Kazuo Kawabe

Magnetoresistance Effect in p-Zn3As2 Single Crystals

Magnetoresistance effect in p -Zn 3 As 2 Single crystals with carrier concentration of ∼7×10 15 ∼10 18 /cm 3 was measured at low temperatures. The undoped crystals usually had ∼10 17 /cm 3 carriers and showed negative magnetoresistance which could be explained by the model of metallic impurity conduction. Specimens containing less carriers (∼7×10 15 /cm 3 ) showed positive magnetoresistance which was characterized by the intermediate impurity conduction. Specimens with p ∼3×10 16 /cm 3 showed magnetoresistance which could be explained by the additive contribution of the negative and positive effect. A possible model is proposed to explain these effects consistently.

Optical Behaviours of Multi-domain Single Crystal of BaTiO_3 in Dependence on Temperature

Apparent absorption edge, transmissivity and true characteristic absorption edge of multi-domain single crystal of barium titanate have been measured in dependence on temperature covering the three transition points, -70°C, 5°C and 120°C. The true characteristic absorption edge shifts linearly with the temperature coefficient quite close to that in ionic crystals which has so far been well known. Both the apparent absorption edge and the transmissivity exhibit quite steep hysteresis curves especially near -70°C. It may be due to the domain structure which is expected to develope when the lattice changes from orthorhombic to rhombohedral near this transition point.

Thermal Transition of Transparency in Ferroelectric Single Crystal of Barium Titanate

The ferroelectric single crystal of barium titanate exhibits a strong absorption in the ultra-violet region at any temperature including the three transition points. The threshold for the ultra-violet absorption shifts gradually towards longer wave-lengths as the temperature rises, and the temperature coefficient of it is about one Angstrom per degree centigrade. In addition to this ultra-violet absorption, the crystal exhibits another absorption in the visible region. The intensity of natural light with wave-lengths in the neighbourhood of 5000 A transmitted through the crystal is measured by means of a multiplier phototube. It shows quite steep and sudden changes near the transition temperatures, especially in thin crystals. Let I (T), I (O) and I (R) be the intensity of transmitted light in the tetragonal, orthorhombic and rhombohedral phases respectively. Then the following relations can be drawn out of the various curves for the thermal transition of transparency. They are \(I(\text{O}){\gtrsim}\frac...

Optical Studies on the Effect of Electric Fields on the Transitions of Barium Titanate

The effect of electric fields on the three transition points of barium titanate was studied experimentally by observing the shift of the curve of the thermal transition of transparency with varying biasing fields. Transparent electrodes, thin films of stannic oxide, were used in order to observe the intensity of light transmitted through crystals to which electric fields were applied. The Curie point is raised up to a critical temperature of 132°C, the required strength of the field being 9 kv/cm, and above this point the transition of the first order no longer takes place. From this fact, the coefficient of P z 4 in the Devonshire expression of the free-energy function is obtained, which supports the value measured by Drougard. The observed shifts of the Curie point and the 5°C transition point are in good agreement with the values calculated by using the Devonshire function of the sixth order. At the -80°C transition, the experimental value is explained by adding the terms of eighth and tenth powers of ...

A Twinning Structure in Crystals of Aniline Hydrobromide, C6 H5 NH3 Br

A new twinning structure of aniline hydrobromide was found by means of an x-ray study. The twin elements are two-fold screw axes parallel to [011] and [0\bar11] through the origin. This type of twinning can be seen, in some cases, by an microscopic observation, especially by etch patterns on a cleaved plane parallel to (100). The formation of the twin will probably due to the difference of the growth rates in different crystallographic directions.