Abstract of Lectures on

AMORPHOUS MATERIALS

Guy J. Adriaenssens

University of Leuven, Belgium.


Amorphous Materials are solids that lack the periodic lattice which is the most characteristic property of crystals. They are mostly obtained from the liquid or gas phase of a compound by sufficiently fast cooling to preclude the nucleation and growth of a crystalline solid. Coupled to the absence of lattice periodicity in the amorphous phase is the fact that the basic Bloch theorem of solid-state physics can no longer be applied and, consequently, that no meaningful reciprocal space can be defined for amorphous materials. Thus, standard solid-state-physics theories become useless. However, this does not mean that the physical properties of amorphous materials are necessarily very different from those of their crystalline analogues.

The chemical bonds that hold solids together, be they silicon, quartz, or arsenic selenide, do not change with the structural phase of the material. This means that atomic arrangements over short distances will be very similar in comparable amorphous and crystalline compounds. All material properties that are mainly determined by short-range interactions will, therefore, also be similar. This includes such things as mechanical and vibrational properties, magnetic properties, or electronic band structures. Properties that benefit from long-range order - such as electrical conductivity - will of course be strongly affected.

The more interesting aspects of amorphous materials are those whereby the inherent disorder of their atomic lattice gives rise to physical phenomena that are not known in the corresponding crystals. Localization of electronic wavefunctions, the possibility of mono-atomic defects, and the appearance of extra low-energy vibrational excitations are such instances. They will be discussed in the lectures.

Practical uses of amorphous materials are both ancient and advanced: window glass as well as optical fibres are amorphous solids, the thin-film transistors that control flat-screen pixels are based on amorphous semiconductors, current re-writable DVDs exploit the ability of some materials to be switched back and forth any number of times between their amorphous and their crystalline phase. Since this last example, and the Te-based compounds that are involved, illuminate many aspects of the specific nature of the amorphous state, it will be discussed to some length. 


Literature: S.R. Elliott, Physics of Amorphous Materials, 2nd ed., Longman Scientific,1990.