Neutron Spin Echo and MuSR Studies of Glassy Dynamics

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.