Charge and Spin
Density Waves; April 1994; Scientific American Magazine; by Brown, Grüner; 7 Page(s)
On a hot July afternoon the Mall in Washington, D.C., is overrun with sightseers. They move earnestly in zigzag patterns carrying their coolers, bouncing from museum to monument to cafeteria. Most of the streets bordering the lawns are flat, and as many tourists stroll in one direction as in the other. Suddenly a drumroll is heard: a marching band is assembling. On the roads, displacing the confused crowd, are gathering serried ranks of uniformed high school students. Soon the band is mustered in neat rows, hardly disturbed even by a child trying to hide between the trumpeters' legs from a pursuing parent. As the tourists watch, the band starts to play and then marches forward with a clash of cymbals.
The wanderers on the Mall imitate rather closely the behavior of electrons in common metals. On cooling to temperatures close to absolute zero, most metals remain in this state; that is, the electrons continue to wander. But in some metals the electrons organize themselves into regular patterns like the ranks of a marching band.