Shunji Takekawa

Stoichiometric Mg:LiNbO 3 as an effective material for nonlinear optics

Photorefractive damage, optical absorption, photoconductivities, and photogalvanic currents of stoichiometric LiNbO(3) single crystals with different Mg doping levels have been investigated. Nominally pure stoichiometric LiNbO(3) shows lower photorefractive damage resistance than congruent crystal; however, stoichiometric crystals doped with MgO of more than 1.8 mol. % exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 8 MW/cm(2) . This remarkable damage resistance can be attributed not only to increased photoconductivity but also to decreased photogalvanic current. Stoichiometric Mg:LiNbO(3) also demonstrates the shortest absorption edge, 302 nm, and a single-domain nature with low scattering losses.

Assignment of the Powder X-Ray Diffraction Pattern of Superconductor Bi2(Sr, Ca)3-xCu2Oy

The powder X-ray diffraction pattern of superconductor Bi2(Sr, Ca)3-xCu2Oy was assigned taking into account the satellite reflections arising from an incommensurate modulated structure. All reflections are indexable with h a*+k b*+l c*+m k, provided that the wave vector k of the modulated structure is b*/4.8. The result shows that many satellites can be observed and some satellite intensities are strong.

Single crystal growth of the superconductor Bi2.0(Bi0.2Sr1.8Ca1.0)Cu2.0O8

Abstract A single crystal of Bi 2.0 ( Bi 0.2 Sr 1.8 Ca 1.0 ) Cu 2.0 O 8 was grown by the floating zone method. This compound melts incongruently and the grown boule is an aggregate of large single crystal grains with growth direction perpendicular to [001]. The crystals tend to grow along the direction [100] and can be easily cleaved along the (001) plane. The grown crystal has orthorhombic symmetry with superstructure and the cell constants a , b and c of the sub-cell are 5.43, 5.43 and 30.63 A, respectively. This crystal reaches zero resistance state at 92 K.

Crystal chemistry of hexaaluminates: β-alumina and magnetoplumbite structures

Abstract Hexagonal aluminates are known to have a layer structure composed of spinel blocks and conduction layers which are stacked alternately. The structural parameters are influenced by the large cations in the conduction layer. Two typical types of hexagonal aluminates, β-alumina and magnetoplumbite, are studied and reviewed from this point of view. The conclusions are that the structure type of hexaaluminates is determined by the charge and radius of the large cations in the conduction layer, and that the conduction layer thickness decreases as the radii of the large cations in the conduction layer decreases and as the population increases. The spinel block thickness increases according to the increase in the amount of Al 3+ defect within the spinel block.

The correlation of MgO-doped near-stoichiometric LiNbO3 composition to the defect structure

Abstract Undoped and MgO-doped LiNbO 3 single crystals with near-stoichiometric compositions were grown from K 2 O-doped melts and Li-rich melts by two different growth techniques. Near-stoichiometric LiNbO 3 single crystals, having large [Li]/[Nb] ratios of 0.988–0.993, showed smaller threshold concentrations of MgO than congruent crystals for the behavior of Curie temperature, OH absorption band and optical damage properties. We discuss the ideal defect structure models, and find that the Li-site vacancy model is consistent with the experimental results for MgO-doped near-stoichiometric LiNbO 3 single crystals.

The crystal structure of barium hexaaluminate phase I (barium β-alumina)

Abstract The crystal structure of barium hexaaluminate phase I (Ba0.79Al10.9O17.14) was determined by single crystal X-ray reflection data. The refinements were carried out by the least-square method to give a final R-value of 0.023. The structure was revealed to be essentially of a β-alumina type and the Ba ion was detected only at the 6h site near the Beevers-Ross site (2d site). The charge compensation for nonstoichiometry was found to be principally effected by the interstitial oxygen due to Frenkel defects of Al ions. From the structural point of view, phase I was referred to as “barium β-alumina.”

Optical Damage Resistance and Refractive Indices in Near-Stoichiometric MgO-Doped LiNbO3

The optical damage resistances of near-stoichiometric LiNbO3 (LN) crystals doped with MgO were investigated at λ=532 nm. Their refractive indices were also measured in the wavelength range from 440 to 1050 nm, from which Sellmeiers equations were derived. While the congruent LN (CLN) crystal required 5 mol% MgO to suppress the optical damage, the near-stoichiometric LN (SLN) crystal doped with 1 mol% MgO showed no optical damage up to the input intensity of 2 MW/cm2. The ordinary refractive indices (no) of MgO-doped SLN crystals decreased with MgO doping concentration up to 4.6 mol%. In contrast, the extraordinary refractive indices (ne) of MgO-doped SLN crystals were found to be almost independent of MgO doping concentration up to 4.6 mol%. With respect to optical device design, it is advantageous that ne of MgO-doped SLN crystal is almost independent of MgO doping concentration up to 4.6 mol% and that the SLN crystal needs only 1 mol% MgO to suppress optical damage.

Magnetization and Spin Correlation of Two-Dimensional Triangular Antiferromagnet LuFe2O4

Magnetization, neutron diffraction and Mossbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.

The crystal structure of lanthanum hexaaluminate

Abstract The least-square refinement of lanthanum hexaaluminate (La0.827Al11.9O19.09) was accomplished using single crystal X-ray diffraction data. The result of the final anisotropic refinement, corresponding to an R-value of 0.039, revealed the structure of a magnetoplumbite type. In the structure interstitial Al ions were found, which were probably formed by a Frenkel defect mechanism. These interstitial Al ions are proposed to be situated in pairs making a bridge between spinel blocks, and to cause Al and La defects in the intermediate layer ( z ⋍ 0.25 ). The nonstoichiometry of lanthanum hexaaluminate is attributed to these defects.

Charge Frustration and Dielectric Dispersion in LuFe2O4

We report the dielectric anomaries at the phase transitions of Lu(Fe 2+ Fe 3+ )O 4 , which is considered as a charge and spin frustrated system. Dielectric constant shows a hump, just above the transition from the high temperature two-dimensional to the low temperature three-dimensional charge ordering. A few glassy behaviors were observed at this transition. The presence of spontaneous polarization in the low temperature phase was confirmed by the measurement of pyroelectricity. A step-wise increase of the dielectric constant with increasing temperature was observed between 150 K and 250 K as in other RFe 2 O 4 . These phenomena are explained by the presence of the polar domains with the ordering of Fe 2+ and Fe 3+ and the motions of the antiphase boundaries between them.