Youichi Murakami

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.

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.

Mode-Locked Fiber Lasers Using Adjustable Saturable Absorption in Vertically Aligned Carbon Nanotubes

We report an adjustable saturable absorber based on vertically aligned single-walled carbon nanotubes (SWNTs). Using the low-temperature alcohol catalytic chemical vapor deposition (CVD) method, high-quality vertically aligned SWNTs were directly synthesized onto quartz substrates. The saturable absorption is adjustable by changing the decline angle of the substrate. We applied it as a mode-locker in the passively mode-locked fiber lasers, and optimized the operation by declining the vertically aligned SWNT sample.

Magnetic Properties of Adsorbed Oxygen in Microporous Carbon

Abstract Microporous activated carbon fibers consisting of a disordered network of micrographites provide a micropore random network available for the adsorption of a large amount of gaseous materials. We investigate magnetic properties of oxygen molecules condensed in the micropore as a molecule-based random magnet. Magnetic susceptibility shows Curie-Weiss temperature dependence with the Weiss temperature of about -20K at low oxygen concentrations below 1% of the saturation concentration. The increase in the oxygen concentration makes the development of the second contribution to the susceptibility that has a broad hump around 60K. The magnetic behavior is explained in term of the formation of antiferromagnetic oxygen clusters on micrographitic domains, where the internal and marginal oxygen molecules in the cluster contribute to the susceptibility with a short range order hump and the Curie-Weiss susceptibility, respectively.

Crystal Structure of Endohedral Metallofullerene La@C 82

Since the success of synthesis the molecular structure of the endohedral metallofullerene is a most interesting subject in relation to the electronic state of metal and carbon cage. Highquality crystals of endohedral metallofullerene La@C82 have been synthesized by the arc discharge and HPLC method. The x-ray powder diffraction and the precession photograph of a tiny single crystal have revealed the cubic structure with the space group I 4 3d and the lattice constant a o = 25.72 ± 0.007 A at room temperature. The structure is characterized by the molecular arrangement in which the molecules align in the [111] direction with the molecular axis orienting in the same [111] direction; this structure suggests the existence of a dipole interaction between the molecules. The metal position in the cage is also discussed on the basis of an ellipsoidal model of molecule.

Mode-locked fiber lasers using vertically aligned carbon nanotubes directly synthesized onto substrates

We demonstrate novel passively mode-locked fiber lasers using vertically aligned carbon nanotubes synthesized using the low-temperature alcohol catalytic CVD method. We found that the laser can be mode locked at wide range of slant angles.

Neutron diffraction study of oxygen monolayers

Abstract An oxygen monolayer physisorbed on graphite, which is regarded as a purely two-dimensional Heisenberg-like antiferromagnet, has been investigated focusing on the magnetic phase transition by magnetic susceptibility and neutron-diffraction measurements. They indicate the existence of magnetic long range order in the dense monolayer ɛ -phase. The average spin value of the oxygen molecule shows a large spin reduction due to quantum and the frustration effects. In the bilayer region, the second oxygen layer, which is not adjacent to the graphite surface, is not found to be ordered magnetically.

Magnetic phase transition of oxygen monolayer films adsorbed on exfoliated graphite

Abstract The magnetic susceptibility (χ) of oxygen monolayer films adsorbed on Grafoil has been investigated in order to elucidate a two dimensional (2D) magnetic phase transition. This system is expected to be a 2D Heisenberg-like antiferromagnet. For the dense phase, in which the molecular axis is perpendicular to a graphite layer, an anomaly of χ was first observed at T N = 11.9 K; which is consistent with the magnetic phase transition temperature reported by the previous structural and neutron experiments. The temperature dependence of the anomaly indicates that a 2D long-range order exists below T N . The coverage dependence of χ suggests that the magnetic ordering occurs in the first layer but not in the second layer at T N . The magnetization is proportional to magnetic field at 0.1 up to 5.5 T, and we cannot observe the spin flop transition.

Magnetic phase transition of oxygen monolayers

An oxygen monolayer adsorbed on graphite, which is expected to be a 2D antiferromagnet, have been investigated by dc magnetic susceptibility measurements. The dense monolayer phase, in which oxygen molecules have a deformed triangular lattice with the molecular axis normal to the graphite surface, shows an anomaly at T N =11.9 K. The temperature dependence of susceptibility χ below T N suggests the presence of an antiferromagnetic long-range order, but no appreciable anisotropy of χ even below T N . The observed coverage dependence of the magnetic anomaly revealed that the first layer adjacent to graphite is responsible for the magnetic transition but the second layer does not contribute to it.

Crystal structures and magnetism of oxygen monolayers on hexagonal boron nitride

Abstract We have measured X-ray diffraction (XRD) and magnetic susceptibility ( χ ) of oxygen monolayers on hexagonal boron nitride (h-BN) as a function of coverage and temperature, and determined the phase diagram, which is discussed compared with that of a graphite substrate. The coverage dependence reveals two distinct phases. A high-density monolayer ζ -phase has a triangular lattice, which is characterized by a magnetoelastic transition to the e -phase at T N =11xa0K, similar to a graphite substrate. On the other hand, for the low-density monolayer λ -phase in which all the molecules are lying on the surface, there appears a structural transition of the first-order to κ -phase, including the change of lattice constants and a simultaneous rotation of molecules. This transition is unique to h-BN and not observed for a graphite substrate. The shrink of the nearest neighbor distance at low temperatures is a result of a magnetoelastic transition and in κ -phase; the magnetic exchange energy ( J ) is expected to be much stronger for the κ -phase ( J =28.7xa0K) than the λ -phase( J =14.5xa0K) and form a quasi-one-dimensional structure ( J nnn / J nn =0.01). Moreover, χ shows an exponential temperature dependence below T C , suggesting the possibility of an energy gap, discussed as a Haldane gap. A remarkably slow kinetic characteristic of logxa0 t is observed both in XRD and χ measurements, which might be related to the domain growth in the quasi-one-dimensional system.