If you've ever had an MRI scan or accelerated a sub-atomic particle to near light speed, then you've experienced the wonders of superconductors. Here's how they work, what they do, and how they can be used in science fiction.
The basic idea of a superconductor is simple: it's a material that conducts electric current with zero resistance. When you send current through a conventional conductor (typically a copper wire), some of the energy is lost as the electrons flowing through the conductor collide with positively charged ions in the metal. That energy is wasted as heat. A superconductor? No resistance. No heat loss. No waste.
A lesser known, but possibly more important, property of superconductors is that they are perfectly diamagnetic. That means that a superconductor cannot be penetrated by a magnetic field. This is what allows superconductors to levitate above magnets, and is exploited for use in magnetic resonance imaging (MRI).
Superconductors aren't necessarily exotic materials. The first superconductor discovered (in 1911) was mercury. Aluminum and tin are superconductors too. Unfortunately, those materials only superconduct at incredibly low temperatures, just a few degrees Kelvin above absolute zero. Several elements act as superconductors at atmospheric pressure and ultra-low temperatures — these are known as Type I superconductors.
Type II superconductors are complex metal compounds and ceramics that superconduct at higher temperatures than Type I superconductors. Mercury thallium barium calcium copper oxide superconducts at 138 K at atmospheric pressure, while other materials exhibit higher transition temperatures (the temperature at which they change from non-superconductor to superconductor) under high pressure. The important thing about Type II superconductors is that many of them have transition temperatures above the boiling point of nitrogen, and are known as "high-temperature superconductors." That means liquid nitrogen can be used for cooling instead of liquid helium, which is more expensive to produce and use.
Why do superconductors superconduct? In the case of the Type I materials, we have a very good idea, based on a theory proposed in 1957 by John Bardeen, Leon Cooper, and John Schrieffer. The BCS Theory explains how the independent electrons in a conventional conductor become "Cooper pairs" of electrons in a superconductor. Picture the lattice of positive ions in a conductor as a grid. As an electron passes through the grid, its negative charge attracts the positive ions. This pulls the grid slightly out of shape, compressing some ions together and creating an area of increased positive charge. The next electron that comes along is pulled toward this area of greater charge, much like race cars drafting each other. The force that keeps the two electrons traveling together is weak — weak enough that the thermal vibrations of the ion lattice overwhelm it except at very low temperatures.
So that explains Type I superconductivity. What about Type II? Answer that, and you can go ahead and book your flight to Stockholm. Scientists have been hammering away at the problem for more than 20 years, and we still don't really understand why Type II superconductors exist. We may be closer to figuring it out, however. Until 2006, all high-temperature superconductors were copper oxides called cuprates. The discovery of iron-based high-temperature superconductors called pnictides gives researchers a different class of materials to compare results to. This should help us figure out what properties are crucial to high-termperature superconduction.
There are a few ways you can use superconductors to add a dose of realistic science to your science-fiction. The Holy Grail of superconductor research is a material that superconducts at room temperature. The implications of such a discovery are legion: perfect power transmission; ultra-fast computers; eternal batteries; incredibly precise scientific equipment.
Superconductor batteries alone could crop up in a number of scenarios. Experiments have proven that a loop of superconductor will carry a charge for years. Theoretically, it could do so for billions of years. So when your protagonist discovers an ancient alien civilization (or returns to ancient Earth) and reactivates long-dormant security mechanisms, they can be powered by batteries that were left fully charged millennia ago, holding their energy in spirals of superconducting material.
Recently, scientists discovered an analog to superconductors — superinsulators. When cooled close to absolute zero, these materials have perfect resistance. They don't allow any current through. Sounds like a good technology for deflector shields, especially if your enemies rely on phasers or other energy weapons.
If you want to get a little more speculative, imagine a material that has negative resistance. That is, as current flows through it, it adds to the current's energy. Perhaps it exploits some strange quantum state, or derives energy from atomic forces (but without the noise and fire and radiation). I'll leave it up to you to see where that idea leads.