Dynamics of Disordered Non-equilibrium Systems: Hysteresis, Noise, and Domain Wall Dynamics in Systems Ranging from Magnets to Earthquakes

本基金支持凝聚态与材料物理的理论研究。对真实材料兴趣的增加引起了对无序系统的极大关注。无序改变了系统的自由能格局，并可能导致亚稳态之间的巨大能量障碍，导致极其缓慢地接近平衡，正如在许多实验中看到的那样。近年来，对明显非平衡效应的研究取得了很大进展，如对外部驱动力（雪崩）的集体反应和系统的内部历史依赖性（滞后）。本项目研究了滞回系统中无序对长尺度行为的不同影响。在长尺度上，在无序量的局部波动被平均掉的地方，被驱动远离平衡的系统通常可以用简单的定律有效地描述，并且普遍的，即，与细节无关的，临界行为已经被检测到。例子包括从磁带中推进畴壁的集体动力学到地震的事件大小分布。应用的方法包括来自动力系统和混沌、临界现象、流体力学和无序系统理论的思想。在一阶相变实验中，当系统处于非平衡状态时，滞回回路是常见的现象。它们可能有一个宏观的跳跃，大致就像在液体的过冷中看到的那样，或者它们可能是平滑变化的，就像在大多数磁铁中看到的那样。在最近与康奈尔大学Sethna小组的合作中，我们研究了非平衡零温度随机场Ising模型作为一阶变换时迟滞行为的模型。随着无序性的增加，人们发现一个跃迁，其中磁化的跳跃（对应于无限雪崩）减少到零。在这个过渡阶段，模型表现出噪声（雪崩）的幂律分布、普遍行为和发散的长度尺度。研究了温度波动和有限场扫描速率对系统的影响。调整扫描频率允许在文献中发现的两个极端情况（远离和接近平衡）之间研究整个实验相关的交叉状态。研究结果将用于解释磁性系统的实验，并探讨与工业应用相关的问题。该基金支持凝聚态物质和材料物理学的理论研究。对真实材料兴趣的增加引起了对无序系统的极大关注。无序改变了系统的自由能格局，并可能导致亚稳态之间的巨大能量障碍，导致极其缓慢地接近平衡，正如在许多实验中看到的那样。近年来，对明显非平衡效应的研究取得了很大进展，如对外部驱动力（雪崩）的集体反应和系统的内部历史依赖性（滞后）。本项目研究磁滞系统（如磁体）中无序对长尺度行为的不同影响

0072783DahmenThis grant supports theoretical research in condensed matter and materials physics. An increased interest in real materials has brought much attention to disordered systems. Disorder changes the free-energy landscape of a system and can lead to large energy barriers between metastable states, resulting in extremely slow approaches to equilibrium, as seen in many experiments. There has been much progress made recently in the study of distinctly nonequilibrium effects, such as collective response to an external driving force (avalanches) and internal history dependence of the system (hysteresis). This project deals with different effects of disorder on long length-scale behavior in hysteretic systems.On long length-scales, where local fluctuations in the amount of disorder are averaged out, systems driven far from equilibrium can often be usefully described by simple laws, and universal, i.e., detail independent, critical behavior has been detected. Examples range from collective dynamics of advancing domain walls in magnetic tapes to event size distributions of earthquakes. The methods applied include ideas from dynamical systems and chaos, critical phenomena, hydrodynamics and disordered systems theory.Hysteresis loops are often seen in experiments at first-order phase transformations when the systems goes out of equilibrium. They may have a macroscopic jump, roughly as seen in the supercooling of liquids, or they may be smoothly varying, as seen in most magnets. In a recent collaboration with the Sethna group at Cornell, we have studied the non-equilibrium zero-temperature random-field Ising model as a model for hysteretic behavior at first-order transformations. As disorder is added, one finds a transition where the jump in the magnetization (corresponding to an infinite avalanche) decreases to zero. At this transition, the model exhibits power law distributions of noise (avalanches), universal behavior and a diverging length-scale. The effect of adding temperature fluctuations and finite field sweep rate to the system will be studied. Tuning the sweep frequency allows the entire experimetally relevant crossover regime to be studied between the two extreme cases that are found in the literature (far from and close to equilibrium). The results will be used to interpret experiments in magnetic systems and to pursue related questions relevant to industrial applications.%%%This grant supports theoretical research in condensed matter and materials physics. An increased interest in real materials has brought much attention to disordered systems. Disorder changes the free-energy landscape of a system and can lead to large energy barriers between metastable states, resulting in extremely slow approaches to equilibrium, as seen in many experiments. There has been much progress made recently in the study of distinctly nonequilibrium effects, such as collective response to an external driving force (avalanches) and internal history dependence of the system (hysteresis). This project deals with different effects of disorder on long length-scale behavior in hysteretic systems such as magnets.***