I stumbled across this neat reference chart for the strengths of various materials while investigating the plausibility of Niven’s variable sword, a weapon made of mono-molecular wire able to slice through almost any object. IIRC there is a scene in Stand on Zanzibar in which a “mucker” (the increased frequency of whom Brunner seems to have accurately predicted) uses a mono-molecular wire trap to decapitate a soldier. A similar trap is important to a pivotal chapter of The Three-Body Problem.
Is it theoretically possible to create the type of mono-molecular wire described in science fiction novels? Given the development of carbon nanotubes for quantum computing, is it likely that such wire will be developed in the future?
Carbon nanotubes are the strongest known material in terms of tensile strength: based upon measurements of a multi-walled nanotube, it is estimated a cable with a cross section of 1 square millimetre would support a weight of 6.4 tonnes.
This doesn’t, however, translate into suitability for the blade of a sword. Wikipedia summarises measurements of radial elasticity as “On the other hand, there was evidence that in the radial direction they are rather soft.” Nanotubes are also not strong under compression, tending to buckle. A nanotube sword would not be very good for stabbing or slashing.
You might be able to make a light, strong, and transparent sword out of an engineered diamonoid structure, but it would have to be thicker to have the required strength. But, as diamond cutting demonstrates, diamond can be cleaved relatively easily along the planes of the crystal, so you may find your diamond sword getting shorter and shorter during swordplay.