Oddities, known as intertwined ordered phases, seem to interfere with superconductivity. “We now have a simple way to understand how they are created and hopefully this understanding will help us to know how to get rid of them,” said Lee.
Most subatomic particles have a tiny magnetic field – a property physicists call “spin” – and electrical resistance happens when the fields of electrons carrying current interact with those of surrounding atoms. Two electrons can join like two bar magnets, the north pole of one clamping to the south pole of the other, and this “Cooper pair” is magnetically neutral and can move without resistance. Lee and Davis propose that this “antiferromagnetic” interaction is the universal cause not only for superconductivity but also for all the observed intertwined ordering. They show how their “unified” theory can predict the phenomena observed in copper-based, iron-based and so-called “heavy fermion” materials.
But if the cause is always the same, why do different materials exhibit different oddities? The difference, they say, is in the varying energy levels of the electrons that are free to carry current, which can be described by a mathematical structure called the ”Fermi surface.”
The new high-temperature superconductors are derived from orderly crystals where the same arrangement of atoms is repeated over and over and the spins of electrons alternate up and down from one unit cell to another. Although this favors antiferromagnetic interaction, electrons are not free to form Cooper pairs. Doping with trace elements distorts the crystal structure and removes some electrons, changing the Fermi surface. Whether Cooper pairing or some other ordering will take place depends on the shape of the Fermi surface, the researchers said.
Heat makes atoms move and can shake Cooper pairs apart, so the holy grail is to design a material where the pairs are bound together so strongly that superconductivity can happen even up to room temperature. It might be possible to describe a Fermi surface that would create that condition, and perhaps then imagine what crystal structure it would require. “Ideally we would like to be able to tell the materials scientist to put elements X, Y and Z together,” Lee said. “Unfortunately we can’t do that yet.”