Absence of strong coupling in YBa₂Cu₃0₇inferred from infrared conductivity

Abstract

THE mechanism of superconductivity in the recently discovered layered copper oxide compounds1 is still unknown. In the conventional theory, electron pairing mediated by the exchange of low-energy bosons leads to superconductivity. If the boson energies are not too large compared to the energy gapΔ (that is, slash h signωo≲ 10Δ), then the interaction of the electron-pair quasiparticles with these bosons leads to an enhancement of the ratio 2ΔKTc (where Tc is the superconducting transition temperature), as well as to modifications to the spectrum of excitations above the energy gap. The term 'strong coupling' is used to denote these phenomena2,3. Infrared spectroscopy can be used to probe these modifications, and thus to measure the spectrum of mediating excitations in strongly coupled superconductors. Here we report infrared conductivity data for YBa2Cu3O7, and show that even up to energies of ≳0.3 eV there is no evidence for a pair-mediating retarded interaction of significant strength. These results suggest that a conventional picture of superconductivity, involving mediation by the exchange of bosons of energy much less than the Fermi energy (EF≈0.5eV), is unlikely to be adequate for YBa2Cu3O7. © 1990 Nature Publishing Group.