Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK.
Nature. 2010 Jul 8;466(7303):221-5. doi: 10.1038/nature09120. Epub 2010 May 19.
The crystal structure of a solid controls the interactions between the electronically active units and thus its electronic properties. In the high-temperature superconducting copper oxides, only one spatial arrangement of the electronically active Cu(2+) units-a two-dimensional square lattice-is available to study the competition between the cooperative electronic states of magnetic order and superconductivity. Crystals of the spherical molecular C(60)(3-) anion support both superconductivity and magnetism but can consist of fundamentally distinct three-dimensional arrangements of the anions. Superconductivity in the A(3)C(60) (A = alkali metal) fullerides has been exclusively associated with face-centred cubic (f.c.c.) packing of C(60)(3-) (refs 2, 3), but recently the most expanded (and thus having the highest superconducting transition temperature, T(c); ref. 4) composition Cs(3)C(60) has been isolated as a body-centred cubic (b.c.c.) packing, which supports both superconductivity and magnetic order. Here we isolate the f.c.c. polymorph of Cs(3)C(60) to show how the spatial arrangement of the electronically active units controls the competing superconducting and magnetic electronic ground states. Unlike all the other f.c.c. A(3)C(60) fullerides, f.c.c. Cs(3)C(60) is not a superconductor but a magnetic insulator at ambient pressure, and becomes superconducting under pressure. The magnetic ordering occurs at an order of magnitude lower temperature in the geometrically frustrated f.c.c. polymorph (Néel temperature T(N) = 2.2 K) than in the b.c.c.-based packing (T(N) = 46 K). The different lattice packings of C(60)(3-) change T(c) from 38 K in b.c.c. Cs(3)C(60) to 35 K in f.c.c. Cs(3)C(60) (the highest found in the f.c.c. A(3)C(60) family). The existence of two superconducting packings of the same electronically active unit reveals that T(c) scales universally in a structure-independent dome-like relationship with proximity to the Mott metal-insulator transition, which is governed by the role of electron correlations characteristic of high-temperature superconducting materials other than fullerides.
晶体结构控制着电子活性单元之间的相互作用，从而决定了其电子性质。在高温超导铜氧化物中，只有一种电子活性 Cu(2+)单元的空间排列方式——二维正方形晶格，可用于研究磁有序和超导性之间的协同电子态竞争。具有超导性和磁性的 C(60)(3-)阴离子的球形分子晶体可以由阴离子的基本不同的三维排列方式组成。在 A(3)C(60)（A = 碱金属）富勒烯中，超导性仅与 C(60)(3-)的面心立方（f.c.c.）堆积有关（参考文献 2、3），但最近具有最高超导转变温度 T(c)的最扩展（参考文献 4）的组成 Cs(3)C(60)已被分离为体心立方（b.c.c.）堆积，它既支持超导性又支持磁性有序。在这里，我们分离出 Cs(3)C(60)的 f.c.c.多晶型物，以展示电子活性单元的空间排列如何控制竞争的超导和磁性电子基态。与所有其他 f.c.c. A(3)C(60)富勒烯不同，f.c.c. Cs(3)C(60)在常压下不是超导体，而是顺磁绝缘体，在压力下变成超导体。在几何上受挫的 f.c.c.多晶型物中（Néel 温度 T(N) = 2.2 K），磁有序发生的温度比基于 b.c.c.的堆积（T(N) = 46 K）低一个数量级。C(60)(3-)的不同晶格堆积方式将 b.c.c. Cs(3)C(60)中的 T(c)从 38 K 改变为 f.c.c. Cs(3)C(60)中的 35 K（在 f.c.c. A(3)C(60)家族中发现的最高值）。相同电子活性单元的两种超导堆积方式的存在表明，T(c)普遍以与 Mott 金属-绝缘体转变接近的结构无关的圆顶状关系缩放，这由除富勒烯以外的高温超导材料的电子相关作用的特征决定。
