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Workshop on "Avalanches in Functional Materials" (AFM)

“功能材料中的雪崩”研讨会（AFM）

基本信息

批准号：
EP/L014793/1
负责人：
Ekhard Salje
金额：
$ 2.95万
依托单位：
University of Cambridge
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2014
资助国家：
英国
起止时间：
2014 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FL014793%2F1
关键词：
Workshop Avalanches Functional Materials AFM

项目摘要

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.

该研讨会将汇集通过实验、统计力学和计算机建模研究“雪崩和冲击”的科学家。急动存在于纳米器件中，其中域边界被外部场激发，例如铁电薄膜，其上通过局部电场写入信息。然后可以将该信息转移到读取设备。问题如下：信息是否因移位而损坏，或者能否实现磁畴壁的连续移位。最近的模拟表明，非常小和薄的设备将遭受“急速”运动和雪崩的形成（例如在雪崩中，一个事件将引发大量次生事件，或者在地震中，每次冲击都可能引发余震）。在这个主题上有丰富的经验分布在许多学科中，但我们还没有将各个社区聚集在一起。这将在拟议的研讨会上进行。急动和雪崩的一个重要研究方面是温度所起的作用。理论工作（分析和模拟工作）集中在低温状态。这里的雪崩是“非热雪崩”。这意味着冲击不会被热激活（而应力和应变条件对于雪崩成核非常重要）。直到 2013 年，人们才清楚地认识到这一结果对实际器件材料具有误导性：在较高温度下，热激活变得非常重要，并且雪崩行为会发生巨大变化。两种热状态之间存在交叉状态，其中具有拉伸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来说变得很重要。虽然对“冲击和雪崩”的理解源于多个专业研究领域，但我们发现不同社区之间的实验方法也有所不同，并且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尚未取得太大进展。典型的是静态和准静态方法，其中外部状态变量绝热地缓慢变化，并观察系统的动态变化。通常，当超过阈值时，这会引起急动。更好的方法是通过大量观察到的雪崩来执行真正的动态测量（共振方法）。此类方法是 2013 年在剑桥开发的，但其他地方没有。原因是影响发生在两个截然不同的时间尺度上。调谐时间（例如升高温度或电场）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）。声学测量和压电测量技术正在开发中，其中谐振频率在 MHz 范围内，而温度变化超过毫K/秒。我们坚信，如果各种实验方法在更广泛的社区内得到更好的了解并有可能在群体之间转移，那么该领域可以取得很大进展。