1. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, USA
2. NEC Research Institute, Princeton, New Jersey 08540, USA
3. H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK
4. Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA

Correspondence to: Pengcheng Dai¹ Correspondence and requests for materials should be addressed to P.D. (e-mail: Email: piq@ornl.gov).

Abstract

One of the most striking properties of the high-transition-temperature (high-T_c) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset^{1, 2, 3, 4}, because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high-T_c superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy⁵. For YBa₂Cu ₃O_6+x, where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6,7,8,9,10,11). Here we show that for underdoped YBa₂Cu ₃O_6.6, where x and T_c are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO₂ planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high-T_c superconductivity. The persistence of a field effect above T_c favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors^{12, 13, 14}, awaiting transition to the superconducting state.