Hiroyoshi Suematsu

Gas adsorption in the inside and outside of single-walled carbon nanotubes

Abstract Adsorption properties of nitrogen and oxygen gases in single-walled carbon nanotube (SWNT) bundles were investigated by the isotherm and X-ray diffraction (XRD) studies. In the as-grown (AG) nanotubes with close-ended caps, both the gases are adsorbed only in the interstitial channels between triangular packed nanotubes. In the heat-treated (HT) nanotubes with open ends, the gases are adsorbed first in the inside of tubes, and next in the interstitial channels. In each site, gases can be adsorbed with the stoichiometory of C 20 N 2 or C 20 O 2 as a monolayer. These results indicate that the inside of nanotube has strong affinity for gas adsorption than the interstitial channels of bundles.

Resistivity of single crystal C60 and effect of oxygen

Abstract A crucial effect of oxygen on resistivity ϱ of a C 60 single crystal and the temperature (160-570K) dependence of ϱ have been studied. The resistivity increases by a factor of 10 4 on absorption of oxygen; the estimated concentration is 4% O 2 eer C 60 . The processes of oxygen absorption and desorption are reversible. The activation energy in ϱ of the oxygen-free crystal is 0.26 and 0.15 eV above and below Tc = 250K , respectively, which is ascribed to an impurity level.

Photoconductivity in Semiconducting Single-Walled Carbon Nanotubes

We have observed the photoconductive response of film samples of single-walled carbon nanotubes for the first time. Two peaks in the photoconductivity excitation spectra around 0.7 and 1.2 eV are observed at room temperature, which can be interpreted as a photocurrent in semiconducting nanotubes. At a low temperature, we found a marked change in the intensity of the spectrum. In this paper, we discuss this temperature dependence and the mechanism of photoconductivity.

Structural, magnetic and superconducting properties of graphite nanotubes and their encapsulation compounds

Abstract X-ray diffraction and magnetic susceptibility measurements have been made for graphite nanotubes and encapsulation compounds of TaC and CeC2;. In the nanotubes the graphite layers expand 2.27% (C0 = 6.852 A) in the interlayer spacing, and only 0.09% in the in-plane direction. The observed (hk0) diffraction peaks show a pronounced asymmetric lineshape of the Warren type, which results from complete stacking disorder. The carbide inside the nanotube, which is a single crystal in most cases, has a lattice parameter very close to that of the carbide in the bulk, indicating no significant effect due to encapsulation. The superconducting Tc of encapsulated TaC is very close to that of the bulk sample. An antiferromagnetic transition has been observed in the CeC2 encapsulation compound; the observed TN is consistent with that found in an earlier neutron study of a bulk sample.

Synchrotron radiation X-ray powder diffractometer with a cylindrical imaging plate

A synchrotron radiation X-ray powder diffractometer for samples of very small amount has been developed to collect high-quality diffraction patterns under extreme conditions, i.e. at low temperature and/or high pressure. A new cylindrical imaging plate (CIP) is used as a detector, in addition to a conventional flat-type imaging plate (FIP). By using the CIP system, the diffraction data in a diffraction angle range −44 ≤ 2θ ≤ 122° are collected with a dynamic range of about 106. The alignment of the diffractometer, measurement and analysis are automatically operated by a workstation. A performance test shows that the CIP system has spatial resolution of about 0.07° with a dynamic range of 106. The diffraction pattern of a standard sample of Si measured by the CIP system has high quality; the refinement of the structure reaches Rw = 3.68% even in the case of a small amount of sample (about 2 µg) and a short exposure time (60 s). Examples of experiments at low temperatures under ambient and high pressures are also presented.

Anomaly of X-ray Diffraction Profile in Single-Walled Carbon Nanotubes

X-ray diffraction (XRD) studies on single-walled carbon nanotube (SWNT) samples prepared by the arc-discharge method were reported. The XRD profile was basically explained to be a result of triangular packing of SWNTs with a lattice constant of 17.1 A and an average nanotube radius of 7.1 A. We found an anomalous change in XRD profiles before and after heat-treatment of the SWNT samples in air at ~350°C. Combined with gravimetric measurements and resistivity measurements, a detailed simulation of the XRD profiles showed that air (oxygen, and/or nitrogen and/or water) can be condensed inside the SWNTs.

SUPERCONDUCTIVITY IN SILICON BASED BARIUM-INCLUSION CLATHRATES

Abstract Clathrates of type Ba 8 TM x Si 46− x (with TM = Au, Ag, Cu) have been investigated. We report on superconductivity in Ba-inclusion silicon based clathrates when doped with transition metals. It is clarified that the transition metals are necessary in order to form the clathrates as major phases, but they destroy the superconductivity. On lowering the amount of TM, an increase in superconducting fraction has been observed, with a maximum at x ≈1. After purification a superconducting fraction of up to 50% was obtained. The critical temperatures range from 5 to 6 K depending on both, x and the type of TM.

Superconductivity in graphite-alkali metal intercalation compounds

Abstract Superconducting properties of pseudo-single crystals of C8K were mainly investigated by the low frequency AC magnetic susceptibility and the electrical resistivity. The measured values of Tc dispersed from 128 mK to 198 mK for 13 samples. The measurements of the superconducting transition under magnetic field revealed a remarkable anisotropy due to the field angle θ measured from the layer plane, i.e., a type I superconductivity was observed for 25°⪅¦θ¦≦ 90dg and a type II for 0°≦¦θ¦⪅25°. The angular dependences of Hc2 and Hc3 were fairly well explained by the effective mass model. The Hc2 versus T curve under magnetic field perpendicular to the c-axis showed a positive curvature in the temperature range near Tc.

Gas Storage in Single-Walled Carbon Nanotubes

Abstract X-ray diffraction (XRD) and electrical resistance measurement on single-walled carbon nan-otube (SWNT) samples prepared by the arc-discharge method are reported. The XRD profile of heat-treated sample indicated that air (oxygen, and/or nitrogen and/or water) can be condensed inside the SWNTs. We also found that the electrical resistance of SWNT soot is significantly affected by exposing to the oxygen gas and humid air.

Ferromagnetism and giant magnetoresistance in the rare-earth fullerides Eu6-xSrxC60

We have studied crystal structure, magnetism, and electric transport properties of a europium fulleride Eu 6 C 6 0 and its Sr-substituted compounds, Eu 6 - x Sr x C 6 0 . They have a bcc structure, which is an isostructure ofother M 6 C 6 0 (M represents an alkali atom or an alkaline-earth atom). Magnetic measurements revealed that magnetic moment is ascribed to the divalent europium atom with S = 7/2 spin, and a ferromagnetic transition was observed at T C = 10-14 K. In Eu 6 C 6 0 , we also confirm the ferromagnetic transition by heat-capacity measurement. The striking feature in Eu 6 - x Sr x C 6 0 is very large negative magnetoresistance at low temperature; the resistivity ratio ρ(H=9 T)/ρ(H=0 T) reaches almost 10 - 3 at 1 K in Eu 6 C 6 0 . Such large magnetoresistance is the manifestation of a strong π-f interaction between conduction carriers on C 6 0 and 4f electrons of Eu.