Electrons and nuclei in tiny wires

我们可以看到和感觉到——甚至听到和闻到——电流在灯泡中的作用。它们加热，发光，最终燃烧殆尽。那么，自然界中最细的导线，比如单原子链和分子连接点，在实验中可以通过高度控制来制造呢？区别不只是大小。原子导线中的电流密度（单位横截面积的电流）可以比灯泡灯丝中的电流密度高出许多数量级。电子和原子核在这些条件下的表现如何，在实验和理论上都是一个悬而未决的问题。多年来，我们小组一直在研究这些系统中的电流如何影响原子核的动力学。关于这些传导系统，我们已经取得了很多成就，也学到了很多东西。根据要解决的问题，世界各地的研究小组提出了一系列方法。日常经验中熟悉的效应也发生在原子尺度上。例如，焦耳加热可以发生在原子导线中，并可能导致局部温度大幅升高。另一个令人兴奋的进展是，我们证明了电流可以对原子施加额外的非保守力场，有可能利用电流作为原子级发动机的驱动器，就像水流驱动水车一样。同时，非保守力作用下的能量传递为纳米级导体的失效提供了潜在的强大机制。原子运动与电子子系统的相互作用可以进一步导致原子核上与速度相关的力。其中之一，有时被称为电子摩擦，是一种有效的阻尼力，由原子运动中由于电子激发而产生的能量损失引起。（反向效应——载流电子的自发声子发射——是焦耳加热的根源，也是理论上最难捕捉的效应之一。）我们在丹麦的合作者提出的另一种与速度相关的力，严格地起源于电流。原子在电流作用下的动力学是所有这些作用相互作用的结果。反过来理解它们对于衡量这些系统作为纳米级器件的稳定性、功能和适用性至关重要，这些系统可以抵御局部加热、电迁移或从电流到原子动力学的巨大能量转移等影响，这些影响可能发生在最近提出的上述非保守电流诱导力下。这些问题中有相当一部分可以通过经典地处理原子运动来理解。然而，在与来自丹麦和中国的同事的合作下，我们已经能够得到一个有效的驱动原子运动的量子刘维尔方程，该方程包含了上述所有效应。这个项目的目的是研究这个方程的性质，并提出它所描述的物理性质。为此，我们将从比较熟悉的原子核的经典描述开始，并进一步了解电流诱导的力对原子导线的影响。然后，我们将继续讨论量子刘维尔方程，并对简单模型系统进行数值求解，同时对它产生的影响进行纸笔分析。战略上，该项目从凝聚态物理和低维系统的电子特性，量子器件，组件和系统以及对量子导体中新型能量转换机制的理解等方面进行了研究。这项工作预计将与我们在尼尔斯玻尔研究所和哥本哈根丹麦技术大学的同事合作。我们将进一步受益于我们与莱顿大学原子尺度导体的领先实验家的长期联系。

We can see and feel - and even hear and smell - what current flow can do in lightbulbs. They heat up, emit light, and eventually burn out. So how about the thinnest wires possible in nature, such as chains of single atoms and molecular junctions, which can be produced experimentally with high degrees of control? The difference is not just size. The current densities (current per unit cross-sectional area) in atomic wires can exceed those in a lightbulb filament by many orders of magnitude. How the electrons and the nuclei behave under these conditions is an open question, to both experiment and theory.For many years our group has worked on the question of how current flow in these systems affects the dynamics of the atomic nuclei. A lot has been achieved and learned about these conducting systems, and a range of methodologies exists, proposed by research groups worldwide, depending on the questions to address. Effects familiar from everyday experience occur at the atomic scale too. For example, Joule heating can take place in atomic wires and can result in a large local temperature rise. Another exciting development was our demonstration that current can exert an additional, non-conservative force field on atoms, with the possibility of using current as the driver for atomic-scale engines, by analogy with how a stream can drive a waterwheel. At the same time, the energy transfer under the non-conservative forces poses a potentially very powerful failure mechanism for nanoscale conductors.The interaction of the atomic motion with the electronic subsystem can lead further to velocity-dependent forces on nuclei. One of them, sometimes termed electronic friction, is an effective damping force, resulting from the loss of energy from the atomic motion due to the excitation of electrons. (The converse effect - spontaneous phonon emission by the current-carrying electrons - is at the root of Joule heating and is one of the most difficult effects to capture theoretically.) Another velocity-dependent force, proposed by our collaborators in Denmark, originates strictly from the current. The dynamics of the atoms under current is the result of the interplay between all these effects. Understanding them in turn is essential in gauging the stability, functionality and applicability of these systems as nanoscale devices, against effects such as local heating, electromigration, or the enormous energy transfer from current to the atomic dynamics that can take place under the recently proposed non-conservative current-induced forces above.A considerable part of these questions can be understood by treating the atomic motion classically. In collaboration with colleagues from Denmark and China, however, we have been able to arrive at an effective driven quantum Liouville equation for the atomic motion under current, that subsumes all of the aforementioned effects.The aim of this project is to study the properties of this equation and bring out the physics it describes. To this end, we will begin with the more familiar classical description of the nuclei, and gain further experience with the effects that current-induced forces can have on atomic wires. We will then move on to the quantum Liouville equation and solve it numerically for simple model systems, together with a pen-and-paper analysis of the effects that it generates.Strategically, the project has aspects from Condensed Matter Physics and the electronic properties of low-dimensional systems, and Quantum Devices, Components and Systems and the understanding of novel energy conversion mechanisms in quantum conductors.This work is envisaged as a collaboration with our colleagues at the Niels Bohr Institute and the Technical University of Denmark in Copenhagen. We will benefit further from our long-standing links with leading experimentalists in atomic-scale conductors at the University of Leiden.