Seiichi Tanuma

Synthesis of low density carbon crystal “carbolite” by quenching of carbon gas

A new carbon crystal named carbolite has been synthesized by quenching high temperature carbon gas on a room temperature substrate. The crystal has a density as small as 1.46 g/cm 3 and is transparent for visible light. The crystal structure is hexagonal, and the dimensions of the unit cell are 1.193 nm in the α-direction and 1.062 nm in the c -direction. Along the c -axis, a parallel array of carbon chains is proposed. The array is made up of a triangular lattice in which the interchain distance is 0.344 nm and the mean interatomic distance within a carbon chain is 0.133 nm. The former value is close to the van der Waals distance between the honeycomb planes of graphite, and the latter value is close to the C-C bond length in the honeycomb plane. Intercalations of potassium, sodium, and iodine atoms are performed.

Effect of intercalation of alkali and halogen species into the low density carbon crystal ‘carbolite’

Abstract A new crystal phase of carbon, ‘carbolite’, was synthesized which consists of a parallel array of carbon chains. The array is made of a triangular lattice of which the inter-chain distance is 0.344 nm and the mean inter-atomic distance is 0.133 nm. The inter-chain space yields intercalations. Potassium, sodium and iodine are shown to intercalate carbolite; the electrical resistance of the material after the reaction is lowered by six orders of magnitude compared to that of initial stage of the intercalated carbolite. Bromine and ICl do not intercalate; instead, they dissolve carbolite.

Anomalous thermoelectric power of Bi/Sb multilayer films and related transport phenomena

Abstract The thermoelectric power of Bi/Sb multilayer films grown on single-crystal Si(111) substrates or on BaF2(111) cleaved surfaces by molecular beam epitaxy method has been measured. An anomalously large negative thermoelectric power was observed on films deposited on Si around room temperature and above, but not on those deposited on BaF2. A possibility of ZT > 1 was suggested.

Magnetic properties of Eu/Se superlattices

Abstract The magnetization of rare-earth superlattices of Eu/Se grown on BaF2(1 1 1) by means of Molecular Beam Epitaxy (MBE) method has been measured in order to clarify the mechanism for the anomalous magnetic properties. Selenium layers in the Eu/Se superlattice act as a separator between Eu layers and the thickness of Se layers was varied from 0.5 to 3 nm, while that of Eu layers was fixed at 1 nm. The crystal structure of samples was characterized by X-ray diffraction and the in situ RHEED observations. A considerably good epitaxial structure was attained for [Eu 1 nm/Se 1 nm] sample, but [Eu 1 nm/Se 0.5 nm] and [Eu 1 nm/Se 3 nm] samples were only polycrystalline. The magnetization process measured at 1.9 K shows that a magnetic phase transition takes place from an antiferromagnetic to a spin-flop state. The critical magnetic field HC is obtained at 3750, 2150 and 1000 G, respectively, for [Eu 1 nm/Se 0.5 nm], [Eu 1 nm/Se 1 nm] and [Eu 1 nm/Se 3 nm] samples. The exchange integral J is dependent on the distance between EuEu layers, i.e., J ∝ d−1.1. The magnetic phase diagram of Eu/Se superlattice has been obtained.

The structure and magnetic properties of Eu-Sr superlattice films

Abstract On epitaxially grown Eu-Sr superlattice films, SQUID measurements of magnetization were made. For the samples with superlattice Eu layer thicknesses of less than 2.5 nm, the Curie-Weiss law held with antiferromagnetic Weiss temperatures of minus several Kelvin, while for the samples with Eu layer thicknesses larger than 5 nm, ferromagnetic phases occurred with two step transitions at 150 and 18 K.

Graphite Intercalation Compounds

A review on the variety of intercalants of graphite intercalation compounds (GIC) as well as on the peculier behavior of intercalants is given. About a handred kinds of intercalants are listed in the case of single-species intercalation. Intercalation takes place as the gaining of free enthalpy by the attraction between intercalants and graphens mainly by the electrostatic interaction due to the charge transfer. The donor type and acceptor type intercalants make GICs electron rich and hole rich conductors respectively. The charge transfer rate varies from unity to, e.g., 0.02. Staging is the most notable behavior of GIC which can make a c-axis superlattice as large periodic distance as more than 50A. Staging kinetics due to Daumas-Herold model, a ‘sliding model’ as the movement of inter-calant sheet and intra-stage fine phase transitions are discussed. Various methods of syntheses of GICs are described. Finally the multi-species intercalation is reviewed giving examples of different methods and different combination of species.

Calorimetry and X-ray powder diffraction studies of carbolite: A new carbon phase

Abstract We report on the specific heat, C(T), measurements of a new carbon phase synthesized by condensation of high temperature carbon gas. For the adequate description of the C(T) dependence, the model of specific heat for quasi-one-dimensional crystals has been applied. By means of X-ray and specific heat measurements it has been found that the material is a metastable phase under the normal conditions, undergoing the transformations to the amorphous state upon heating above T≈350 K.

Dynamic of transformation under the heat treatment of a new carbon phase carbolite studied by calorimetry and X-ray powder diffraction

Abstract We report on the specific heat, C(T) , measurements of a new carbon phase carbolite in the temperature ranges 4.5K T T C(T) dependence does not follow the Debye law. For the adequate description of the measured low temperature C(T) dependence, the chain-like crystal structure of the carbolite crystals has been considered, and the model of specific heat for quasi-one-dimensional crystals has been used. By means of X-ray and high temperature specific heat measurements it has been found that carbolite is a metastable phase under the normal conditions, undergoing the irreversible complicated transformations to the amorphous state upon heating above T ≈350K.

MICROWAVE SURFACE RESISTANCE OF YBa2Cu3O7-δ

The magnetic field dependence of microwave surface resistance of both single crystals and sintered polycrystals of YBa2Cu3O7-δ has been measured at 24 GHz. The resistance decreases below superconducting transition temperature and reaches constant residual resistance. The value is much lower for single crystals than for polycrystals. Furthermore polycrystals showed low field magnetoresistance and high field one with hysteresis which resembles the magnetization behavior of typical type II superconductor. The single crystal, however, showed nearly no magnetoresistance. The magnetoresistance observed is proved to be a inherent phenomena to sintered polycrystal which is composed of grains and contains many voids.