Ichiroh Nakada

The Effects of Temperature and Electric Field for the Photo-Generation of Free Carriers in Anthracene

In order to study the generation of free carriers in an anthracene crystal, the measurement of the pulsed photoconduction has been performed. The number of free holes and electrons produced in a crystal after a flash illumination has been found to depend both on the temperature and the electric field. The number of generated holes increases exponentially with the rise of the temperature. At the higher temperatures it shows a tendency of saturation. The temperature dependency is not so remarkable for electrons. The number of holes and electrons increases with the field strength; yet in the higher field region the tendency of the saturation appears. The mobility of holes at the room temperature is 0.8 cm 2 /V.s and decreases as the temperature is raised. The mobility of electrons at the room temperature is 0.4 cm 2 /V.s and its variation with temperature is small. A model of energy levels is proposed for the carrier generation.

Superconductivity of Y1Ba2(Cu1-xFex)3Oy

A systematic study on the superconductivity of Y1Ba2(Cu1-xFex)3Oy has been made down to 30 mK for various x values. The critical temperature decreases linearly with x. The complete Meissner effect is retained down to 30 mK, up to about x=0.10. The crystal structure of the superconducting phase was found to transform from the orthorhombic to the tetragonal between x=0.02 and 0.05. It is concluded that a branching effect of a one-dimensional chain of Cu1-O through Fe impurities is essential for this structure transition. The supercurrent is considered to flow on the two-dimensional sheets of Cu2.

The Optical Absorption of Anthracene Crystal near 4000 A

The absorption spectrum of anthracene single crystal was measured in the range from 3950 A to 4400 A, i.e., the long wave-length tail of A 1 g – B 2 u transition. For the change of temperature from -180°C to 80°C, the absorption increases markedly both for a- and b-polarized light. The contour in the absorption tail can be separated into two parts. The one is the absorption obeyed to the Urbach rule, and the other is the absorption due to the transition from the hot level in the ground state to the first excited state. In the former absorption at a temperature Range above -80°C, the constant values of σ in the Urbach formula are obtained; 1.68 in a-polarized light and 0.88 in b-polarized light. In the latter absorption, the peaks at 4125 A and 4145 A were observed with a- and b-polarized light, respectively. The common thermal activation energy of 0.16 eV is obtained.

Two Crystal Phases in the Superconducting Ba-Y-Cu-O System and Their Reactivity to Water

Crystals with a stoichiometry either Ba2YCu3-xO7-y or Ba4YCu2O8-y are a high tc superconductor. The former is well known. With respect to the latter the crystal structure was determined to be tetragonal with lattice parameters a=5.78 A and c=7.99 A. It was found that both of them reacted with water. The former was slowly decomposed by warmed water at about 35°C, while the latter was destroyed at room temperature in an instant.

The Trapping of Photo-Carriers in Anthracene Crystals by Tetracene

The trapping of free electrons and holes in anthracene single crystals doped with tetracene has been measured by the pulsed photoconduction method. Tetracene behaves as a shallow hole trap located 0.40 eV ±0.02 eV above the hole band. At the same time it also acts both as a shallow and a deep electron traps. The shallow electron trap is located at 0.17±0.02 eV below the electron band. Both for electrons and holes the capture cross-section of shallow traps is proportional to the tetracene content. The quantum efficiency for photo-carrier generation in anthracene does not change by the existence of tetracene which is a remarkable quencher for photo-fluorescence. From this result it is deduced that the photo-carriers are generated by multi-exciton processes, and that the generation of carriers at the surface interacting with diffusing-out singlet excitons makes negligible contribution for photoconduction.

Crystal Structure of Superconductor Ba2YCu2.89O6.80

Crystal structure of Ba2YCu2.89O6.80 sigle crystal was determined by the X-ray diffraction method at room temperature. The space group is Pmmm with lattice parameters a = 3.8699(8) A, b = 3.8667(8) A and c = 11.688(2) A. A specific structure was found in the c-plane sandwiched between two barium layers. The Cu-O bond perpendicular to the plane 1.828(6) A is shorter than the normal bond 1.95 A. Within the plane oxygen atoms show an extremely large thermal vibration.

Exciton Dynamics in Isotopically Mixed Crystals of Anthracene

The amalgamation type 0–0 excitons in the isotopically mixed crystals of normal and deuterated anthracene are investigated by simultaneous measurement of reflection, fluorescence, and pre-resonant Raman spectra. The observed features of the bulk and surface excitons are interpreted in terms of the theoretical band structure for the vibronic excitons calculated on the basis of the dynamical coherent potential approximation. The effect of cluster formation on the dynamical properties of excitons in these mixed crystals is also discussed by the Bethe-Peierls approximation.

Preparation of Some Chalcogenide Spinel Single Crystals and Their Electronic Properties

Chalcogenide spinel single crystals such as FeCr2S4, CoCr2S4 and some mixed crystals were grown by the chemical vapour deposition method with CrCl3 as a transport agent. The surface microstructure was observed under an interference contrast microscope and a Michelson interference microscope. It was found that the (111) face of FeCr2S4 single crystal was covered with a group of steps 7.6±0.7 A high. This is a half-elementary step in the direction. This gives an evidence that the crystals has grown by the spiral growth mechanism by Burton-Cabrera-Frank theory. Resistivity, Hall effect and thermoelectric power were measured with p-type FeCr2S4 and n-type FeCr2S4 substituted by Cu and an energy level scheme for crystalline FeCr2S4 was proposed. Between the conduction band and the valence band, from lower to higher, there exist Fe2+-band, Cr2+-level and S-valence level.

Superconductivity of Y1Ba2(Cu1-xAlx)3Oy and Y1Ba2(Cu1-xZnx)3Oy

The superconducting transition temperatures and the lattice parameters of Y 1 Ba 2 (Cu 1- x Al x ) 3 O y and Y 1 Ba 2 (Cu 1- x Zn x ) 3 O y compounds were measured. The crystal symmetry was found to transform from the orthorhombic form to the tetragonal in the Al compound when x is increased, while the orthorhombic structure is maintained in the Zn compound.

Unit cell step structures on vapour-grown Cr2S3 single crystals

Abstract Step structures on the (001) plane of as-grown crystalline Cr 2 S 3 that had been prepared by the chemical vapour deposition method have been observed by interference contrast microscopy. Two kinds of steps, with heights of 10.1 and 5.1 A were found. These are respectively due to the unit cell and a half of the unit cell of the crystal. Unit cell steps were seen to occur successively from a screw dislocation with Burgers vector normal to the surface equal to half of a unit cell. The generated steps with height equal to a half of the unit cell coagulated soon into a step with height of one unit cell. By passing other screw dislocations on the surface the unit cell steps combine with them just as expected from the screw dislocation model for monomolecular steps. The unit cell steps split spontaneously into half unit cell steps forming a fork-like step structure. The successively drawn out forking structure weaves an interlaced step pattern on the surface. However, the two interlaced steps annihilate when they meet.