Electrons and nuclei in tiny wires

我们可以看到、感觉到——甚至听到和闻到——电流在灯泡中的作用。它们发热、发光，并最终烧毁。那么，自然界中可能的最细导线（例如单原子链和分子连接）怎么样？它们可以通过高度控制的实验方法生产出来吗？区别不仅仅是大小。原子线中的电流密度（每单位横截面积的电流）可以超过灯泡灯丝中的电流密度多个数量级。对于实验和理论来说，电子和原子核在这些条件下的行为方式都是一个悬而未决的问题。多年来，我们的小组一直致力于研究这些系统中的电流如何影响原子核的动力学问题。关于这些传导系统，我们已经取得了很多成就，也学到了很多知识，并且存在一系列由世界各地的研究小组根据要解决的问题提出的方法。日常经验中常见的效应也发生在原子尺度上。例如，焦耳热可能发生在原子线中，并可能导致局部温度大幅上升。另一个令人兴奋的进展是我们证明电流可以对原子施加额外的非保守力场，通过类比溪流如何驱动水车，可以使用电流作为原子级发动机的驱动器。同时，非保守力下的能量转移为纳米级导体带来了潜在的非常强大的失效机制。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的速度相关力。其中之一，有时称为电子摩擦，是一种有效的阻尼力，是由于电子激发导致原子运动能量损失而产生的。 （相反的效应——载流电子自发声子发射——是焦耳热的根源，也是理论上最难捕获的效应之一。）我们在丹麦的合作者提出的另一种与速度相关的力，完全源于电流。电流下原子的动力学是所有这些效应相互作用的结果。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、功能性和适用性至关重要，以应对局部加热、电迁移或在上述最近提出的非保守电流诱导力下可能发生的从电流到原子动力学的巨大能量转移等影响。这些问题的很大一部分可以通过经典地处理原子运动来理解。然而，通过与丹麦和中国的同事合作，我们已经能够得出电流下原子运动的有效驱动量子刘维尔方程，该方程包含所有上述效应。该项目的目的是研究该方程的性质并揭示它所描述的物理原理。为此，我们将从更熟悉的原子核经典描述开始，并进一步了解电流感应力对原子线的影响。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，并对其产生的影响进行纸笔分析。从战略上讲，该项目涉及凝聚态物理和低维系统的电子特性，以及量子器件、组件和系统以及对量子导体中新型能量转换机制的理解。这项工作预计将与我们在尼尔斯玻尔研究所和丹麦技术大学的同事合作进行。哥本哈根。我们将进一步受益于我们与莱顿大学原子尺度导体领域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.