Novel Quantum Transport Phenomena in Molecular Junctions

凭借其巨大的可变性，单分子器件为通过纳米结构实现量子传输的新效应提供了一个迷人的舞台。这种效应的一个自然来源在于电子自由度与少数明确定义的分子振动的耦合。这种耦合是特定于分子的，而不是像量子点这样的传统纳米结构，在传统纳米结构中，人们通常处理声子模的连续体。人们期待新的量子输运机制，至少有两个原因：(I)输运电流可以激发非平衡分子振动。我们预测，这可能会导致许多电子以自相似层次雪崩的形式流动。(Ii)分子能级的极化子位移可以使电荷能量重整化为负值，类似于非晶态半导体中的负U中心。最近，我们发现通过这种负U分子的输运是由电子对的隧穿所主导的。我们建议通过一个双管齐下的研究计划来研究这些有趣的通过分子结的输运模式。首先，我们将努力了解这些情况下的传输，超越了速率方程体系，这有望揭示更多的物理现象。其次，我们将超越以前研究中使用的分子连接的最小模型，努力实现对可能的实验系统的更现实和更完整的描述。最后，我们还将进一步探索分子结中量子输运的新模式。

By virtue of their enormous variability, single-molecule devices offer a fascinating arena for realizing novel effects in quantum transport through nanostructures. A natural source for such effects lies in the coupling of the electronic degrees of freedom to few, well-defined molecular vibrations. This coupling is specific to molecules as opposed to conventional nanostructures such as quantum dots, where one commonly deals with a continuum of phonon modes. One expects new regimes of quantum transport for at least two reasons: (i) The transport current can excite nonequilibrium molecular vibrations. We predicted that this can lead to current flow in the form of a self-similar hierarchy of avalanches of many electrons. (ii) Polaron shifts of the molecular levels can renormalize the charging energy to negative values, analogous to negative-U centers in amorphous semiconductors. Very recently, we showed that transport through such negative-U molecules is dominated by tunneling of electron pairs.We propose to study these interesting modes of transport through molecular junctions by pursuing a two-pronged research program. First, we will push to understand transport in these situations beyond the rate-equation regime, which is expected to uncover additional physics. Second, we will go beyond the minimal model of molecular junctions employed in previous studies in an effort to achieve a more realistic and complete description of possible experimental systems. Finally, we will also search for further novel modes of quantum transport in molecular junctions.