In the last two decades low-dimensional (low-d) physics has matured into a major branch of science. Quite generally we may define a system with restricted dimensionality d as an object that is infinite only in one or two spatial directions (d = 1 and 2). Such a definition comprises isolated single chains or layers, but also fibres and thin layers (films) of varying but finite thickness. Clearly, a multitude of physical phenomena, notably in solid state physics, fall into these categories. As examples, we may mention: ¿ Magnetic chains or layers (thin-film technology). ¿ Metallic films (homogeneous or heterogeneous, crystalline, amorphous or microcristalline, etc.). ¿ I-d