What is superconductivity?

Good Vibrations

If heat increases resistance, then cranking down the thermostat ought to decrease it, right? Well, it does, within limits. In normal conductors, resistance falls as the thermometer drops, but it never disappears. Superconductors work a bit differently.

As a superconductor cools, it follows a similar curve of gradually dropping resistance until it reaches its particular critical temperature; then, abruptly, all resistance disappears. It's as if resistance were slowly losing a tug-of-war with conductance and then, frustrated, let go of the rope. Actually, the substance undergoes a phase transition. Like ice melting into water, the conventional material assumes a new state, one with zero resistance.

To understand what's going on here, we need to make a few modifications to our atomic jungle gym. Specifically, we need to start taking magnetism into account.

When the atoms in a conductor give up electrons, they become positively charged ions, causing a net attraction between the atomic lattice and the negatively charged electrons passing through it. In other words, as if vibrations and deformations weren't bad enough, the tennis balls we're throwing through our oscillating jungle gym are magnets. You might assume that this would increase their chances of encountering resistance while passing through our wobbly grid, and you'd be right -- for normal conductors. Superconductors, however, use it to their advantage.

Picture a pair of tennis balls thrown through the grid, one hot on the other's tail. As the first ball passes through the positively charged lattice, it attracts the surrounding atoms toward it. By bunching up, these atoms create a local area of higher positive charge, which increases the force pulling the second electron forward. Consequently, the energy spent to get through, on average, breaks even.

Like square dancers, these Cooper pairs form and break up constantly, but the overall effect perpetuates itself down the line, enabling electrons to zip through the superconductor like greased lightning.

Cooper pairs are named for physicist Leon N. Cooper who, with John Bardeen and John Robert Schrieffer, advanced the first successful model explaining superconductivity in conventional superconductors. Their achievement, known as the BCS Theory in their honor, garnered them the 1972 Nobel Prize in physics.

Superconductivity refused to remain pinned down for long, however; soon after the BCS Theory achieved traction in the field, researchers began discovering other superconductors -- such as high-temperature superconducting copper-oxides -- that broke the BCS model.

In this next section, we'll look at what sets these exotic superconductors apart from the rest.