Presentations

単位格子中にペロブスカイト関連キャリアブロック層を有する、混合アニオン化合物 Sr2ScMPO3 (M = Mn, Ni)を新規に合成し、結晶構造、電気的性質、熱起電力を明らかにした。Sr2ScMPO3 (M = Mn, Ni)は、いずれも正方晶であり、M = Mnの場合は、絶縁的、Niの場合は金属的な電気的性質を示す。M = Niの20-300 Kでの熱起電力の大きさは、|10| VK1以下である。

GdFeAsO1-xFxの結晶構造、及び電気的、磁気的性質を、フッ素濃度を体積べガード則を基準として定量しその電子磁気状態を明らかにした。GdFeAsOの電気抵抗率は、150 K 付近(Tanom)で温度の低下とともに、急激に減少し、低温で残留抵抗を示す。一方、フッ素を配合した試料は最高で47 K付近の超伝導転移を示した。57Feメスバウワ分光により母相の反強磁性相を低温で確認した。これらの結果は先行研究と矛盾しない。

Wurtzite boron nitride (w-BN) and cubic boron nitride (c-BN) have interested researchers because of their high hardness and chemical inertia to ferrous materials. Moreover, recent theoretical calculations verify that w-BN is harder than diamond [1]. However, since w-BN phase exists in the low-temperature and high-pressure range between h-BN and c-BN phases [2,3], it was difficult to obtain pure w-BN excluding h-BN and c-BN as second phase. We demonstrate an optimal temperature to obtain single phase w-BN at relatively low pressure (around 10 GPa) by in situ Energy Dispersive X-ray diffraction (EDXRD) under HPHT at the synchrotron radiation facility, SPring-8. w-BN appears under 10(1) GPa and an optimal temperature to obtain w-BN single-phase exists at 1000 C, and h-BN and c-BN are mixed under and over this temperature. [1] Zicheng Pan, Hong Sun, Yi Zhang and Changfeng Chen, Phys. Rev. Lett. 102, 055503 (2009). [2] F. P. Bundy and R. H. Wentorf, Jr., J. Chem. Phys. 38, 1144 (1963). [3] T. Taniguchi, Special Issue of The Review of High Pressure Science and Technology. 18, 34 (2008). (In Japanese.)

SmFeAsO1-xFx, SmFeAs1-x PxO1-yFy (Sm1111)の多結晶試料について, magnetic Jcを拡張Beanモデルを用いて求め, 4端子法により測定される電圧降下の閾値を定義して求まるtransport Jcと比較した同一試料の磁束密度分布をSQUID顕微鏡により直接観察し, 多結晶体における磁場侵入長の算出を試みた. 5 Kにおけるmagnetic JcはSmFeAsO1-xFx (x = 0.082) が〜13kA cm-2, SmFeAs1-xPxO1-yFy (x=0.02, y = 0.10)が〜12 kA cm-2であった. 多結晶試料の磁場侵入長をLondonの理論[1]を仮定し, 算出すると単結晶等で報告されるIntrinsicな値[2]に比べ数桁大きい値を示した.学会では, この値を多結晶の結晶粒界の影響を考慮し解釈する.

これまでに報告された鉄系高温超伝導体の内,は最も高い超伝導転移温度、上部臨界磁場、及び不可逆磁場を示すのはいずれも混合アニオン化合物である。しかしながら、これらの混合アニオン化合物を利用した超伝導線材の臨界電流密度は、ThCr2Si2構造を示すBa1-xKxFe2As2を初めとするいわゆる122系に比べて, 数桁低い値にとどまる。鉄系超伝導体の歴史と現状を紹介し、混合アニオン化合物の電子磁気相図の重要性と、今後の展望を概説する。

Introduction In 2004, the boron-doped diamond (BDD) was found to be a superconductor [1]. On the other hand, it is known that diamond surface is easily modified by chemical treatment [2], and physical properties, such as surface conductivity, could be modulated by the surface modification [3]. Along these lines, it is expected that superconducting properties of BDD could be modulated by surface modification. Experimental Section The as-deposited BDD was hydrogenated, and then oxidized electrochemically to obtain the oxygen-terminated BDD. Thus obtained oxidized BDD was re-hydrogenated to obtain the hydrogen-terminated BDD. For each termination, superconducting properties were examined with transport and magnetic measurements. Results and Discussion In the transport measurements, although the critical temperature unchanged, critical current density (JC) decreased after oxidation and recovered after hydrogenation. In the magnetic measurements, the JC were determined from M-H curves using the extended Bean model. As is the case with the transport measurements, the JC could be modulated in a reversible manner between the hydrogen- and oxygen-terminated BDD (Fig.). This is because the volume fractions of the superconducting phases changed by surface modification. References [1] Ekimov, E. A. et al. Nature 2004, 428, 542. [2] Szunerits, S. et al. J. Solid State Electrochem. 2008, 12, 1205. [3] Maier, F. et al. Phys. Rev. Lett. 2000, 85, 3472.

Ternary or quaternary compounds Cu2–TM–A–Ch4 (TM; transition metal, A; group 14 or 15 elements, Ch; chalcogen) are promising p-type thermoelectric materials because of their heavy but still conducting valence band, which is composed of Cu 3d and Ch 3p orbitals. Lattice thermal conductivity should be suppressed by cation disorder, however, coexistence of transition metals such as Cu and Zn on quaternary compounds complicate the understanding of the details of cation disorder by means of conventional X-ray diffraction. In this work, We demonstrate the crystal structure and thermoelectric properties of kuramite Cu3Sn(Se,S)4. Structural analysis revealed that polycrystalline samples crystallize in tetragonal I–42m space group. The 2a site was occupied by Cu only, while 2b and 4d sites were occupied by Cu and Sn partial disorder. A decrease of electrical conductivity (rho) with temperature indicates that kuramite is metal or degenerate semiconductor, which is in contrast to most of other stannite–type compounds with band gap of 0.96–3.25 eV. At the conference, we will also report the alloy effect on thermal conductivity of Cu3Sn(Se,S)4 solid solution.

Ae2MFePnO3 (so-called 21113 systems) with perovskite-type layers such as Ae2MO3, where Ae denotes alkaline earth metals, M does Sc, Ti, Cr, V and other transition metal atoms and Pn does As and P shows superconductivity at T < 46 K. Sr2VFeAsO3-d is a representative compound in 21113 systems. [Zhu et al, Phys. Rev. B (2009); Ogino et al, Supercond. Sci. Technol. (2009); Ogino et al, Supercond. Sci. Technol. (2009)] Although the oxygen deficiency (d) as a function of Tc is still controversial in Sr2VFeAsO3-d, many samples have been reported as superconductors with Tc = 24-37 K. In this study, a polycrystalline Sr2VFeAsO3-d (d = ~0.1- 0.6) were prepared by a solid state reaction using an alumina tube in a sealed silica tube. DC electrical resistivity was measured by a four-probe technique. Magnetization measurements were performed on a superconducting quantum interference device. 57Fe Mossbauer spectra were obtained using conventional equipment. The electronic and magnetic phase diagram of Sr2VFeAsO3-d is elucidated.

クロムイオンによる正方格子を有するLnCr2Si2Cは金属的な電気的性質を示す金属間化合物である。アーク溶融による試料合成を行い、それぞれの組成の炭素(C)欠陥による磁性の変化、Siサイトへのホウ素(B)ドープによる磁性の変化を調べ、Siの5 at%程度Bに置換したSmCr2Si2Cは4 Kの低温で遍歴強磁性を示すことがわかった。

単位格子中にペロブスカイト構造酸化物を有し、異方性の非常に大きい鉄系超伝導体Sr2VFeAsO3-dの磁性、電気的性質の酸素欠陥との相関しらべ、超伝導相と磁性相は非共存であることを明らかにした。d = 0.5 付近では鉄は自発磁化を示し、Spin density waveとして振る舞う。またFeの磁性相と超伝導相は非共存である。

二次元近藤格子を有するCeFePOのFeサイトにRuとCrを部分置換することで、ゼロ磁場下では常磁性を示すCeFePOをTc = 8-15 Kの強磁性体に転移させることに成功した。すなわち、二次元近藤格子の量子相転移は、5-20at%程度のRuドープ/Crドープのそれぞれで引き起こすことができる。

鉄系超伝導体の基本構造であるZrCuAsSi型構造 (いわゆる1111構造)を示す高温超伝導体の内、未だに電子磁気状態相図の不明なGdFeAsO1-xFxの超伝導転移温度のフッ素濃度依存性を明らかにした。超伝導転移温度(Tc)は x > 0.65付近より現れる。Tcはフッ素濃度とともに上昇し、そのTcは48 K付近に上昇する。