| 研究実績の概要 |
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). In this research, we are using a new building block for molecular superconductors beyond C60. This is [Li@C60], an endohedral metallofullerene, which incorporates a Li+ ion inside the C60- cage. We have developed a scalable method to obtain neutral Li+@C60(-) by chemical reduction of Li+@C60 using decamethylferrocene. 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. However, although this preparative route does not demand long reaction times, it leads to poorly crystalline materials. This is unlike electrolytic reduction routes, which afford very crystalline materials but in small quantities. Nonetheless this allows the in-depth structural characterization, which has unveiled a highly symmetric hexagonal crystal structure comprising disordered dimer units in analogy with (C59N)2 or molecular dihydrogen. To date, 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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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
1: 当初の計画以上に進展している
理由
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 a good pace for this research.
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| 今後の研究の推進方策 |
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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