Superconductivity is well known to be a state where a material has no resistance to an electrical current passing through it. But what is less well known is that there are different types of superconductor. Type I superconductors are those referred to most often. They can allow current to flow on their surface with no resistance and repel weak enough magnetic fields but if the field gets too strong the superconducting nature breaks down. Type II superconductors also have resistanceless flow on their outer surface but when a strong magentic field gets applied to them, a current also flows on the interior. Then there is type 1.5. To understand these the concept of coherence length is important. Coherence length is the depth to which Cooper Pairs will propagate and so also the depth where current will flow without resistance. In type I this length is greater than the penetration depth of the magnetic field; In type II it is less; and in type 1.5 there are two coherence lengths and so is both. This means by using type 1.5 superconductors it is possible to switch between exterior and interior current.
A paper has just been published on the chimerical superconductor lithium iron astatine (LiFeAs), which details its strange and unique qualities gained by being part of this group. This substance is very important as LiFeAs is a superconductor without requiring any doping and so can be completely pure while experiments are done on it. This amazing discovery was that the dualistic nature of the chemical meant that is had two transition temperatures instead of the standard one. It appears that temperature can be used switch between the conducting states, a much simpler method than an applied field. This means it is plausible that other things could be used to switch between states and develop even more complex systems.