Title: Kinetic Friction of Structurally Superlubric 2D Material Interfaces Show Chinese title

Title: 结构超润滑二维材料界面的静摩擦力

Abstract: The ultra-low kinetic friction F_k of 2D structurally superlubric interfaces, connected with the fast motion of the incommensurate moir\'e pattern, is often invoked for its linear increase with velocity v_0 and area A, but never seriously addressed and calculated so far. Here we do that, exemplifying with a twisted graphene layer sliding on top of bulk graphite -- a demonstration case that could easily be generalized to other systems. Neglecting quantum effects and assuming a classical Langevin dynamics, we derive friction expressions valid in two temperature regimes. At low temperatures the nonzero sliding friction velocity derivative dF_k/dv_0 is shown by Adelman-Doll-Kantorovich type approximations to be equivalent to that of a bilayer whose substrate is affected by an analytically derived effective damping parameter, replacing the semi-infinite substrate. At high temperatures, friction grows proportional to temperature as analytically required by fluctuation-dissipation. The theory is validated by non-equilibrium molecular dynamics simulations with different contact areas, velocities, twist angles and temperatures. Using 6^{\circ}-twisted graphene on Bernal graphite as a prototype we find a shear stress of measurable magnitude, from 25 kPa at low temperature to 260 kPa at room temperature, yet only at high sliding velocities such as 100 m/s. However, it will linearly drop many orders of magnitude below measurable values at common experimental velocities such as 1 {\mu}m/s, a factor 10^{-8} lower. The low but not ultra-low "engineering superlubric" friction measured in existing experiments should therefore be attributed to defects and/or edges, whose contribution surpasses by far the negligible moir\'e contribution. Show Chinese abstract

Abstract: 二维结构超润滑界面的超低动能摩擦力F_k，与非共格莫尔图案的快速运动有关，通常因其随速度v_0和面积A的线性增加而被引用，但到目前为止从未被认真地处理和计算。 在这里，我们这样做，并以扭曲的石墨烯层在体石墨顶部滑动为例，这是一个可以很容易推广到其他系统的示范案例。 忽略量子效应并假设经典Langevin动力学，我们推导出在两种温度范围内的摩擦表达式。 在低温下，通过Adelman-Doll-Kantorovich类型的近似，显示出非零的滑动摩擦速度导数dF_k/dv_0等同于一个双层结构，其基底受到一个解析得出的有效阻尼参数的影响，代替了半无限基底。 在高温下，摩擦力与温度成正比，这符合涨落-耗散理论的解析要求。 该理论通过不同接触面积、速度、扭转角度和温度的非平衡分子动力学模拟得到验证。 使用6^{\circ }-扭转的石墨烯作为原型，在Bernal石墨上，我们发现剪切应力具有可测量的大小，从低温下的25 kPa到室温下的260 kPa，但仅在高速滑动如100 m/s时出现。 然而，在常见的实验速度如1 {\mu }m/s时，它会线性下降许多数量级，低于可测量值，降低了一个数量级10^{-8}。 因此，现有实验中测量到的低但不是超低的“工程超润滑”摩擦应归因于缺陷和/或边缘，它们的贡献远远超过微不足道的莫尔贡献。