For nearly four decades, one question has stalked condensed matter physics: what actually binds electrons into superconducting pairs in the cuprates, the copper oxide materials that superconduct at temperatures far beyond anything conventional theory can explain? In conventional superconductors, described by the Bardeen–Cooper–Schrieffer (BCS) framework, the answer is elegant and settled. Electrons exchange virtual phonons, quantized vibrations of the crystal lattice, and this interaction welds them into Cooper pairs. The same electron–phonon coupling that creates the pairs also scatters electrons in the normal, resistive state, which means the critical temperature and the strength of a characteristic linear-in-temperature scattering rate rise and fall together through a single coupling constant. In the cuprates, no such relationship had ever been convincingly identified, leaving the pairing mechanism shrouded in uncertainty. Now, a study published in Nature Physics by Caitlin Duffy of Radboud University and the University of Grenoble Alpes, together with S. J. Tu, Q. H. Chen, Kui Jin, Nigel Hussey and colleagues in China, the Netherlands and the United Kingdom, reports the clearest experimental demonstration yet that in electron-doped cuprates, superconductivity is glued by magnetic spin fluctuations rather than lattice vibrations.
The team focused on lanthanum cerium copper oxide, La2−xCexCuO4, known as LCCO, an electron-doped cuprate in which cerium atoms donate electrons to the copper oxide planes. Measuring resistivity and magnetoresistance across the entire overdoped region of the phase diagram, they found a striking correlation between the strength of scattering by antiferromagnetic spin fluctuations and the superconducting transition temperature. As doping increased and the transition temperature fell from near its optimum of about 25 kelvin to zero, the strength of the anisotropic, temperature-linear scattering dropped by a full order of magnitude, mirroring the decline in superconductivity. In other words, the two quantities tracked each other just as the electron–phonon coupling constant links transition temperature and scattering in ordinary metals, but here the coupling was to magnetic, not lattice, excitations.
What makes this result remarkable is not merely the correlation itself, which had been glimpsed before, but the experimental rigor with which it was isolated. In overdoped cuprates, the coefficient of the linear-in-temperature resistivity, usually labeled A1, is known to fall in step with the critical temperature, and a similar relationship has been reported in organic Bechgaard salts and in the iron-based superconductor BaFe2Co2As2. But resistivity is a blunt instrument. It depends on carrier density, impurity content and band structure as well as on scattering, so it cannot on its own reveal whether the temperature-linear term really reflects a coupling whose strength varies. Alternative interpretations have flourished. Some researchers argued that linear resistivity in cuprates reflects universal Planckian dissipation, a quantum bound at which the scattering rate reaches roughly kBT divided by the reduced Planck constant, so that A1 would be fixed by thermodynamic quantities rather than by any pairing glue. Others questioned whether the scattering rate in cuprates is truly linear in temperature at all. Without a way to extract the scattering rate itself, the debate stalled.
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