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翻译了一些富勒烯超导的维基条目

已有 2452 次阅读 2011-5-9 20:54 |个人分类:格物|系统分类:科研笔记

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英文地址: http://en.wikipedia.org/wiki/Fullerene#Superconductivity

中文地址:http://zh.wikipedia.org/wiki/%E5%AF%8C%E5%8B%92%E7%83%AF#.E5.AF.BC.E7.94.B5.E6.80.A7

 

The increase of transition temperature with the unit-cell volume had been believed to be evidence for the BCS mechanism of C60 solid superconductivity, because inter C60 separation can be related to an increase in the density of states on the Fermi level, N(εF). Therefore, there have been many efforts to increase the interfullerene separation, in particular, intercalating neutral molecules into the A3C60 lattice to increase the interfullerene spacing while the valence of C60 is kept unchanged. However, this ammoniation technique has revealed a new aspect of fullerene intercalation compounds: the Mott-Hubbard transition and the correlation between the orientation/orbital order of C60 molecules and the magnetic structure.[61]

 

C60固体超导性的[[BCS理论]]认为,[[超导转变温度]]随着[[晶胞体积]]的增加而升高,因为C60分子间的间隔与[[费米能级]]N(εF)的态密度的升高相关,因此科学家们做了大量的工作试图增加富勒烯分子间的距离,尤其是将中性分子插入A3C60[[晶格]]中来增加间距同时保持C60的价态不变。不过,这种氨化技术意外地得到了新奇的富勒烯插入复合物的特别的性质:[[Mott-Hubbard转变]]以及C60分子的取向/轨道有序和磁结构的关系。

The C60 molecules compose a solid of weakly bound molecules. The fullerites are therefore molecular solids, in which the molecular properties still survive. The discrete levels of a free C60 molecule are only weakly broadened in the solid, which leads to a set of essentially nonoverlapping bands with a narrow width of about 0.5 eV.[53] For an undoped C60 solid, the 5-fold hu band is the HOMO level, and the 3-fold t1u band is the empty LUMO level, and this system is a band insulator. But when the C60solid is doped with metal atoms, the metal atoms give electrons to the t1u band or the upper 3-fold t1g band.[62] This partial electron occupation of the band leads to sometimes metallic behavior. However, A4C60 is an insulator, although the t1u band is only partially filled and it should be a metal according to band theory.[63] This unpredicted behavior may be explained by theJahn-Teller effect, where spontaneous deformations of high-symmetry molecules induce the splitting of degenerate levels to gain the electronic energy. The Jahn-Teller type electron-phonon interaction is strong enough in C60 solids to destroy the band picture for particular valence states.[61]

C60固体是由弱相互作用力组成的,因此是分子固体,并且保留了分子的性质。一个自由的C60分子的分立能级在固体中只是很弱的弥散,导致固体中非重叠的带间隙很窄,只有0.5eV。未掺杂的C60固体,5hu带是其HOMO能级,3倍的t1u带是其空的LUMO能级,这个系统是带禁阻的。但是当C60固体被金属原子掺杂时,金属原子会给t1u带电子或是3倍的t1g带。带的部分电子占据有时会呈现金属性质。虽然它的t1u带是部分占据的,按照BCS理论A4C60t1u带是部分占据的应该有金属性质,但是它是一个绝缘体,这个矛盾可能用Jahn-Teller效应来解释,高对称分子的自发变形导致了它的兼并轨道的分裂从而得到了电子能量。这种Jahn-Teller型的电子-声子作用在C60固体中非常强以致于可以破坏了特定价态的价带图案。

A narrow band or strongly correlated electronic system and degenerated ground states are important points to understand in explaining superconductivity in fullerene solids. When the inter-electron repulsion U is greater than the bandwidth, an insulating localized electron ground state is produced in the simple Mott-Hubbard model. This explains the absence of superconductivity at ambient pressure in caesium-doped C60 solids.[59] Electron-correlation-driven localization of the t1uelectrons exceeds the critical value, leading to the Mott insulator. The application of high pressure decreases the interfullerene spacing, therefore caesium-doped C60 solids turn to metallic and superconducting.

窄带隙或强电子相互作用以及简并的基态对于理解并解释富勒烯固体的超导性非常重要。电子相互斥力比带宽大时,简单的Mott-Hubbard模型会产生绝缘的局域电子基态,这就解释了常压时铯掺杂的C60固体是没有超导性的。电子相互作用驱动的t1u电子的局域超过了临界点会生成Mott绝缘体,而使用高压能减小富勒烯相互间的间距,此时铯掺杂的C60固体呈现出金属性和超导性。

A fully developed theory of C60 solids superconductivity is still lacking, but it has been widely accepted that strong electronic correlations and the Jahn-Teller electron-phonon coupling[64] produce local electron-pairings that show a high transition temperature close to the insulator-metal transition.[65]

关于C60固体的超导性还没有完备的理论,但是BCS理论是一个被广泛接受的理论,因为强电子相互作用和Jahn-Teller电子-声子偶合能产生电子对,从而得到较高的绝缘体-金属转变温度。



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