I recall Arthur C. Clark writing in "Fountains of Paradise" about a possible "positronium" cable, which as I remember was chains of H2 nuclei - essentially pure proton-proton bonds. This would be the ultimate in tensile strength, with a free-hanging length of >50 miles (again - as I recall). Clark includes a synopsis of the technology at the end of the book which is quite interesting.
[Unfortunately, there's no good reason to believe that such stuff is
possible. In the meantime we'll just have to get along with graphite
and its cousins, e.g. buckytubes.
--JoSH]