What neutrons do tell us about the nature of (spin) glasses?

A common feature of all glassy systems is the decisive role of dynamics in the evolution of macroscopic properties, while the short and eventually par tial long-range order shows little difference between the different phases. Our recent neutron scattering results on a Cu-Mn spin glass dramatically i llustrate this point, and provide unambiguous evidence against recent sugge stions that behaviour in Cu-Mn is driven by spin density waves rather than glassy dynamics. Recent findings in Au-Fe provide the first circumstantial evidence for the theoretically predicted extremely long correlation lengths at the spin-glass transition. Recent experimental advances opened up the w ay to investigate dynamics in glasses and liquids on extended length scales , where only spin correlations could be studied by now. (C) 2000 Elsevier S cience B.V. All rights reserved.