While obtaining theoretical predictions for dissipation during sliding motion is a difficult task, one regime that allows for analytical results is the so-called noncontact regime, where a probe is weakly interacting with the surface over which it moves. Studying this regime for a model crystal, we extend previously obtained analytical results and confirm them quantitatively via particle based computer simulations. Accessing the subtle regime of weak coupling in simulations is possible via use of Green-Kubo relations. The analysis allows us to extract and compare the two paradigmatic mechanisms that have been found to lead to dissipation: phonon radiation, prevailing even in a purely elastic solid, and phonon damping, e.g., caused by viscous motion of crystal atoms. While phonon radiation is dominant at large probe-surface distances, phonon damping dominates at small distances. Phonon radiation is furthermore a pairwise additive phenomenon so that the dissipation due to interaction with different parts (areas) of the surface adds up. This additive scaling results from a general one-to-one mapping between the mean probe-surface force and the friction due to phonon radiation, irrespective of the nature of the underlying pairwise interaction. In contrast, phonon damping is strongly nonadditive, and no such general relation exists. We show that for certain cases, the dissipation can even decrease with increasing surface area the probe interacts with. The above properties, which are rooted in the spatial correlations of surface fluctuations, are expected to have important consequences when interpreting experimental measurements, as well as scaling with system size.

中文翻译：

---

非接触摩擦：声子阻尼的作用及其非普遍性

虽然获得滑动运动过程中耗散的理论预测是一项艰巨的任务，但允许分析结果的一种方式是所谓的非接触方式，其中探针与其移动的表面弱相互作用。在研究模型晶体的这种机制时，我们扩展了先前获得的分析结果，并通过基于粒子的计算机模拟对它们进行了定量确认。通过使用 Green-Kubo 关系可以访问模拟中弱耦合的微妙机制。该分析使我们能够提取和比较已发现导致耗散的两种典型机制：声子辐射，即使在纯弹性固体中也普遍存在，以及声子阻尼，例如，由晶体原子的粘性运动引起。虽然声子辐射在较大的探头表面距离处占主导地位，声子阻尼在小距离上占主导地位。此外，声子辐射是一种成对相加现象，因此与表面不同部分（区域）的相互作用造成的耗散加起来。这种附加缩放是由平均探针表面力与声子辐射引起的摩擦之间的一般一对一映射产生的，而与潜在成对相互作用的性质无关。相比之下，声子阻尼是强非可加性的，不存在这种一般关系。我们表明，在某些情况下，耗散甚至可以 这种附加缩放是由平均探针表面力与声子辐射引起的摩擦之间的一般一对一映射产生的，而与潜在成对相互作用的性质无关。相比之下，声子阻尼是强非可加性的，不存在这种一般关系。我们表明，在某些情况下，耗散甚至可以 这种附加缩放是由平均探针表面力与声子辐射引起的摩擦之间的一般一对一映射产生的，而与潜在成对相互作用的性质无关。相比之下，声子阻尼是强非可加性的，不存在这种一般关系。我们表明，在某些情况下，耗散甚至可以随着探针相互作用的表面积增加而减少。上述特性植根于表面波动的空间相关性，预计在解释实验测量以及随系统大小缩放时具有重要意义。