Numerical Modelling of Spin Ice Dynamics

你为什么不能把北极从磁铁上扯下来呢？如果你试着把一个磁铁切成两半，你会得到两个更小的磁铁，每个都有北极和南极。重复这个过程，你最终得到一个有磁矩的原子，它同样有两个极性。从本质上讲，答案是，虽然有带电荷的粒子（电“单极子”，如电子），但在我们的宇宙中没有带磁性的粒子（磁单极子）。然而，越来越多的证据表明，这并不是事情的全部。1997年，一种神秘的新型磁铁被发现，称为自旋冰。我们的主要理论认为，这些晶体中单个离子的磁场自我定向，从而产生局部的磁化源和磁汇，这与磁单极子惊人地相似。最近的数值和实验工作表明，磁单极子在自旋冰晶周围的磁场波动中产生独特的特征，这种特征被称为“粉红噪声”。这个项目旨在了解单极子是如何产生粉红噪声的；如何利用这种理解来明确地证明单极子的存在；以及我们如何将这种理解应用于开发技术应用。这项工作将结合数值方法（动力学蒙特卡罗）与分析建模。主题将包括噪声谱，分形渗透簇，循环擦除随机漫步，失衡动力学和玻璃行为。虽然这项工作本质上是理论性的，但也有与实验同事合作的余地，该项目将由两名实验学家共同监督。为自旋冰开发的模型将适用于人工自旋冰结构，由S. Ladak博士团队开发的纳米级磁畴阵列，它捕捉了自旋冰的关键特征。G. Klemencic博士的团队正在使这些相同的结构具有超导性，从而提供了将量子效应纳入工作的可能性。该项目包括为个人发展提供大量外部资金，包括公开演讲和媒体培训。如果需要的话，还可以提供3个月的工资来进行工业实习。两者都是监管团队对负责任创新承诺的一部分。

Why can't you pull the north pole off a magnet? If you try to cut a magnet in half you get two smaller magnets, each with a north and south pole. Repeat this process and you ultimately end up with a single atom with a magnetic moment, which again has both poles. Essentially, the answer is that while there are particles with electric charge (electric 'monopoles', such as electrons) there are no magnetically charged particles -- magnetic monopoles -- in our universe.However, there is growing evidence that this is not the whole story [1]. In 1997 a mysterious new type of magnet was discovered, called a spin ice. Our leading theories suggest that the magnetic fields of individual ions in these crystals orient themselves so as to create local sources and sinks of magnetization, which bear a striking resemblance to magnetic monopoles. Recent numerical [2] and experimental work [3] indicates that magnetic monopoles produce unique signatures in the fluctuations of the magnetic field around spin ice crystals, signatures known as 'pink noise'.This project is aimed at understanding how monopoles generate pink noise; how that understanding might be used to definitively prove the monopoles' existence; and how we might put this understanding to use in developing technological applications. The work will combine numerical approaches (kinetic Monte Carlo) with analytical modelling. Topics will include noise spectra, fractal percolation clusters, loop erased random walks, out-of-equilibrium dynamics, and glassy behaviour. While the work is theoretical in nature, there is scope to engage with experimental colleagues, and the project will be co-supervised by two experimentalists. The models developed for spin ices will be applicable to the artificial spin ice structures, arrays of nanometre-scale magnetic domains which capture the key features of spin ices, developed by the group of Dr S. Ladak. These same structures are being made superconducting by the group of Dr G. Klemencic, offering the possibility to include quantum effects into the work.The project includes substantial external funding for personal development, including public speaking and media training. There is also funding available for 3 months' salary to undertake an industrial placement if desirable. Both are part of the supervisory team's commitment to Responsible Innovation.