The effect of the pseudogap on suppressing high energy inelastic neutron scattering in superconducting YBa2Cu3O6.5

TL;DR

This study investigates high-energy spin fluctuations in YBa2Cu3O6.5, revealing a suppression of high-energy excitations linked to the pseudogap, with new observations of a high-energy gap and a doping-dependent peak.

Contribution

It provides new insights into high-energy spin dynamics and their relation to the pseudogap in cuprate superconductors, including the discovery of a high-energy gap and a doping-dependent excitation.

Findings

Isotropic momentum dependence at high energies

Significant reduction in spin excitation intensity above ~100 meV

Observation of a doping-dependent peak around 400 meV

Abstract

We have measured the spin fluctuations in the YBa2Cu3O6.5 (YBCO6.5, Tc=59 K) superconductor at high-energy transfers above ~ 100 meV. Within experimental error, the momentum dependence is isotropic at high-energies, similar to that measured in the insulator for two dimensional spin waves, and the dispersion extrapolates back to the incommensurate wave vector at the elastic position. This result contrasts with previous expectations based on measurements around 50 meV which were suggestive of a softening of the spin-wave velocity with increased hole doping. Unlike the insulator, we observe a significant reduction in the intensity of the spin excitations for energy transfers above ~ 100 meV similar to that observed above ~ 200 meV in the YBCO6.35 (Tc=18 K) superconductor as the spin waves approach the zone boundary. We attribute this high energy scale with a second gap and find agreement with measurements of the pseudogap in the cuprates associated with electronic anomalies along the antinodal positions. In addition, we observe a sharp peak at around 400 meV whose energy softens with increased hole doping. We discuss possible origins of this excitation including a hydrogen related molecular excitation and a transition of electronic states between d levels.