 was studied using a dc superconducting quantum interference device magnetometer and 13C NMR. We found that χsSQ has a peculiar temperature (T) dependence behaving as (1-CT2) with a positive constant C∼(1×10-6) deg-2, contrary to the almost T independent χsNMR. These observations indicate a possibility that there exist Kondo-like impurities, whose Kondo temperature is ∼500 K and whose content is ∼0.001 spins per carbon. On the basis of these studies, the lattice constant dependence of the intrinsic spin susceptibility was established to be dχs/da0=(5.7±0.4)×10-4 emu/mol C60/Å in A3C60 superconductors where A is an alkali metal

Synthesis of single wall carbon nanotubes by using arc discharge technique in nitrogen atmosphere

Single wall carbon nanotubes (SWNTs) were prepared with arc discharge technique using Ni/Co carbon composite rod in He, Ar, and N${_2}$ atmosphere, respectively. The yield and the diameter distribution of them were compared with each other. The results show that N${_2}$ atmosphere at low pressure gives the highest yield for the formation of SWNTs, almost comparable to that obtained with laser furnace technique. It also declares that He atmosphere seems to make SWNTs having smaller diameter distribution than those obtained in N${_2}$ and Ar atmosphere. These findings were summarized and used for the discussion related to the formation mechanism of SWNTs

Formation of single-wall carbon nanotubes in Ar and nitrogen gas atmosphere by using laser furnace technique

The formation of single-wall carbon nanotubes (SWNTs) by using laser vaporization technique in different ambient gas atmosphere was investigated. SWNTs were prepared with Rh/Pd (1.2/1.2 atom%)-carbon composite rod in Ar and nitrogen gas atmosphere, respectively. Raman spectra of raw carbon materials including SWNTs and photoluminescence mapping of dispersed SWNTs in a surfactant solution demonstrate that the diameter distribution of SWNTs prepared in Ar atmosphere is narrower than those obtained by using CVD technique (e.g. HiPco nanotube), even when the ambient temperature is as high as 1150 ○C. It was also found that nitrogen atmosphere gives wider diameter distribution of SWNTs than that obtained with Ar atmosphere. Furthermore, the relative yield of fullerenes (obtained as byproducts) is investigated by using HPLC (high-performance liquid chromatography) technique. It was found that the relative yield of higher fullerenes becomes lower, when nitrogen is used as an ambient gas atmosphere. Based on these experimental findings, a plausible formation mechanism of SWNTs is discussed.

Mono-dispersed single-walled carbon nanotubes made by using arc-burning method in nitrogen atmosphere

Single-walled carbon nanotubes made by using arc-burning technique in nitrogen atmosphere were dispersed in sodium cholate (SC) solution, and the diameter and chirality distribution of semiconductive SWNTs was investigated by UV-VIS-NIR and Raman spectroscopy, and photoluminescence mapping technique. In the typical formation condition, the diameter distribution of them is found to be relatively narrow (1.2 nm–1.4 nm in diameter), less chirality dependent, almost the same as the diameter distribution of SWNTs obtained by using laser-furnace technique