Workshop on "Avalanches in Functional Materials" (AFM)

该研讨会将汇集从事“雪崩和颠簸”实验、统计力学和计算机建模的科学家。在畴边界由外场激发的纳米器件中，比如在铁电薄膜中，存在着由局部电场写入的信息。然后，这些信息可以被移动到一个读取设备。问题是：信息是否被移位损坏，或者是否可以实现域壁的连续移位。最近的模拟表明，非常小而薄的设备会受到“突然”运动和雪崩形成的影响（比如在雪崩中，一个事件会引发许多次要事件，或者在地震中，每次冲击都会引发余震）。在这个问题上，许多学科都有丰富的经验，但我们还没有把不同的社区聚集在一起。这将在提议的研讨会中发生。温度所起的作用是研究跳闸和雪崩的一个重要方面。理论工作（分析和模拟工作）集中在低温状态。这里的雪崩是“热的”。这意味着弹跳不会被热激活（而应力和应变条件对雪崩的成核非常重要）。直到2013年，人们才清楚地认识到，这一结果对真正的器件材料具有误导性：在更高的温度下，热激活变得重要，雪崩行为发生了巨大变化。在两种热状态之间是一个交叉状态，其中以拉伸指数为主导的动力学速率定律。许多材料的交叉点似乎在室温附近，因此这种效应不是一种奇怪的现象，而是对许多设备应用变得重要。虽然对“突发事件和雪崩”的理解源于众多专业研究领域，但我们发现不同社区之间的实验方法也不同，迄今为止，在将实验方法从一个社区传播到另一个社区方面还没有取得太大进展。典型的是静态和准静态方法，其中外部状态变量缓慢地绝热变化，并观察系统的动态变化。通常情况下，当一个阈值被克服时，就会产生抖动。一个更好的方法是用大量观察到的雪崩进行真正的动态测量（共振方法）。这种方法是2013年在剑桥开发出来的，但在其他地方没有。原因是这些影响发生在两个非常不同的时间尺度上。调谐时间（例如提高温度或电场）必须非常缓慢（有时需要数周），而对抖动的测量必须非常快（抖动具有与声音传播速度相关的固有时间尺度）。声学测量和压电测量的技术正在开发中，谐振频率在兆赫兹范围内，温度变化在毫克/秒以上。我们坚信，如果在更广泛的社区内更好地了解各种实验方法并可能在各群体之间转移，则可以在这一领域取得很大进展。

The workshop will bring together scientists who work on 'avalanches and jerks' experimentally, in statistical mechanics, and by computer modelling. Jerks exist in nano-devices where domain boundaries are excited by external fields such as in ferroelectric thin films which have information written on them by local electric fields. This information can then be moved to a reading device. The issue is the following: is the information corrupted by the shift or can a continuous shift of domain walls be achieved. Recent simulations have shown that very small and thin devices will suffer from 'jerky' movements and the formation of avalanches ( such as in snow avalanches where one event will trigger a multitude of secondary events or as in earth quakes where each shock can trigger an after-shock). There is tremendous experience in this subject distributed over many disciplines but we have not yet brought the various communities together. This will happen in the proposed workshop.An important research aspect for jerks and avalanches is the role played by temperature. Theoretical work ( analytical and simulation work) has focused on the low temperature regime. Here the avalanches are 'a-thermal'. This means that jerks will not be thermally activated (while stress and strain conditions are very important for the nucleation of avalanches). Only in 2013 has it become clear that this result is misleading for real device materials: at higher temperatures the thermal activation becomes important and the avalanche behaviour changes dramatically. Between the two thermal regimes is a cross-over regime where kinetic rate laws with stretched exponential dominate. The crossover point for many materials seems to be around room temperature so that this effect is not a curiosity of a strange phenomenon but becomes important for many device applications.While the understanding of 'jerks and avalanches' stems from a multitude of specialised research areas, we find that the experimental approaches are also different between the various communities and not much progress has been made so far to disseminate experimental approaches from one community to another. Typical are static and quasi-static approaches where an external state variable is changed adiabatically slowly and the dynamic change of the system is observed. Typically this gives rise to a jerk when a threshold value is overcome. A much better approach would be to perform truly dynamic measurements with a large number of observed avalanches (resonance methods). Such methods are developed in Cambridge in 2013 but nowhere else. The reason is that the effects happen on two very different time scales. Tuning times (e.g ramping up temperature or electric fields) have to be very slow (and take sometimes weeks) while the measurement of a jerk has to be very fast (a jerk has an intrinsic time scale related to the speed of sound propagation). Techniques are being developed for acoustic measurements and piezoelectric measurements where the resonance frequencies are in the MHz range while temperature is changed over milliK/sec. We strongly believe that much progress in this field could be made if the various experimental approaches were better known within a wider community and potentially transferred between groups.