CNR-NANO, Via Campi 213a, 41125, Modena, Italy.
SISSA & CNR-IOM Democritos, Via Bonomea 265, 34136, Trieste, Italy.
Nat Commun. 2017 Nov 13;8(1):1461. doi: 10.1038/s41467-017-01660-8.
Fifty years ago Walter Kohn speculated that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons-electron-hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. Here we show that this excitonic insulator is realized in zero-gap carbon nanotubes by performing first-principles calculations through many-body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings call into question the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulators.
五十年前，沃尔特·科恩推测，零带隙半导体可能会不稳定，容易自发产生激子——电子-空穴对，它们由库仑吸引力束缚在一起。重建的基态将打开一个能隙，打破了底层晶格的对称性，这是电子相关性的真正结果。在这里，我们通过第一性原理计算和多体微扰理论以及量子蒙特卡罗方法，证明了这种激子绝缘体在零带隙碳纳米管中得以实现。激子有序调节了两个碳子晶格之间的电荷，从而打开了一个可以实验观测到的能隙，这个能隙的大小与管半径成反比，并与轴向磁场的强度弱相关。我们的发现对碳纳米管的 Luttinger 液体范式提出了质疑，并提供了实验区分激子和 Mott 绝缘体的测试方法。
