Chapter 3 serves as an introduction to glasses and in doing so focuses on what is characteristic of glasses in general. To briefly summarize, the phenomenological common features are an extremely long or perpetual state of nonequilibrium, disobeying time homogeneity (aging), and memory effects. Structurally, the common features are interaction and disorder, the combination of which leads to frustration. As far as experiments are concerned, one can excite the glass with some generalized force – thermodynamically an intensive quantity – and measure a responding dynamical variable (an extensive quantity) or a generalized susceptibility. For example, in structural glasses, one measures deformation or viscosity responding to stress, in spin glasses one measures magnetization or magnetic susceptibility responding to applied magnetic fields, and in electron glasses one measures conductivity or electrical susceptibility in response to applied electric fields. Alternatively, one can measure slow relaxation of this variable after cooling from high temperature.

In relation to Chapter 3, this chapter is primarily intended to point out the more subtle differences between the electron glass and other glasses, with some particular attention to the spin glass, which is a close relative of the electron glass and is the most widely studied glass.

Structural glasses

Before all else, it is useful to start with structural glasses, which, in more than one sense, is the mother of other glasses. The silica glass (“window glass”) is by far the oldest glass that humans took interest in, mainly as an object of fashion.