Physicists Listen to Electrical Noise to Probe High-Temperature Superconductor Mystery
Researchers in Munich and Utrecht measured current "shot noise" in a copper-oxide superconductor and found evidence that electron pairs form at higher temperatures than those at which the material superconducts. The result suggests the bottleneck is getting pairs to move in step, not forming them.
A team of physicists led by Milan Allan of Ludwig Maximilian University of Munich in Germany and Ingmar Swart of Utrecht University in the Netherlands has published a study in the journal Science on high-temperature superconductors. Superconductors carry electricity without resistance, but most work only a few degrees above absolute zero. In the late 1980s, copper-based materials called cuprates were found to superconduct at temperatures reachable with affordable liquid nitrogen. Nearly 40 years later, scientists still disagree about why.
The missing electrons
At the heart of the puzzle is a state called the pseudogap. Above the temperature at which cuprates become superconducting, some electrons seem to vanish: measurements find far fewer of them at the energies where they should appear. Two main explanations exist. One holds that electrons freeze into patterns such as charge density waves, with superconductivity developing separately. The other links the effect closely to superconductivity, proposing that electrons form pairs, known as Cooper pairs, which carry current in a superconductor.
Listening to noise
The team measured shot noise, the tiny fluctuations produced by the arrival of individual charges in a current. Paired electrons produce more noise. Using a scanning tunneling microscope, they examined crystals of a cuprate known as BSCCO, cooled to a few degrees above absolute zero. Getting the instrument sensitive enough took about four years.
The extra noise began rising right at the local pseudogap energy, at every spot measured and in samples of different chemical composition. In magnetic fields of up to six tesla, which suppress pairing, the noise fell back toward the single-electron level, as expected if pairs were responsible.
What it means
The measured pairing energy is well above the temperature at which the material superconducts. Swart said the picture is that pre-formed pairs appear first, and phase coherence, where pairs move together, sets in only at lower temperatures. Allan said this could mean the critical temperature might be pushed higher, but both researchers caution it will not be that simple. The experiment does not rule out that frozen patterns are intertwined with pairing, and the technique loses sensitivity as temperature rises, so it could not probe above the superconducting temperature.
