膨胀A-$_3$-C $_{60}$富勒烯非常规超导性质的模拟
Modeling the Unconventional Superconducting Properties of Expanded A$_3$C$_{60}$ Fullerides
摘要：
三价碱金属富勒烯A $_3$ C $_{60}$，其中C $_{60}$是一个完善的分子超导体家族。电子配对具有s波对称性，并且是由于标准的电子-声子耦合，特别是通过Jahn-Teller分子内C $_{60}$振动。对这些系统重新感兴趣的一个来源是强烈的电子-电子排斥的迹象，这尤其出现在C $_{60}$ -C $_{60}$距离扩大的化合物中。在几种化合物中，T $_c$初始增加后，进一步膨胀导致超导性下降，并最终消失，有利于Mott绝缘态。我们从理论上研究了三轨道哈伯德模型，包括声子介导的相互作用，使用动力学平均场理论，这是特别合适的，由于所有的相互作用的局部性质。我们研究了该系统作为一个功能的比例的分子内排斥$U$的电子带宽$W$，增加的$U/W$代表的晶格膨胀的主要影响。相图接近实际材料，在莫特转变之前有一个圆顶形的超导区。该模型预测的非常规性质包括：（i）正常相中的赝能隙;（ii）超导开始时的动能和直流电导率的增益;（iii）尽管有很强的相关性，但仍有规则的自旋磁化率和比热;（iv）出现一个以上的能量尺度来控制重整化单粒子色散。这些预测如果得到证实，将确立富勒烯，特别是膨胀富勒烯，作为强关联超导体更广泛家族的成员。
英文摘要：
The trivalent alkali fullerides A$_3$C$_{60}$, where C$_{60}$ are a well established family of molecular superconductors. The electron pairing has s-wave symmetry and is due to standard electron-phonon coupling, in particular by Jahn-Teller intramolecular C$_{60}$ vibrations. A source of renewed interest in these systems are indications of strong electron-electron repulsion, which emerges especially in compounds where the C$_{60}$-C$_{60}$ distance is expanded. In several compounds after an initial increases T$_c$, further expansions leads to a decline of superconductivity and its eventual disappearance in favor of a Mott insulating state We theoretically study a three-orbital Hubbard model including the phonon-mediated interaction using Dynamical Mean-Field Theory, which is particularly suitable due to the local nature of all the interactions. We studied the system as a function of the ratio of intra-molecular repulsion $U$ over the electron bandwidth $W$, the increase of $U/W$ representing the main effect of lattice expansion. The phase diagram is close to that of actual materials, with a dome-shaped superconducting region preceding the Mott transition. Unconventional properties predicted by this model include: (i) a pseudogap in the normal phase; (ii) a gain of kinetic energy and of d.c. conductivity at the onset of superconductivity; (iii) regular spin susceptibility and specific heat despite strong correlations; (iv) the emergence of more than one energy scale governing the renormalized single particle dispersion. These predictions, if confirmed, would establish fullerides, especially the expanded ones, as members of the wider family of strongly correlated superconductors.
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