Coupling strength of charge carriers to spin fluctuations in high-temperature superconductors

In conventional superconductors, the most direct evidence of the mechanism responsible for superconductivity comes from tunneling experiments in which a clear image of the electron-phonon interaction is revealed. The observed structure in the current voltage characteristics at the phonon energies can be used to measure, through inversion of the Eliashberg equations, the electron phonon spectral density \alpha^2F(\omega). The coherence length in conventional materials is long and the tunneling process probes several atomic layers into the bulk of the superconductor. On the contrary, in the high-T_c oxides, particularly for c-axis tunneling, the coherence length can be quite short and in an optical experiment or in neutron scattering experiments the bulk of the sample is probed. Therefore, these spectroscopies become the methods of choice for the investigation of mechanisms of high-T_c superconductivity. Accurate reflectance measurements in the infrared range and precise polarized neutron scattering data are available for a variety of oxides. In this paper we show that conducting carriers studied by means of infrared spectroscopy reveal strong coupling to a resonance structure in the spectrum of spin fluctuations examined with neutron scattering. The coupling strength inferred from experiment is sufficient to account for high values of T_c which signals the prominent role of spin excitations in the superconductivity of oxides.