Physicists listened to electrical noise to crack a superconductor mystery

Cuprate superconductors can carry electricity without resistance at temperatures far above those tolerated by conventional superconductors, but the reason remains unclear. A central puzzle is the “pseudogap”: above the superconducting transition, measurements show fewer electrons at certain energies than expected. Some explanations attribute this to electrons freezing into patterns; another proposes that electrons pair up before the material becomes superconducting, with those pairs only later beginning to flow in a coordinated way.

A team tested for pairing by measuring shot noise - the tiny fluctuations produced as charges cross between a scanning-tunneling-microscope tip and a cuprate crystal. Paired electrons should create stronger noise than individual electrons. In BSCCO samples, the extra noise began at the energy of the pseudogap, tracked local changes in that energy, and diminished under magnetic fields that suppress pairing. The results indicate that pairing contributes to the pseudogap, though they do not rule out a role for frozen patterns too. The measured pairing energy exceeds the superconducting transition temperature, supporting a sequence in which pairs form first and coordinated flow, or phase coherence, develops later. This suggests that enabling existing pairs to move together may be a bigger obstacle than forming pairs, but it does not mean room-temperature superconductivity is within easy reach. The technique has yet to measure the relevant noise above the transition temperature, leaving open what limits superconductivity and how the two stages are connected.