Workshop on "Avalanches in Functional Materials" (AFM)

研讨会将汇集科学家谁的工作“雪崩和抽搐”实验，在统计力学，并通过计算机建模。在纳米器件中，畴界被外场激发，如在铁电薄膜中，信息被局部电场写在其上。该信息然后可以被移动到阅读设备。问题是：信息是否被移位破坏，或者是否可以实现畴壁的连续移位。最近的模拟表明，非常小和薄的设备将遭受“不稳定”的运动和雪崩的形成（例如在雪崩中，一个事件将触发多个次级事件，或者在地震中，每个地震都可能触发余震）。在这个问题上有大量的经验，分布在许多学科，但我们还没有把各个社区聚集在一起。这将发生在拟议的研讨会。一个重要的研究方面的急动和雪崩的作用是由温度。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。这意味着冲击不会被热激活（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，这一结果对真实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。两个热制度之间是一个交叉制度，其中动力学速率定律与拉伸指数占主导地位。许多材料的交叉点似乎在室温附近，因此这种效应并不是一种奇怪的现象，而是对许多设备应用很重要。虽然对“急动和雪崩”的理解源于许多专业研究领域，我们发现，不同社区之间的实验方法也不同，迄今为止，在传播实验方法方面没有取得多大进展，从一个社区到另一个社区。典型的是静态和准静态方法，其中外部状态变量缓慢地改变，并且观察到系统的动态变化。典型地，当超过阈值时，这引起急动。一个更好的方法是对大量观测到的雪崩进行真正的动态测量（共振法）。这种方法是2013年在剑桥开发的，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，升温或电场）必须非常慢（有时需要数周），而急动度的测量必须非常快（急动度具有与声音传播速度相关的固有时间尺度）。正在开发用于声学测量和压电测量的技术，其中谐振频率在MHz范围内，而温度以毫K/秒变化。我们坚信，如果各种实验性方法在更广泛的社区中得到更好的了解，并有可能在群体之间推广，就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.