A type of matter first predicted by Eugene Wigner in 1934 has been discovered at unusually high temperatures. In most metals, electrons behave like a gas and move randomly through the structure. However, Wigner predicted that when the electron density is low enough, the electron gas will freeze into a lattice with ferromagnetic properties. According to computer simulations, these Wigner crystals should only form at temperatures below 100 Kelvin or so, but the first Wigner crystal to be observed experimentally has been found at a temperature of 600 K. It was discovered in lanthanum-doped calcium hexaboride by a team of researchers from the US, Switzerland and Argentina (Nature 397 412).
Normal metals have an equal number of electrons with spin “up” and spin “down”. However, when there are more spins pointing up than down, or vice versa, the metal becomes a ferromagnet. Furthermore, the unpaired electrons responsible for the magnetism are pinned by the metal lattice and cannot move. This is what happens in iron. In lanthanum-doped calcium hexaboride, however, the unpaired electrons responsible for the magnetism are free to move. “We don’t know what the practical implications of this are yet, ” said team member Roy Goodrich of Louisiana State University, “but a lot of people will be looking at this.”
