Kenichi Imaeda

Intermolecular energy-band dispersion in oriented thin films of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) by angle-resolved photoemission

Angle‐resolved ultraviolet photoemission spectra using synchrotron radiation were measured for oriented thin films of bis(1,2,5‐thiadiazolo)‐p‐quinobis(1,3‐dithiole) (BTQBT) on graphite. From the photon energy dependence of normal emission spectra, the energy‐band dispersion of π‐bands were observed for the highest (HOMO) and next highest (NHOMO) bands. This is the first observation of intermolecular dispersion in a single‐component organic molecular crystal. The results demonstrate that the BTQBT molecules have a strong intermolecular interaction, which can be derived from the introduction of a covalent interaction between sulfur atoms in addition to the usual intermolecular interaction by van der Waals forces.

Evolution of superconductivity of KxC60 upon K-doping : microwave low-field signal and ESR study

Abstract By means of low magnetic-field micowave absorption (Low-Field Signal) and conventional ESR, we have studied the evolution of superconductivity in bulk KxC60 dependent on the variation of K- doping temperature and time. The first LFS as an occurrence of superconductivity is observed in a doping time as short as td = 0.5 h with a discrete Tc ∼ 19 K, and the volume of the superconducting phase increases on further dopings, while Tc changes slightly. The sample in a superconducting state exhibits strong ESR absorptions, which indicates the presence of a non-superconducting metallic phase coexisting with a superconducting phase in inhomogeneously doped KxC60. These phases are closely correlated and disappear at the same time by overdoping.

Structure and superconductivity of single crystalline C60

Abstract We have succeeded in synthesis of C 60 single crsytals with orthorhombic lattice structure contrary to the known cubic form. The structure of the orthorhombic form was confirmed by a four-circle X-ray diffractometer. The intense microwave absorption signal in low magnetic field was detected on the K-doped C 60 single crystals with the onset of superconductivity at Tc = 17.7 K.

Synthesis, characterization and physical properties of incommensurate layered compounds/(RES)xTaS2 (RE=Rare earth metal)

Abstract Structures and physical properties have been investigated for a series of new ternary sulfides (RES) x TaS 2 (RE=Pr, Sm, Nd, Gd, Dy and Er) with an incommensurate RES-TaS 2 stacking structure. The lattice constants decrease monotonically with increasing the atomic number of rare earths, indicating that RE is trivalent in these compounds. An antiferromagnetic transition is found in (GdS) 1.20 TaS 2 and (DyS) 1.22 TaS 2 with Neel temperatures of 4.1 and 1.7K, respectively, which are strongly depressed in comparison with those of corresponding rare earth monosulfide. (SmS) 1.19 TaS 2 exhibits weak magnetic behavior indicating the divalent state of Sm, though trivalence is expected from the lattice constant.

New superconducting sodium-nitrogen-C60 ternary compound

Abstract We have prepared sodium-doped C60 utilizing the thermal decomposition of sodium azide (NaN3). The strong low-magnetic field microwave absorption with a clear hysteresis was observed in the samples of NanC60 (n=3∼4), indicating the superconducting nature. By means of a SQUID measurement, the superconducting transition temperature is found to be around 12 K. Significantly, elemental analysis detected a nitrogen which was surely produced in the process of the thermal decomposition of NaN3. We suppose that this new superconducting phase will be NaxNyC60 where x is between 3 and 4, and y is unknown. This is the first superconducting material in Na-doped C60 without other alkali metals.

Hall-effect observation in the new organic semiconductor bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole)(BTQBT)

Single crystals of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole)(BTQBT) were grown either by recrystallization from a nitrobenzene solution or by sublimation in nitrogen atmosphere The electrical resistivities were 1.2 × 103Ω cm and 2.7 × 105Ω cm at room temperature for a crystal grown by recrystallization and that obtained by sublimation, respectively. These values are remarkably low for a single-component organic crystal. In addition, the BTQBT crystals have a small anisotropy in resistivity (ρ⊥/ρ∥, ≈ 2), which is ascribed to strong intermolecular interactions inherent in the crystal structure. They show a Hall effect which is an unusual observation in organic semiconductors. The sign of carriers was determined to be positive and the Hall mobility was found to be ca. 4 cm2 s–1 V–1 at room temperature.

Fluorine-graphite HOPG intercalation compounds

Abstract Fluorine-graphite intercalation compounds C x F (15>×≥5.6) have been obtained by exposing HOPG to fluorine with available highest purity, at 50°C for periods exceeding 30 days. The identity period and the intercalant thickness of pure stage-2 compound C 6.4 F are, respectively, 9.46 and 2.76 A. In-plane electrical conductivities of C x F are ca. 8–10 times that of the pristine HOPG. The c-axis resistivity of the stage-2 compound vs. temperature shows a maximum at 194 K; metallic-like behavior at lower temperatures and semiconductor-like behavior at higher ones. The compound also shows an anomalous c-axis thermal expansion at 130 K and a drastic motional narrowing in the linewidth of 19 F-NMR at 150 K, suggesting a kind of in-plane phase transition of intercalated fluorine.

Microwave low-field absorption and ESR in Kx(C60)1-y(C70)y and KxC70

Abstract Superconductivity (SC) in K-doped molecular alloys of C 60 and C 70 are studied by means of the nonresonant microwave low-field signal (LFS) and conventional ESR, and a LFS-search for SC in K x C 70 is performed. The critical temperature of K x (C 60 /C 70 ) determined at the onset of hysteretic LFS, decreases to T c =16K (at about 10% C 70 ), and T c =10K (at about 25% C 70 ), compared with T c =19K in pure K x C 60 suggesting the decrease of electron-phonon coupling. A strong multicomponent g=2 ESR signal appears and coexists with LFS at low T for SC composition of K x (C 60 /C 70 ) indicating presence of a non-SC metallic phase. At overdoping LFS disappears and a single ESR line is enormously broadened to Δ H pp (300K) = 80G. In K x C 70 doped under the same conditions the LFS is not detected, indicating absence of SC above 5K, while a narrow ESR single line appears.

Microwave Spectroscopy of Fullerene-Based Molecular Superconductors in Low Magnetic Filed: Mx C60 (MK, Rb, I2, Ga, In), Mx Cn (n 70,76,78,84,90) and Mx(C60)1-y(C70)y(MK, Rb)

Abstract We have used the combined microwave spectroscopy: the nonresonant microwave absorption in low-magnetic field (so-called LFS) together with conventional ESR for the search and study of superconductivity (SC) in the new family of fullerene molecular solids: C60, C70, C76, C78, C84 C90, and their alloys ((C60)1-y(C70)y), doped by different metals (K, Rb, In, and Ga) and iodine I2. These experiments fall into two categories: 1) Search for SC in higher fullerenes and study of the evolution of SC properties upon doping of pure (one component) fullerenes including Cm thin films. 2) Investigation of the Tc change in molecular alloys of Mx (C60)1-y(C70)y type. In the first category of experiments we discuss the evolution of LFS and ESR spectra upon variation of Rb-doping conditions: temperatures of doping: Td (C60), Td(M), and doping time td, and as a result we find the optimal conditions for vapour phase Rb-doping. Here we demonstrate that SC Tc appears and disappears with doping abruptly at a discrete v...

Superconductivity in NaH intercalated C60

Abstract We have found a new superconducting (SC) phase with the nominal composition (NaH) 4 C 60 , which was prepared by direct intercalation of sodium hydride (NaH) in C 60 . In a SQUID magnetization measurement, the SC transition temperature was found to be 15 K and the SC volume fraction was estimated to be 65%. The large value of the volume fraction suggests the creation of a single phase. This was confirmed by the powder X-ray profile showing a single f.c.c. phase with a lattice constant a=14.356(3) A . Hydrogen in the SC sample was analyzed by thermal desorption spectroscopy.