Workshop on "Avalanches in Functional Materials" (AFM)

这次研讨会将汇集那些通过实验、统计力学和计算机建模来研究“雪崩和急流”的科学家。跳跃存在于纳米器件中，其中的磁畴边界由外场激发，例如在铁电薄膜中，其上有由局部电场写入的信息。然后可以将这些信息移动到阅读设备上。问题是：信息是否被这种转变破坏了，或者是否可以实现域墙的持续转变。最近的模拟表明，非常小和很薄的设备将受到“急速”运动和雪崩形成的影响(例如在雪崩中，一个事件将触发多个次要事件，或者像在地震中，每一次地震都会触发余震)。在这一主题上有丰富的经验，分布在许多学科，但我们还没有把不同的社区聚集在一起。这将发生在拟建的工作室中。对于急流和雪崩，一个重要的研究方面是温度所起的作用。理论工作(分析和模拟工作)主要集中在低温区域。在这里，雪崩是a级的。这意味着急流将不会被热激活(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，这一结果对于真实的设备材料具有误导性：在更高的温度下，热激活变得重要，雪崩行为发生了戏剧性的变化。在这两个热区域之间是一个交叉区域，其中具有扩展指数的动力学速率定律占主导地位。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变得重要。虽然对“颠簸和雪崩”的理解源于许多专门的研究领域，但我们发现，不同社区的实验方法也不同，到目前为止，在将实验方法从一个社区传播到另一个社区方面没有取得太大进展。典型的方法是静态和准静态方法，在这种方法中，外部状态变量绝热地缓慢变化，并观察系统的动态变化。通常情况下，当阈值被超过时，这会引起抖动。一种更好的方法是用大量观测到的雪崩(共振法)进行真正的动态测量。这种方法是2013年在剑桥开发的，在其他地方是没有的。原因是，影响发生在两个非常不同的时间尺度上。调谐时间(例如，增加温度或电场)必须非常慢(有时需要数周)，而对急速的测量必须非常快(急速具有与声音传播速度相关的固有时标)。正在开发声学测量和压电测量技术，其中共振频率在MHz范围内，而温度变化在毫克/秒以上。我们坚信，如果在更广泛的社区内更好地了解各种试验方法，并有可能在不同群体之间转移，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.