Elastic constants and observation of significant elastic softening in superconducting Bi2Sr2CaCu2O8 single crystals

Signatures of the electronic nature of pairing in high-T(c) superconductors obtained by non-equilibrium boson spectroscopy

Understanding the pairing mechanism that gives rise to high-temperature superconductivity is one of the longest-standing problems of condensed-matter physics. Almost three decades after its discovery, even the question of whether or not phonons are involved remains a point of contention to some. Here we describe a technique for determining the spectra of bosons generated during the formation of Cooper pairs on recombination of hot electrons as they tunnel between the layers of a cuprate superconductor. The results obtained indicate that the bosons that mediate pairing decay over micrometre-scale distances and picosecond timescales, implying that they propagate at a speed of around 10⁶ m s⁻¹. This value is more than two orders of magnitude greater than the phonon propagation speed but close to Fermi velocity for electrons, suggesting that the pairing mechanism is mediated by unconventional repulsive electron–electron, rather than attractive electron–phonon, interactions.

The mechanism of high-temperature superconductivity remains a subject of debate. Krasnov et al. describe a technique for measuring the spectra of bosons generated during the formation of Cooper pairs in a cuprate, the results of which suggest that the process is governed by electron–electron interactions.