High-T_c superconductivity in the cuprates arises in close proximity to a charge-density wave (CDW) phase. A challenge in the field is to understand how both phenomena compete and whether, behind pure competition, there is a more involved relationship between them. To tackle this question, experiments have used temperature, magnetic field, hole-doping or uniaxial strain as tuning parameters. The effect of hydrostatic pressure, on the other hand, is controversial. Here, using nuclear magnetic resonance (NMR) in YBa₂Cu₃O_y, we find that the short-range CDW in the normal state as well as the long-range CDW in high fields are weakened only slightly at a pressure of 1.9 GPa. Quantitative analysis of the data supports the hypothesis that virtually all of the increase in Tc upon increasing pressure arises from a gradual decrease of CDW strength up to ~15 GPa. We propose explanations for some of the conflicting conclusions drawn from different experiments and suggest that they may actually shed light on the CDW.

Work done with I. Vinograd, R. Zhou, H. Mayaffre, S. Krämer (LNCMI Grenoble), R. Liang, W.N. Hardy, D.A. Bonn (University of British Columbia)