Superconductivity

Push electricity through an ordinary wire and some of it always leaks away as heat. That waste is called resistance, and every normal metal has it. It is why phone chargers get warm and why power lines lose energy on the way to your house.

But cool certain materials down far enough, close to the coldest temperatures there are, and something remarkable happens all at once. Their resistance does not just get small. It drops to nothing. Zero. Start a current flowing in a loop of superconductor and it will keep going round and round for years without fading, because there is nothing to slow it down.

There is a second trick that comes with it. A superconductor shoves magnetic fields out of itself. Rest a small magnet just above one, and instead of falling it hangs there in mid-air, floating. That floating is the clearest sign you are looking at a superconductor and not just a very cold lump of metal.

The catch, for now, is the cold. Most superconductors only work when chilled to hundreds of degrees below zero, which is expensive. If we ever find one that works at room temperature, it would change how we move and use electricity everywhere.

Resistance in a normal metal comes from electrons bumping into things. The atoms of the metal sit in a regular lattice, but that lattice is always jiggling with heat, and it has flaws. As electrons drift through carrying the current, they scatter off all that jiggling and lose energy to it, which shows up as heat. Cool the metal and the jiggling calms down, so resistance falls a little, but for an ordinary metal it never reaches zero.

A superconductor does something different and sudden. Below a sharp critical temperature, written \(T_c\), the electrons stop behaving as a crowd of independent particles. They join up into pairs, called Cooper pairs, and all those pairs lock into a single shared quantum state that moves together. You cannot nudge one electron and scatter a tiny bit of energy off it, because it is no longer acting alone. To disturb the current you would have to disturb the whole coordinated state at once, and at low temperature there is not enough energy around to do that. So the current flows on with no resistance at all.

The magnetic trick has a name too: the Meissner effect. A superconductor does not merely block magnetic fields, it actively expels them, setting up surface currents that cancel the field inside. Those same currents push back on a nearby magnet and hold it up. In many practical superconductors the field can thread through in tiny quantised tubes that get pinned in place, which locks the magnet in position rather than just floating it, and that pinning is what makes stable levitation possible.

Try the model above. As the temperature drops past \(T_c\), the resistance curve falls off a cliff to zero, the scattering stops, and the magnet lifts off the surface.

BCS theory. The microscopic explanation, from Bardeen, Cooper and Schrieffer in 1957, is that a subtle attraction can arise between electrons that normally repel. As one electron moves through the lattice it tugs the positive ions slightly toward it, leaving a faint trail of extra positive charge that a second electron is drawn to. This phonon-mediated attraction binds electrons into Cooper pairs with opposite momenta and spins. Because the pairs are bosonic in character, they can all occupy one coherent ground state described by a single macroscopic wavefunction.

The energy gap. Breaking a Cooper pair costs a minimum energy, the superconducting gap \(\Delta\). At temperatures where the available thermal energy is well below \(\Delta\), single-electron scattering is forbidden: there is no low-energy excitation to scatter into. That gap is why the resistance is not merely small but identically zero, and why the transition is so sharp. As \(T\) rises toward \(T_c\), the gap shrinks and closes, and superconductivity ends.

Persistent currents and flux quantisation. The macroscopic wavefunction has a phase, and requiring it to be single-valued around a loop forces any trapped magnetic flux to come in discrete units, the flux quantum \(\Phi_0 = h/2e\). The factor of \(2e\) is direct evidence that the carriers are pairs. A current set up in a superconducting ring persists because changing it would require unwinding that phase everywhere at once.

Type I, type II, and what is still open. Type-I superconductors expel field completely until it destroys the state; type-II superconductors let field in as pinned vortices over a wide range, which is what makes the strong magnets in MRI scanners, maglev trains and fusion reactors practical. The neat BCS story explains the classic low-temperature metals. It does not fully explain the high-temperature copper-oxide superconductors discovered in 1986, whose pairing mechanism remains one of the important open problems in physics, and the search for a room-temperature superconductor continues.