Atsuko Nakayama

Anomalous helium-gas-induced spin-lattice relaxation and the evidence for ultra micropores in microporous carbon

The spin-lattice relaxation mechanism of dangling bond spins in activated carbon fibers (ACF) with huge specific surface areas (∼3000m2/g) was investigated by ESR measurements in the presence of the gases, He, Ne, Ar, H2, N2, and O2. The introduction of helium gas remarkably enhances the spin-lattice relaxation rate, suggesting the participation of the collisional process of helium atoms in the spin relaxation from the dangling bond spins to the lattice. Taking into account an exceptionally large condensation of helium gas, this proves that ACF has ultra micropores which can accommodate only the small diameter helium atoms, resulting in a novel molecular sieve effect.

Pressure-induced phase transition of Bi2Te3to a bcc structure

The pressure-induced phase transition of bismuth telluride, Bi2Te3, has been studied by synchrotron x-ray diffraction measurements at room temperature using a diamond-anvil cell (DAC) with loading pressures up to 29.8 GPa. We found a high-pressure body-centered cubic (bcc) phase in Bi2Te3 at 25.2 GPa, which is denoted as phase IV, and this phase apperars above 14.5 GPa. Upon releasing the pressure from 29.8 GPa, the diffraction pattern changes with pressure hysteresis. The original rhombohedral phase is recovered at 2.43 GPa. The bcc structure can explain the phase IV peaks. We assumed that the structural model of phase IV is analogous to a substitutional binary alloy; the Bi and Te atoms are distributed in the bcc-lattice sites with space group Im-3m. The results of Rietveld analysis based on this model agree well with both the experimental data and calculated results. Therefore, the structure of phase IV in Bi2Te3 can be explained by a solid solution with a bcc lattice in the Bi-Te (60 atomic% tellurium) binary system.

Magnetic properties of activated carbon fibers

Abstract Magnetic susceptibilities and ESR spectra were measured for pitch-based activated carbon fibers (ACF) having huge specific surface areas (≈3000m2/g). These fibers consist of ca. 10A micro-pores with which dangling bond spins are associated. Under an atmospheric condition, a broad weakly temperature dependent ESR signal (ΔH≈800G) was observed, while evacuation of the sample generates a narrow ESR signal (ΔH≈40G) with Curie law behavior superimposed upon the broad signal. From the experimental results for gas adsorption and heat-treatment effects, the narrow signal and the broad signal are respectively assigned to isolated dangling bond spins and conduction electron spins interacting with dangling bond spins directly attached to graphitic skeletons.

Metallization and superconductivity in hexaiodobenzene under high pressure

Iodanil (C 6 I 4 O 2 ) is known as a unique organic crystal showing metallization and superconductivity under high pressure. Hexaiodobenzene (C 6 I 6 ) has a similar molecular structure to iodanil and is a good example to study the mechanism of the pressure-induced metallization of organic monomolecular crystals. We observed an insulator-to-metal transition at 35 GPa for hexaiodobenzene and superconductivity around 2 K in the metallic phase and these value was found to be closed to those of iodanil.

ESR study of activated carbon fibers: preliminary results

We have carried out Electron Spin Resonance (ESR) measurements on activated carbon fibers (ACF) with specific surface areas (SSA) of 3000 and 2000 m[sup 2]/g. The ESR spectrum of ACF fibers in air is extremely broad (500 to 1000 Gauss), and the spin susceptibility decreases rapidly with decreasing specific surface area. Also measured was the ESR signal of the desorbed fibers in vacuum. As a result of desorption, the broad peak decreases slightly in intensity, and a narrow ([approx]65 Gauss at room temperature) peak appears. We report results on the temperature dependence of both peaks. The narrow peak is interpreted as due to spins associated with dangling bonds, whereas we attribute the broad peak to the conduction carrier spins which is broadened by the boundary scattering process ([ital T][sub 1] contribution) and the dipolar broadening process ([ital T][sub 2] contribution) associated with the dangling bond spins.

Anomalous Spin-Lattice Relaxation Induced by Helium Gas in Microporous Carbon

Microporous activated carbon fibers with huge specific surface area (3000 m 2 /g) have dangling bond spins at the peripheries of micro-graphitic domains. Spin-lattice relaxation of the dangling bond spins was studied by ESR measurements in the presence of various gases, He, Ne, Ar, H 2 , N 2 , O 2 . The introduction of helium gas strongly enhances the spin-lattice relaxation rate 1/ T 1 , suggesting the contribution of the collisional process to the spin-lattice relaxation mechanism. The introduction of a simple model on the basis of the electric dipole-dipole interaction makes the calculation of 1/ T 1 possible and the obtained result can explain the experimentally observed result.

Pressure-induced topological phase transition in the polar semiconductor BiTeBr

We performed X-ray diffraction and electrical resistivity measurement up to pressures of 5 GPa and the first-principles calculations utilizing experimental structural parameters to investigate the pressure-induced topological phase transition in BiTeBr having a noncentrosymmetric layered structure (space group P3m1). The P3m1 structure remains stable up to pressures of 5 GPa; the ratio of lattice constants, c/a, has a minimum at pressures of 2.5 - 3 GPa. In the same range, the temperature dependence of resistivity changes from metallic to semiconducting at 3 GPa and has a plateau region between 50 and 150 K in the semiconducting state. Meanwhile, the pressure variation of band structure shows that the bulk band-gap energy closes at 2.9 GPa and re-opens at higher pressures. Furthermore, according to the Wilson loop analysis, the topological nature of electronic states in noncentrosymmetric BiTeBr at 0 and 5 GPa are explicitly revealed to be trivial and non-trivial, respectively. These results strongly suggest that pressure-induced topological phase transition in BiTeBr occurs at the pressures of 2.9 GPa.

Collapse of CuO Double Chains and Suppression of Superconductivity in High-Pressure Phase of YBa2Cu4O8

The crystal structure and electrical resistivity of YBa2Cu4O8 (Y124) were studied under high pressure up to 18 GPa using diamond-anvil cells, respectively, in order to clarify its conduction mechanism. Y124 causes the first-order phase-transition into the orthorhombic Immm at pressure around 11 GPa. The high-pressure phase (HPP) also shows the superconductivity, while the manner of temperature dependence of electrical resistance and the pressure dependence of transition temperature, Tc, drastically change above 11 GPa. The CuO2 plane persists in HPP but the CuO double chains collapse with the phase transition and transform into three-dimensional Cu–O network, resulting in the renewal of conduction system.

Novel Structure of Microporous Activated Carbon Fibers and Their Gas Adsorption

Activated carbon fibers are a kind of microporous carbon. Using dangling bond spins attached to the peripheries of the micropores, we investigated the microporous structures in relation to the heat-treatment and gas adsorption effects. Functional groups weakly bonded to the graphitic backbone are removed by the heat-treatment at moderate temperatures 200-400°C, resulting in the generation of a variety of dangling bond spins. The heattreatment above 500°C brings about homogenization of the dangling bond spins. For gas adsorption, the introduction of helium gas strongly enhances the spin-lattice relaxation rate for the dangling bond spins. In addition to a remarkably large condensation of helium gas in the microporous region, the enhancement proves the presence of ultra-micropores which can accommodate only the smallest diameter helium atoms.

Magnetic and electronic properties of ferrocene-doped activated carbon fibers

Magnetic and electronic properties of ferrocene-doped activated carbon fiber (Fc-ACF) are investigated to clarify a mechanism of ACF micrographite network. The emergence of the super-exchange interaction between spins originated from ferrocenyl (Fc) through hopping electrons on ACF and a valence fluctuation phenomenon of iron atoms are recognized, which may be interpreted by a model that Fc plays a role of bridges between micrographite for hopping-electrons.