Evidence for structural phase transitions and large effective band gaps in quasi-metallic ultra-clean suspended carbon nanotubes

We report evidence for a structural phase transition in individual suspended metallic carbon nanotubes by examining their Raman spectra and electron trans- port under electrostatic gate potentials. The current-gate voltage characteristics reveal anomalously large quasi-metallic band gaps as high as 240 meV, the largest reported to date. For nanotubes with band gaps larger than 200 meV, we observe a pronounced M-shape profile in the gate dependence of the 2D band （or G＇ band） Raman frequency. The pronounced dip （or softening） of the phonon mode near zero gate voltage can be attributed to a structural phase transition （SPT） that occurs at the charge neutrality point （CNP）. The 2D band Raman intensity also changes abruptly near the CNP, providing further evidence for a change in the lattice symmetry and a possible SPT. Pronounced non-adiabatic effects are observed in the gate dependence of the G band Raman mode, however, this behavior deviates from non-adiabatic theory near the CNP. For nanotubes with band gaps larger than 200 meV, non-adiabatic effects should be largely suppressed, which is not observed experimentally. This data suggests that these large effective band gaps are primarily caused by a SPT to an insulating state, which causes the large modulation observed in the conductance around the CNP. Possible mechanisms for this SPT are discussed, including electron-electron （e.g., Mott） and electron-phonon （e.g., Peierls） driven transitions.

我们由在静电的门潜力下面检验他们的拉曼系列和电子运输在单个推迟的金属性的碳 nanotubes 为结构的阶段转变报导证据。当前门的电压特征象 240 兆电子伏一样高揭示反常地大的伪金属性的乐队差距，最大迄今为止报导了。为有比 200 兆电子伏大的乐队差距的 nanotubes，我们在 2D 乐队的门依赖观察显著 M 形状侧面(或 G 乐队) 拉曼频率。显著剧降(或弄软) 零扇门附近的声子模式，电压能被归因于发生在费用中性点(CNP ) 的结构的阶段转变(SPT ) 。2D 乐队拉曼紧张也在 CNP 附近突然地变化，在格子对称和可能的 SPT 换换花样提供进一步的证据。然而，显著非断热的效果在 G 乐队拉曼模式的门依赖被观察这行为在 CNP 附近从非断热的理论背离。为有比 200 兆电子伏大的乐队差距的 nanotubes，非断热的效果应该大部分被压制，它没试验性地被观察。这个数据建议这些大有效乐队差距被 SPT 首先引起到一个绝缘的状态，它引起在 CNP 附近在传导力观察的大调整。为这 SPT 的可能的机制被讨论，包括电子电子(例如，莫特) 并且电子声子(例如， Peierls ) 驾驶转变。