Synthesis and phase control of fullerene based superstructured materials

在本财年，我们试图揭示富勒烯基化合物的普遍性和独特性，处理层状氮化物超导体，这种超导体也表现出带绝缘体到超导体的转变，类似于碱金属插层富勒化物的情况。我们成功地制备了一系列Li含量连续变化的样品(x&lt；0.12)，并在BL2B-02上用同步辐射X射线衍射技术证实了这些样品是单相的。这与富勒烯化合物的情况形成了鲜明的对比，富勒烯化合物只有线相而得不到中间相，清楚地表明了富勒烯化合物的反常固相化学。此外，我们还测量了层状氮化物超导体在磁场中的比热，发现正常态的索末菲常数γ很小(=1.0MJ/molK^2)。利用这个值和几个能带计算得到的费米能级的态密度，我们可以得到很小的电声子耦合常数λ(=0.22)。这一事实与在拉曼散射光谱中观察到的较窄的声子线是一致的。这一事实也与富勒烯分子的超导性形成了鲜明的对比，在富勒烯分子发生形变的地方，超导电性是由具有中等电子-声子耦合强度的高频声子介导的，这表明了Jahn-Teller相互作用的重要性。理论上指出与简并LUMO轨道耦合的Jahn-Teller相互作用在富勒化物中是重要的，我们的结果与此是一致的。

In this fiscal year, we have tried to reveal universality and unique points in fullerene based compounds, dealing with layered nitride superconductors that also exhibit band insulator to superconductor transition similarly to the case of alkali metal intercalated fullerides. We have succeeded in preparing a series of samples with continuously changed Li-content (x<0.12), and have confirmed that these are of single phase by using synchrotron X-ray diffraction technique at BL2B-02, SPring 8. This fact makes striking contrast with the case of fullerides where only line phases exist and the intermediate phases can not be obtained, clearly indicating the anomalous solid state chemistry of fullerene compounds. Also, we have performed the specific heat measurements of layered nitride superconductors in magnetic fields, and have found that the normal-state Sommerfeld constant γ is very small (=1.0 mJ/mol K^2). We can obtain very small electron-phonon coupling constant λ(=0.22) by using this value and the density of states at the Fermi level predicted by several band calculations. This fact is in accord with the relatively narrow phonon lines observed in Raman scattering spectra. This fact also makes striking contrast with the fullerides where superconductivity is mediated by high frequency phonon with intermediate electron-phonon coupling strength, indicating the importance of Jahn-Teller interaction where the fullerene molecules show deformation. It is theoretically pointed out that Jahn-Teller interaction that couples with the degenerate LUMO orbitals is important in fullerides, and our result is consistent with this.