Disentangling the Electronic and Phononic Glue in a High- T c Superconductor

The phenomenon of superconductivity, in which a material suddenly (below a certain transition temperature T c ) becomes a perfect conductor with zero electrical resistance, can be roughly explained in terms of Bose-Einstein condensation of pairs of electrons. In conventional superconductors, the formation of these so-called Cooper pairs is mediated by lattice deformations (phonons), but this mechanism is insufficient to explain the high T c of cuprate superconductors. Other mechanisms, such as magnetic fluctuations, have been proposed which originate with the electrons themselves rather than the lattice. Dal Conte et al. (p. 1600 ) used time-resolved optical spectroscopy of an optimally doped cuprate to show that the temporal evolution of the reflectivity is consistent with the electronic contribution being dominant and is able to account for the high T c by itself. A time-resolved optical technique resolves the influence of lattice dynamics on electron pairing in a cuprate. Unveiling the nature of the bosonic excitations that mediate the formation of Cooper pairs is a key issue for understanding unconventional superconductivity. A fundamental step toward this goal would be to identify the relative weight of the electronic and phononic contributions to the overall frequency (Ω)–dependent bosonic function, Π(Ω). We performed optical spectroscopy on Bi 2 Sr 2 Ca 0.92 Y 0.08 Cu 2 O 8+δ crystals with simultaneous time and frequency resolution; this technique allowed us to disentangle the electronic and phononic contributions by their different temporal evolution. The spectral distribution of the electronic excitations and the strength of their interaction with fermionic quasiparticles fully account for the high critical temperature of the superconducting phase transition.