2020 Fiscal Year Research-status Report
Development of new high Tc superconductors through internal and external dual-direction doping of carbon superatoms
| Project/Area Number |
18K18724
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| Research Institution | Osaka Prefecture University |
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Principal Investigator |
プラシデス コスマス 大阪府立大学, 工学(系)研究科(研究院), 教授 (90719006)
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| Project Period (FY) |
2018-06-29 – 2022-03-31
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| Keywords | Superconductivity / Molecular nanocarbons / Endohedral fullerene / Dual doping / Magnetism |
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| Outline of Annual Research Achievements |
Most superconductors have simple structures built from atoms, but superconductors made from molecules arranged in solid structures also exist.
Prominent examples are those of nanocarbon superatoms, the fullerenes (C60) - they show the highest superconducting transition temperature, Tc (38 K) and do not lose their zero resistance performance even under extremely high magnetic fields (>90 Tesla). However, they have now reached their upper limit of performance. In this research, we are attempting to remove this roadblock by using a new building block for molecular superconductors beyond the C60 nanocarbon molecule. This is [Li@C60], an endohedral metallofullerene, which incorporates a Li+ ion inside the C60- cage. We have now developed a scalable method to obtain neutral Li+@C60(-) by chemical reduction of Li+@C60 using decamethylferrocene. The preparative route does not demand long reaction times unlike electrolytic reduction routes. Investigation of solid [Li@C60] revealed the presence mainly of dimers (Li@C60)2, together with the co-existence of a small fraction of the EPR-active monomer form. These results added pieces of important information on the chemistry of the endohedral metal fulleride [Li@C60] as an emerging metal/carbon hybrid. At present, we have achieved a full structural characterization of the structural properties of the endohedral metallofullerene as a function of temperature down to liquid helium temperatures and as a function of pressure up to applied hydrostatic pressures of 12 GPa.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
Superconductors have no electrical resistance and carry electricity without losing energy. The development of new materials in order to achieve
transition temperatures to zero-resistance as high as possible is at the extreme forefront of current challenges in materials science. C60
superconductors played leading role in materials research in the last 30 years achieving a robust zero-resistance state at record temperatures and surviving at extremely high magnetic fields. But they have reached their upper limit. Here we are facing the challenge of surpassing the past performance of C60 superconductors. We are targeting to achieve this by developing the uncharted field of high-symmetry superatomic carbon frameworks with metal ions inside the cages and using unprecedented mechanisms of electronic control by dual-direction internal and external electron doping. This is a challenging proposal because there are simply no systems of this type created before and, if and when made, theory predicts superb performance. Currently we have achieved the first milestone of producing and characterizing in the bulk the parent neutral lithium endohedral C60 fullerene both as a function of temperature and pressure - this constitutes the starting material, the synthon of our eventual targets and confirms that we are progressing at an excellent pace for this research.
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| Strategy for Future Research Activity |
Our research plan follows two complementary procedures: (i) to develop the new synthetic chemistry needed, and (ii) to combine it with advanced structural and physical property measurements and feedback from theory. The research will include: [1] Synthesis of dual-direction-doped A+n[Li+@C60(n+1)-] phases (A = alkali metal; n = 1-6). This will define the full range of valences and electronic ground states in C60 cages dually-electron-doped internally and externally. [2] Physical control of structure and properties. Application of pressure will be used to drive insulator-to-metal transitions and trigger the emergence of superconductivity out of non-superconducting A+n[Li+@C60(n+1)-] precursors away from half filling of the conduction band. [3] Electronic and magnetic ground states in the new materials. The strong interplay between crystal and electronic structure requires the use of many advanced experimental techniques at both ambient and elevated pressures. We have the expertise to employ the full range of experimental techniques to investigate crystal structure (synchrotron X-ray & neutron diffraction), electronic
structure (magnetometry, transport properties, specific heat) and dynamics (NMR/muSR/EPR & IR/Raman spectroscopy) throughout the project duration. The integrated study of structure and electronic properties in the normal and superconducting states will be the basis for theoretical understanding of the new metallic/superconducting ground states.
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| Causes of Carryover |
The original targets of this research project were rapidly achieved and we were proceeding in targeting the availability of the endohedral metallofullerene materials in large-quantities to allow full exploratory synthesis. The next steps involved more elaborate structural and physical characterization as a function of temperature as well as the utilization of high pressure to enhance the properties. Progress along these lines was severely impeded by the coronavirus pandemic. We anticipate that the significant resources necessary to proceed with the implementation, including those carried over will allow completion of the project.
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