Applied magnetic field enhances superconductivity

22 Sep

Metals conduct electricity better at low temperatures. Superconductivity is a phenomenon exhibited by some materials where the resistance of that material drops to zero, meaning that it conducts electricity very well, below a certain critical temperature. And I mean VERY well…like electrical current can run around forever in there. (Picture shows levitation of a magnet due to an induced magnetic field by the superconductor beneath).

Superconductivity is a very useful feature, but the problem is that the critical temperature is usually extremely low. It is a bit impractical to work with something that has to be kept at such a low temperature. There are high-temperature superconductors, but we are talking 90K (-300 degrees F),which is still pretty cold. Scientists have been looking for ways to increase the critical temperature of materials to make superconductivity more applicable.

For a minute, let’s look at the theory. There are a couple different ways of explaining the phenomenon of superconductivity. BCS theory describes it as a “condensation” of elections into bound, boson-like, states due to electron-phonon (quantum mechanical characterization of vibration) interaction. Which pretty much means that the electrons pair up because of the attractions and repulsions caused by lattice vibrations. You can kind of think of it classically in that the electron will pull the positive parts of the lattice towards itself, causing a net positive density around the first electron, which then attracts another electron. These are known as Cooper pairs–Cooper is the “C” in BCS.

According to BCS theory, applied magnetic fields should lower the superconductivity by disrupting these pairs of electrons. It would break up the symmetry. There have been some cases where an applied magnetic field slightly enhances the superconductivity, but never actually raises the critical temperature. It was thought that the magnetic field suppressed the negative effects of paramagnetic impurities in the material, causing the slight increase in superconductivity. This paper, however, shows that applied magnetic field can actually raise the critical temperature of the material, while the presence of impurities still decreases it, which completely flies in the face of accepted theory.