1. Ferroelectric liquid crystals
2. For Neumann's principle, see e.g. J. F. Nye, Physical Properties of Crystals, Oxford 1957, p 20. Nye's version is adopted here. This axiomatic principle exists in many formulations. A beautiful one is due to P. Curie, who independently found the principle, which is therefore sometimes also referred to as the Curie principle
3. de Gennes , P. G.The Physics of Liquid Crystals321Oxford second printing 1975, p