A glass is a system that does not reach thermodynamic equilibrium during any reasonable experimental time. Therefore, the traditional well-established statistical mechanics methods cannot be applied to such systems. In particular, two concepts that are fundamental to statistical mechanics, namely, ergodicity and the fluctuation-dissipation theorem, cannot be utilized in discussing properties of glasses. This chapter will discuss in more detail the problems inherent in applying these two concepts to glasses and will outline some more of the basic properties of glassy systems. Among glasses, spin glasses are the most similar to electron glasses so their main properties and proposed models will be reviewed. The chapter ends with an introduction to two-level systems, which is suggested as the possible origin of some of the properties of glasses, in particular, those of electron glasses.

The modern concept of glass

Fundamental scientific interest in glasses started a few decades ago with the pioneering works of Anderson and Mott in disordered (i.e., noncrystalline) solids. For fairness' sake, it should be mentioned that much earlier Schrödinger tried to raise attention to such systems when he conceived of the “aperiodic solid” as a probable candidate for carrying the genetic code. He also emphasized the long-term stability of such structures. The advent of DNA proved him of course right.

The first known glass is the common window glass.Awell-known feature typical of this glass has been that it reaches a crystalline state extremely slowly, if ever.