| 研究課題/領域番号 |
26400323
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| 研究機関 | 国立研究開発法人物質・材料研究機構 |
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研究代表者 |
KOLODIAZHN Taras 国立研究開発法人物質・材料研究機構, 超伝導物性ユニット, 主幹研究員 (80469767)
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| 研究期間 (年度) |
2014-04-01 – 2017-03-31
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| キーワード | tetragonal bronze / thermal conductivity / thermoelectric power / metal insulator / charge density wave / Anderson localization / low-dimensional systems |
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| 研究実績の概要 |
Solid solution of Ba6-xSrxNb10O30 crystalizes in filled tetragonal tungsten bronze (TTB) structure in high-symmetry P4/mbm space group. Our results indicate that the Ba and Sr ions show complete solid solution without miscibility gap. Our most recent results indicate that the Sr end member show lower crystal symmetry than the rest of the Ba6-xSrxNb10O30 compounds.
We were able to prepare Sr6Nb10O30 end member single crystal from the melt. The crystal structure analysis of the single crystal indicated that it crystalize in the Amam space group with distorted TTB structure. We attribute these distortions to the small size of the Sr ion located in the pentagonal TTB channels. In particular, the Amam structure shows doubling of the unit cell along the c direction as well as distortions in the ab plane which lead to an increase in the a and b unit cell parameters.
Measurement of the electron transport of the Ba6-xSrxNb10O30 solid solution reveals an unusual metal to insulator (MIT) transition which occurs upon contraction of the unit cell. While the Ba-rich compounds are metallic the Sr-rich compounds show an insulating ground state. It is noteworthy that the MIT occurs at x = 3, where the glass-like low-temperature thermal conductivity starts to appear.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
The project is progressing as planned without any major delays.
While trying to explain this unusual MIT in Ba6-xSrxNb10O30 solid solution we considered several possible scenarios including symmetry-driven Mott transition, MIT driven by critical electron concentration, and finally, MIT driven by disorder/ random field Anderson localization. Based on the analysis of the experimental data supplemented by calculations of critical electron concentration, we come to conclusions that Anderson localization due to random electric fields is the main driving force for MIT in Ba6-xSrxNb10O30 solid solution. Another important finding of this project was the discovery of the low-temperature superconductivity at Tc < 1.6 K in the Ba-rich members of the Ba6-xSrxNb10O30 solid solution. Analysis of specific heat of the superconducting phase transition points to the conventional s-wave superconductivity in the title compounds with no evidence of the nodal lines or nodal points.
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| 今後の研究の推進方策 |
Because of the potential interest in the Ba6-xSrxNb10O30 compounds as high-temperature thermoelectrics, our main future efforts will be focused on the high-temperature thermal diffusivity and thermopower measurements. High-temperature specific heat will be measured and analyzed in view of the two high-temperature phase transitions observed in the Sr6Nb10O30 end member of this TTB series. Furthermore, the first-principles calculation of the effect of the structural distortions on the electron density of states has been initialized and will be completed by the end of the project. Some of the budget funds were transferred from FY2015 to FY2016 because the author could not attend the Fall meeting of the Materials Research Society for health reasons.
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| 次年度使用額が生じた理由 |
Some of the budget funds were transferred from FY2015 to FY2016 because the author could not attend the Fall meeting of the Materials Research Society for health reasons.
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| 次年度使用額の使用計画 |
The funds will be used to attend the Fall Meeting of European Society for Materials Science.
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