Superconductors

Ever notice the back of your smartphone getting warm after you use it for a while? That's because electrons running through the wires keep bumping into the atoms inside the material, creating friction heat. The force that gets in the way of electrons is called electrical resistance. Because of resistance, some of the electricity made at a power plant (about 3 to 4% in Korea) is lost as heat before it ever reaches your home.

Inside a superconductor, something amazing happens. When you cool the material way down, at some point the resistance that used to block the electrons vanishes completely. The electrons glide through like skaters on smooth ice. No heat is produced at all, and you reach a state where electrical resistance is exactly zero.

What if you made a ring out of a superconductor and sent electricity around it once? Even after you remove the battery, the current keeps circling for years without losing energy. This is called a persistent current, and it lets you run extremely strong currents with no wasted energy.

When people think of superconductors, the first image is often a mysterious disk floating in midair above a magnet. Many assume it's just magnets pushing each other away, but something more special is going on. A superconductor pushes magnetic fields coming from outside right out of its body.

When you bring a magnet close to an ordinary metal, the magnetic field passes straight through it. A superconductor is different. The moment it cools down, it drives out the magnetic field that was inside and blocks outside fields from getting in. (Many superconductors, called Type II, do let some of the field seep in as thin threads when the field gets strong.) This was discovered in 1933 by Meissner and Ochsenfeld, and it's usually called the Meissner effect (or the Meissner-Ochsenfeld effect).

A superconductor doesn't just get pushed away from a magnet. It can also lock into place at a fixed distance in midair. That locked-in, stuck-in-place feeling comes not just from repulsion but from flux pinning, where some of the magnetic field lines get trapped inside the superconductor. By the way, Japan's superconducting maglev trains and the MRI (magnetic resonance imaging) machines in hospitals don't use this floating effect. They rely on the fact that zero resistance lets you run very strong currents to build a powerful electromagnet.

To be a bit more precise, superconductivity is the quantum mechanics of the tiny world showing up at a scale we can actually see. Normally, electrons all carry the same negative charge (-), so they strongly repel each other. But at very low temperatures, something unexpected happens. When an electron passes by, nearby positive ions get pulled toward it, briefly creating a patch of positive charge. That patch attracts a second electron that follows, and the two electrons pair up. The vibrations of the material don't get in the way of pairing. Instead they act as a matchmaker. This explanation fits conventional superconductors. For high-temperature copper-oxide superconductors, how the electrons pair up isn't fully understood yet.

These paired electrons are called Cooper pairs. A lone electron easily bumps into surrounding atoms, but Cooper pairs, holding hands two by two, move together like one giant wave. So when an obstacle shows up, they just flow smoothly around it without colliding and travel with no resistance.

The catch is that Cooper pairs are only weakly bound, so they break easily if things warm up even a little. The temperature limit below which superconductivity appears is called the critical temperature, and for most superconductors it's an extreme cold of below minus 100 degrees Celsius (minus 148°F). That's exactly why scientists around the world are racing to find a room-temperature superconductor, one that works at everyday temperatures without expensive cooling equipment.