Neutron scattering as a probe of unconventional superconductivity in YBa2Cu3O7 (invited) (abstract)

Abstract

We have carried out a variety of neutron scattering experiments on the high temperature superconductor YBa2Cu3O7 which elucidate the unconventional microscopic properties of the cuprates and illustrate the power and versatility of neutron scattering as a probe of novel materials. Small angle magnetic neutron diffraction is thus far the only technique capable of revealing the structure of the vortex lattice in the cuprate superconductors in magnetic fields of several tesla. We have shown that in YBa2Cu3O7 the vortex lattice in this field range has an oblique structure when the field is applied along the c axis (perpendicular to the CuO2 layers). Recent theoretical work has attributed this unusual structure to the unconventional (d-wave) pairing state realized in the cuprates. We will also discuss the rich behavior of the vortex lattice when the field is tilted with respect to the c axis. We have used inelastic neutron scattering to elucidate the magnetic and lattice vibrational excitations in YBa2Cu3O7. Magnetic and phonon scattering were separated both by detailed calculations of phonon dynamical structure factors and by neutron polarization analysis. A novel resonant magnetic excitation at wave vector q=(π/a,π/a) and energy ℏω=40 meV appears in the superconducting state. The simplest model for this resonance relies on quasiparticle pair creation and implies a sign reversal of the energy gap function on the Fermi surface. Alternative models will also be discussed. We have also observed a strongly wave vector dependent renormalization of the energy and lifetime of a specific phonon in the superconducting state of YBa2Cu3O7. Since the phonon self-energy is sensitive to the anisotropies of the energy gap and Fermi surface via the electron–phonon interaction, our data contain important information about these parameters.