Neutron Spin Echo and MuSR Studies of Glassy Dynamics

玻璃的性质和玻璃跃迁被诺贝尔物理学奖得主菲利普·安德森描述为物理学中最有趣的未解决问题之一。尽管玻璃制造技术已经被人们熟知和开发了数千年，但人们对液体如何以及为什么冷却成玻璃状而不是有序的晶体状态却知之甚少。人们相信，通过研究原子在过渡接近时的动力学行为的演变，可以找到玻璃化转变本质的线索：然而，向有序状态的过渡伴随着随时间指数变化的弛豫，自150多年前Kohlrausch的工作以来，人们已经知道无序和玻璃化系统的弛豫是非指数的。虽然这种非（或拉伸的）指数弛豫似乎无处不在，但其起源仍然存在争议。事实上，目前还不清楚它是由并行过程还是分层过程产生的，还是两者的结合。在某些方面，自旋玻璃是一种相对简单的模型系统，可以用来研究玻璃态的行为。在自旋玻璃中，磁性自旋的方向而不是它们的位置冻结成随机状态。我们最近在各种自旋玻璃上对中子和介子束的测量似乎表明，一种基于概率考虑的新理论可能提供一个适当的模型来描述玻璃温度以上的弛豫。此外，我们还证明了该模型与最近基于合作无序或非广泛熵的热力学理论之间的联系，这些理论在描述地震和龙卷风等现象时也很有用。如果这种联系可以被证明，我们将有一个方案，通过该方案，各种类型的自旋玻璃和结构玻璃（如易碎玻璃和强玻璃）可以被理解，表征和参数化，根据它们的基本热力学性质。因此，本建议的目的是利用中子自旋回波和介子自旋弛豫技术来扩展我们对自旋玻璃的初步、有限的调查，以严格测试新的概率弛豫理论的适用性及其与非广泛熵理论的关系，从而建立一种理解玻璃化转变的通用热力学方法，并为Kohlrausch弛豫提供解释。

The nature of glass and the glass transition has been described by Physics Nobel Laureate Philip Anderson as one of the most interesting unsolved problems in physics. Despite the fact that the technology of glass manufacture has been known and exploited for thousands of years, relatively little is understood about how and why a liquid cools to a glassy rather than a well-ordered crystalline state. It is believed that that clues to the nature of the glass transition can be found by studying the evolution of the dynamical behaviour of the atoms as the transition is approached: whereas the transition to an ordered state is accompanied by relaxation which is exponentially dependent on time, it has been known since the work of Kohlrausch over 150 years ago that disordered and glassy systems relax non-exponentially. Although this non- (or stretched-) exponential relaxation seems to be ubiquitous, its origin remains controversial. Indeed it is still not clear whether it arises from parallel or hierarchical processes, or a combination of both.In some ways spin glasses, in which the orientation of the magnetic spins rather than their positions freeze into a random state, are relatively simple model systems with which to study glassy behaviour. Our recent neutron and muon beam measurements on a variety of spin glasses seem to indicate that a novel theory based upon probabilistic considerations may offer an appropriate model with which to describe the relaxation above the glass temperature. Moreover we have also demonstrated links between this model and recent thermodynamic theories based upon cooperative disorder , or non-extensive entropy, which are also useful in characterising phenomena such as earthquakes and tornadoes. If this connection can be proven we will have a scheme by which various types of both spin and structural glasses (eg fragile and strong glasses) can be understood, characterised and parameterised in terms of their fundamental thermodynamic properties. The aim of this proposal is therefore to use neutron spin echo and muon spin relaxation techniques to extend our preliminary, limited, survey of spin glasses to test rigorously the applicability of the new probabilistic relaxation theory and its relationship to the theory of non-extensive entropy in order to establish a universal thermodynamic approach to understanding the glass transition and to provide an explanation for Kohlrausch relaxation.