2016 Fiscal Year Annual Research Report
Electron and phonon transport in single crystal tetragonal niobium bronzes
| Project/Area Number |
26400323
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| Research Institution | National Institute for Materials Science |
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Principal Investigator |
KOLODIAZHN Taras 国立研究開発法人物質・材料研究機構, 機能性材料研究拠点・強相関物質グループ, 主幹研究員 (80469767)
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| Project Period (FY) |
2014-04-01 – 2017-03-31
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| Keywords | Tetragonal bronze / thermal conductivity / thermoelectric power / metal-insulator / charge density wave / Anderson localization |
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| Outline of Annual Research Achievements |
Solid solution of Ba6-xSrxNb10O30 crystalizes in filled tetragonal tungsten bronze (TTB) structure described by a general formula A12A24B12B28C4O30. Our results indicate that the Ba and Sr ions show complete solid solution without miscibility gap. Some preferential occupancy of the A1 site with Sr and A2 site with Ba ion has been detected. Our most recent results indicate that the Sr end member show lower crystal symmetry than the rest of the Ba6-xSrxNb10O30 compounds. The Ba-end member crystallizes in the highest symmetry P4/mbm space group (a = b = 1.25842(18) nm and c = 0.39995(8) nm) and so do all the compositions with 0 < x < 5.
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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