Dispersion of Magnetic Excitations in Optimally Doped SuperconductingYBa2Cu3O6.95

Abstract

Detailed neutron scattering measurements of ${\mathrm{Y}\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{u}}_3{\mathrm{O}}_{6.95}$ found that the resonance peak and incommensurate magnetic scattering induced by superconductivity represent the same physical phenomenon: two dispersive branches that converge near 41 meV and the in-plane wave vector ${\mathbf{q}}_{\mathrm{A}\mathrm{F}}=(\pi /a,\pi /a)$ to form the resonance peak. One branch has a circular symmetry around ${\mathbf{q}}_{\mathrm{A}\mathrm{F}}$ and quadratic downward dispersion from $\approx 41\mathrm{m}\mathrm{e}\mathrm{V}$ to the spin gap of $33\pm 1\mathrm{m}\mathrm{e}\mathrm{V}$. The other, of lower intensity, disperses from $\approx 41\mathrm{m}\mathrm{e}\mathrm{V}$ to at least 55 meV. Our results exclude a quartet of vertical incommensurate rods in $\mathbf{q}$-$\omega$ space expected from spin waves produced by dynamical charge stripes as an origin of the observed incommensurate scattering in optimally doped YBCO.