A collective mode of electrons predicted to exist in high-temperature superconductors, but difficult to observe in experiments has been identified by physicists at the Massachusetts Institute of Technology (MIT). The finding could advance our understanding of these materials, they say. According to the Bardeen–Cooper–Schrieffer (BCS) theory, superconductivity occurs when electrons in a material overcome their mutual electrical repulsion to form electron pairs. These Cooper pairs, as they are known, can then travel unhindered through the material as a supercurrent without scattering off phonons (quasiparticles arising from vibrations of the material’s crystal lattice) or other impurities. Cooper pairing is characterized by a tell-tale energy gap near the Fermi level, which is the highest energy level that electrons can occupy in a solid at absolute zero temperature.…