The mysterious Twilight Zone of superconductors

Superconductors conduct electricity perfectly, but only at an extreme low temperature. Just above that temperature is a 'pseudogap' where neither regular nor low temperature rules apply. A group of scientists are fixing to find out why.

Superconductors are used shamelessly in medicine, science, and technology, and they put up no resistance. That's the definition of a superconductor, after all, a material that has no resistance - to electricity, at least. Most materials will to some extent resist the flow of electricity. Superconductors let electrons flow perfectly.

Electrons in superconductors can break all the rules because they stop behaving like electrons. Electrons have half-integer spins, and have to obey the Pauli Exclusion principle. They can't occupy the same electrical state at the same time. In superconductors, they behave like bosons - particles with full integer spins that can occupy the same electrical state. They do this by pairing up in Cooper Pairs. Two electrons link up, two half-spins make a whole spin, and suddenly the electrons can occupy the same 'space' perfectly well. Though they behave like other kinds of particles, while retaining certain electron properties. This makes them lose all resistance to electricity and enter a state of superconductivity.

But there is a price. Superconductors don't reveal themselves under normal conditions. They have to drop below a certain temperature in order to work. In even in high-temperature superconductors this temperature is only about twice the temperature of liquid nitrogen. It turns out, though, that high-temperature superconductors don't just turn superconductivity on and off. Between their superconducting, low-temperature state and their regular, normal-temperature state lies a 'pseudogap'.

It's name is derived from the 'gap' between the energy level at which electrons team up in pairs and the the energy level at which they break free and act like separate electrons again. In the pseudogap energy level, electrons aren't teaming up to become Cooper Pairs, but the material shows that they don't behave like regular, single electrons, either. What exactly happened during this phase was a mystery. Some thought that during the pseudogap phase electrons paired up, but not in great enough numbers to make the material a superconductor. The energy dropped, more electrons paired up, and the material slid slowly into superconductivity. Others thought that the pseudogap was an entirely different state. It was not a mix of the regular material and the paired-up material, but something different.

It looks like the 'something different' theory is correct. Ruihua He, at The Lawrence Berkeley National Laboratory, conducted a series of tests and found that all of them point the existence of a phase transition - a specific state of matter - separate from both the regular-temperature state and the superconducting state. He claims that the electrons in the pseudogap 'organize themselves' in a way that influences Cooper Pairs, but neither He nor any of the other scientists working with him know what that new organization is, nor whether it helps or hurts the superconductivity of the material. Whether they are conducive to superconductivity or not, this discovery is exciting. Electrons are behaving in a new way, and figuring out that way might make room-temperature superconductors a possibility.

Via Science and Physics World.