Eriko Ohshima

Generation of terahertz radiation using zinc oxide as photoconductive material excited by ultraviolet pulses

Terahertz (THz) radiation generated from photoconductive antenna fabricated on a single crystal zinc oxide (ZnO) is presented. The THz-radiation power is saturated at bias voltages above 800V∕cm and the obtained spectrum extends up to 1 THz. Moreover, ZnO is found to be highly transparent in the visible, near-infrared, mid-infrared and THz frequency regions. The results depicted here will categorically unravel the prospects of using ZnO as a material for integrated active optics.

Structure analysis of oxygen-deficient TlSr2CuOy by neutron diffraction and high-resolution electron microscopy

Abstract The crystal structure of orthorhombic TlSr 2 CuO y was determined by Rietveld analysis of neutron powder diffraction data with the aid of electron diffraction and high-resolution electron microscopy. We found that oxygen vacancies are formed along the a -direction in a CuO 2− z sheet, which is consistent with a superstructure of twice the subcell along the b -direction. CuO 6 octahedra and CuO 4 square planes alternate along the b -direction by sharing their corners in accordance with the ordered oxygen vacancies in the CuO 2− z sheet. An apical oxygen atom in the CuO 6 octahedron is nearer to a TlO layer than an oxygen atom shared by a TlO 6 octahedron and the CuO 4 plane. A crystal-structure image obtained by high-resolution electron microscopy could be successfully reproduced by computer simulation using structure parameters obtained by neutron powder diffraction.

High-pressure phases of V2O5 : an application of an in situ X-ray observation method to high-pressure synthesis of materials

The high-pressure and high-temperature behavior of V 2 O 5 with a layered-orthorhombic structure was studied by both in-situ X-ray diffraction observations and recovery experiments. Two quenchable high-pressure phases (HPP1 and HPP2) were observed in the pressure and temperature range up to 11 GPa and 700°C. The phase transition to the HPP1 was observed by elevating the temperature above 300°C in the pressure range up to 7GPa. The HPP1 was found to have an orthorhombic cell (a = 7.100(7) A, b = 6.294(5) A and c = 3.578(2) A at 0.1 MPa and 27°C). The phase transition to the HPP2 was also observed in an elevating temperature process above 9 GPa. Unit cell parameters of the HPP2 have not yet been determined.

Structural properties of the copper oxide carbonate Ba4CaCu2O6+δCO3 (δ≃0)

Abstract The structure parameters of Ba4CaCu2O6 + δCO3 were refined by Rietveld analysis of angle-dispersive neutron powder diffraction data. The stoichiometric metal composition was confirmed by X-ray microanalysis. Iodometry showed that the amount of excess oxygen, δ, was nearly zero. The carbon content and infrared absorption spectrum of the compound gave evidence for the inclusion of CO32− ions in its crystal lattice. Ba4CaCu2O6CO3 is tetragonal with the P4/mmm space group and lattice parameters of a = 5.7879(2) A and c = 8.1409(3) A . Ba atoms occupy positions corresponding to the A site in ABO3 whereas Ca, Cu and C atoms are located at positions corresponding to the B site. Two copper sites are contained in the structure of Ba4CaCu2O6CO3: Cu(1) and Cu(2). If weak bonds between the Cu atoms and O atoms in the CO32− ion are neglected, Cu(1) and Cu(2) are taken as coordinated to four O(1) atoms and two O(2) atoms, respectively. Bond-valence sum calculations showed that the oxidation state of Cu is 2.48 for Cu(1) and 1.66 for Cu(2). The splitting and large thermal parameters of O atoms contained in the CO32− ion revealed its highly disordered orientation.

Synthesis and structure of Ba4CaCu2Oy(CO3)

Abstract New materials with the chemical composition of Ba 4 CaCu 2 O y (CO 3 ) were synthesized. Two tetragonal polymorphs with different oxygen content were obtained by furnace cooling and quenching to room temperature. The proposed crystal structure model similar to the previously reported Greaves phase of Ba 4 CaCu 2.24 O 6.96 (CO 3 ) 0.5 was refined by X-ray Rietveld analysis.

Synthesis and superconductivity of oxycarbonates of the Tl-1201 phase

Abstract TlBa x Sr 4- x Cu 2 (CO 3 )O y oxycarbonates with the Tl-1201 structure were synthesized over a wide range of x (1.0≤ x ≤3.2) and examined by X-ray powder diffraction, electron diffraction, high-resolution electron microscopy and magnetometry methods. New phases with the Tl-1201 structure having 6 and 7 times periodicities along the b -axis were isolated. The former was stable for a wide Ba-rich range (1.8≤ x ≤3.2), and the latter was observed for a smaller Ba content range (1.2≤ x ≤1.6). The phase stability was found to depend on the synthesis condition. The maximum T c of the as-synthesized specimen in this series occurred at about 70 K around x =2.0. The Meissner volume fraction of the as-synthesized specimen ( x =2.4) was 30% at 5 K.

Synthesis of oxycarbonate solid solutions (Tl1−xHgx)Sr2Ba2Cu2(CO3) (x=0−1.0) and superconductivity

Abstract Solid solution superconductors (Tl 1− x Hg x )Sr 2 Ba 2 Cu 2 (CO 3 )O y were synthetized for x =0−1.0. The crystal structures, especially its superstructure and superconducting properties, were examined by X-ray powder diffraction, electron microscopy, resistivity and susceptibility measurements. The three different crystallographic phases have been observed; Phases I and II have an 8 times commensurate and 7.5 times incommensurate crystal structure and their propagation vector is along [010]. Phase III has a 4.7-4.5 times incommensurate structure along the [110] direction. The discontinuous change at about x =0.7−0.8 is due to the oxygen deficiency with increasing Hg content T c also suddenly dropped from 70 to 60 K at about x =0.7−0.8.

Structural disorder and charge transfer in the superconductor (Pb0.5Cu0.5)(Sr0.5La0.5)2CuO5 + δ

Abstract Two samples of (Pb 0.5 Cu 0.5 )(Sr 0.5 La 0.5 ) 2 CuO 5 + δ with different oxygen contents ( δ = 0.03 and 0.13) showed superconducting transition temperatures somewhat different from each other: 24 K for the fully oxidized sample and 35 K for the deoxidized sample. The structural parameters of these samples were refined by the Rietveld analysis of neutron powder diffraction data to clarify the structure/property relationship in this nonstoichiometric oxide. In the oxidized sample, excess oxygen partially occupies an interstitial site at (0.151, 1 2 , 0) in a (Pb 0.5 Cu 0.5 )O 1 + δ layer, whereas this site proved to be vacant in the deoxidized sample. All the atoms on this layer are displaced from ideal rock-salt-type positions to achieve more favorable bonding environments. The length of a CuO bond on the CuO 2 conduction sheet is decreased from 1.8985 to 1.8915 A upon oxygen removal. This finding provides unambiguous evidence for the idea that the hole concentration on the CuO 2 sheet increases with releasing excess oxygen from the (Pb 0.5 Cu 0.5 )O 1 + δ layer.

Superconductivity due to charge transfer to the CuO2 sheet from the (Tl,Pb) O layer in Tl0.5Pb0.5Sr2−xNdxCuO5−δ

Abstract The superconductivity of Tl 0.5 Pb 0.5 Sr 2− x Nd x CuO 5−δ was observed for x ≥ 0.3, although the calculated Cu valence assuming Tl 3+ and Pb 4+ is slightly above or below 2.0. The binding energy of the Tl and Pb 4f core levels measured by XPS shifted to higher energy than those of Tl 3+ and Pb 4+ in the reference materials, showing that the Tl and Pb valences are lower than 3 + and 4 +, respectively. These observations strongly suggest that holes in the CuO 2 sheet are created by charge transfer from the (Tl,Pb)O layer, similar to the double-layered TlBa cuprates. In contrast, the temperature variation of electrical conductivity of Tl 0.5 Pb 0.5 Sr 2 CuO 5−δ changed from metallic to semiconducting with increasing oxygen deficiency, δ, and no superconductivity was observed through any control of δ. Although XPS measurement also suggested that Tl and Pb valences lowered with increasing oxygen deficiency, δ, the reason why the system did not show superconductivity can be understood by the depletion of oxygen from the CuO 2 plane during deoxygenation.

Valence analysis of Pb and Cu and superconductivity of (Pb,Cu)(Sr,La)2CuOy

Abstract The Pb 1201 phases, Pb 0.5 Cu 0.5 Sr 2− x La x CuO y , with x = 1.0–1.2 and Pb 0.6 Cu 0.4 Sr 2− x La x CuO y , with x = 0.9–1.2 have been prepared at 1010–1015°C for 1.7–2.0 hours in flowing oxygen. To evaluate the Cu valence in the CuO 2 sheet, the Pb valence is separately determined by K 2 Cr 2 O 7 titration, in addition to the oxygen content determination by iodometry. The T c (onset) rises as the oxygen content decreases due to release of the overdoping, but was not affected by La 3+ substitution for Sr 2+ and by the Pb content in the (Pb,Cu)O layer. The explanation is that the Cu valence in the CuO 2 sheet decreases as the oxygen content decreases, while the Pb valence is reduced by La 3+ substitution or the Pb content increases.