Phase Transitions in Nanofriction

对于我们理解摩擦，认识到促进能量耗散的基本机制，即参与从动能到热的转换，是相当重要的。在微观尺度上，这个问题是密切相关的假设，即现实的界面形成为多粗糙接触，其中一个单一的粗糙通常被认为是最基本的建筑块的摩擦。在扫描探针显微镜中，这样的一个单一的粗糙接触之间形成的尖端和样品和技术，因此被认为是一个理想的模型系统，研究纳米级originations.However，虽然测量定量摩擦值是相对简单的，它仍然是一个挑战，以确定能量耗散的机制。在这里，一个新的方法来解决这个问题在于在相变的摩擦现象的分析。在固态物理学中，相变通常是一个非常感兴趣的领域，并且已经很好地分析了各种材料的相变。这开辟了使用外部控制参数（例如温度）诱导材料性质的明确定义的变化的可能性，并且将这些变化与伴随的界面摩擦的变化相关联。这种方法对于所谓的电子摩擦的情况变得特别明显，它解决了电子在能量耗散过程中的作用。例如，可以分析超导体，其中电阻在特定阈值温度以下消失，与摩擦相关的效应（例如电子-声子耦合）也发生变化。同时，还可以分析结构相变。在这种情况下，扫描探针显微镜可以用作一种机械光谱学，其中尖端和样品之间的力相互作用允许探测相变期间材料内的变化。使用这种方法，一般的能量耗散机制可以分析，而相变本身也可以是interest.Despite这种潜力，相变迄今为止很少有针对性的纳米摩擦实验，这部分是由于实验的挑战，执行原子力显微镜测量作为温度的函数。到目前为止，甚至一级相变和二级相变之间的根本区别都还没有在实验上得到解决。总之，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相变相关的影响，其空间分辨率在纳米范围内。

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.