Structural basis for the fast phase change of Ge2Sb2Te5: Ring statistics analogy between the crystal and amorphous states

The three-dimensional atomic configuration of amorphous Ge2Sb2Te5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge2Sb2Te5 can be regarded as "even-numbered ring structure," because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge–Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast crystallization speed of the material

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 Eu6C60 and its Sr-substituted compounds, Eu6-xSrxC60. They have a bcc structure, which is an isostructure of other M6C60 (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 TC = 10 - 14 K. In Eu6C60, we also confirm the ferromagnetic transition by heat capacity measurement. The striking feature in Eu6-xSrxC60} is very large negative magnetoresistance at low temperature; the resistivity ratio \rho(H = 9 T)/\rho(H = 0 T) reaches almost 10^{-3} at 1 K in Eu6C60. Such large magnetoresistance is the manifestation of a strong pi-f interaction between conduction carriers on C60 and 4f electrons of Eu.Comment: 5 pages, 4 figure

Thermal expansion of single-walled carbon nanotube (SWNT) bundles: X-ray diffraction studies

Thermal expansion coefficient in single-walled carbon nanotube bundles was determined as (-0.15±0.20)×10-5 (1/K) for the tube diameter and (0.75±0.25)×10-5 (1/K) for the triangular lattice constant by means of x-ray scattering between 300 K to 950 K. The value for the intertube gap was (4.2±1.4)×10-5 (1/K), which is larger than 2.6×10-5 (1/K) for the c-axis thermal expansion in graphite. The results reveal the presence of a remarkably larger lattice anharmonicity in nanotube bundles than that of graphite. The small value for the tube diameter is consistent with the seamless tube structure formed by a strong covalent bond between carbon atoms comparable to that in graphite

Magnetotransport of carbon nanotubes: magnetic-field-induced metal-insulator transition

We have measured magnetotransport of an individual multi-walled carbon nanotube. Though the resistance without magnetic field increases with decreasing temperature (non-metallic) from room temperature down to 2 K, it becomes metallic below 30 K by applying a magnetic field (H ≒ 4 T) perpendicular to the nanotube axis. The transverse magnetoresistance (TMR) below 30 K decreases with increasing magnetic field, and after reaching minimum value around 4 T, it increases. On the other hand, under the intense magnetic field above 6 T the TMR does not show continuous increase but tends to be saturated. The results can be explained by the theoretical prediction of the magnetic-field-induced metal-insulator transition of semiconducting carbon nanotubes

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

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_N_2 or C_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

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 phase transition in the ammoniated alkali C_<60> compound (NH_3)K_3C_<60>

X-ray diffraction measurements of (NH_3)K_3C_60 have revealed a structural phase transition at T_s=150 K, which is attributed to the orientational order-disorder transition of the K-NH_3 pair at the octahedral site of the C_60 lattice. The low-temperature phase has a face-centered-orthorhombic structure derived by doubling the unit lattice vectors of the high-temperature phase along three axes. The superlattice intensity increases continuously below T_s, which means that the transition is second order. At 100 K<T<T_s, a negative thermal expansion is observed along the a and b axes. This is closely related to the contraction of K-N interatomic distance. We also discuss the relation between the superconductivity and the local symmetry of C_60 in the crystal

C_<70> Molecular Stumbling inside Single-Walled Carbon Nanotubes

We report the structural study of C_-one-dimensional (1D) crystal formed inside single-walled carbon nanotubes (SWNTs). X-ray diffraction measurements were performed between 100 K and 999 K on C_-encapsulated SWNTs with an average diameter of 1.37 nm. Two different domains, where the dominant alignment of C_ molecular long axis is standing or lying with respect to the tube axis, were observed; the ratio of the standing to the lying C_-domains is roughly 7:3. Thermal expansion of interfullerene distance gave no evidence for orientational phase transitions, not as in the 3D crystals. Instead, the long-range order of the standing C_-alignment was thermally destroyed with an activation energy of 39±4 meV. The results imply the importance of either the 1D fluctuation or the tube-C_ interaction at finite temperatures