Phase Transitions in Nanofriction

为了我们理解摩擦，认识促进能量耗散的基本机制，即从动能到热量的转化，是相当重要的。在微观尺度上，这个问题与假设密切相关，即现实界面是由多粗糙面接触形成的，其中单个粗糙面通常被认为是摩擦的最基本组成部分。在扫描探针显微镜中，在尖端和样品之间形成了这样的单粗糙接触，因此该技术被认为是研究纳米尺度摩擦起源的理想模型系统。然而，虽然测量定量摩擦值相对简单，但确定能量耗散机制仍然是一个挑战。在这里，解决这个问题的新方法在于分析跨相变的摩擦现象。在固体物理中，相变通常是一个非常有趣的领域，相变已经对各种各样的材料进行了很好的分析。这开辟了利用外部控制参数（如温度）诱导材料性能明确变化的可能性，并将这些变化与界面摩擦的伴随变化联系起来。这种方法在所谓的电子摩擦的情况下变得特别明显，它解决了电子在能量耗散过程中的作用。在这里，例如，超导体可以进行分析，其中电阻在特定阈值温度下消失，并且与摩擦相关的效应（例如电子-声子耦合）也同样发生变化。同时，还可以对结构相变进行分析。在这种情况下，扫描探针显微镜可以作为一种机械光谱学，其中尖端和样品之间的力相互作用可以探测材料在相变过程中的变化。利用这种方法，可以分析一般的能量耗散机制，同时也可以对相变本身感兴趣。尽管有这种潜力，但到目前为止，纳米级摩擦实验很少针对相变，部分原因是由于原子力显微镜作为温度函数进行测量的实验挑战。到目前为止，即使是一阶和二阶相变之间的根本区别也没有在实验上得到解决。总而言之，这个项目为纳米摩擦学的基本问题提供了新的见解。此外，应用扫描探针显微镜还可以表征与纳米范围内空间分辨率的相变有关的影响。

For our understanding of friction, it is of considerable importance to recognize the fundamental mechanisms, which facilitate energy dissipation, i.e. which are involved in the conversion from kinetic energy to heat. On the microscopic scale, this problem is closely related to the assumption, that realistic interfaces are formed as multi-asperity contacts, where a single asperity is usually considered as the most fundamental building block of friction. In scanning probe microscopy, such a single-asperity contact is formed between the tip and the sample and the technique is thus considered to provide an ideal model system to study the nanoscale origins of friction.However, while measuring quantitative friction values is relatively straightforward, it still remains a challenge to identify the mechanisms of energy dissipation. Here, a new approach to address this question lies in the analysis of friction phenomena across phase transitions. In solid state physics, phase transitions are usually an area of great interest and phase transitions are already very well analyzed for a large variety of materials. This opens up the possibility to induce well-defined changes of the material properties using an external control parameter, like e.g. temperature, and correlate these changes to the accompanying variations of the interfacial friction. This approach becomes particularly apparent for the case of the so-called electronic friction, which addresses the role of electrons for the energy dissipation process. Here, e.g. superconductors can be analyzed, where the electrical resistance vanishes below a certain threshold temperature and effects relevant for friction like e.g. electron-phonon coupling are changing likewise.At the same time, also structural phase transitions can be analyzed. In this case, scanning probe microscopy can be utilized as a kind of mechanical spectroscopy, where the force interaction between tip and sample allows to probe the changes within the materials during the phase transition. Using this approach, general energy dissipation mechanisms can be analyzed, while also the phase transition itself can be of interest.Despite this potential, phase transitions have so far rarely been targeted by nanoscale friction experiments, which is partially due to the experimental challenge to perform atomic force microscopy measurements as a function of temperature. Up to now, even the fundamental differences between first and second order phase transitions have not yet been addressed experimentally.All in all, this project thus offers new insight into very fundamental questions of nantribology. In addition, applying scanning probe microscopy also allows to characterize the effects related to phase transitions with a spatial resolution in the nanometer range.