1. Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA
2. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
3. National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA

Correspondence to: W. P. Halperin¹ Correspondence and requests for materials should be addressed to W.P.H. (e-mail: Email: w-halperin@northwestern.edu).

Abstract

Puzzling aspects of high-transition-temperature (high-T_c) superconductors include the prevalence of magnetism in the normal state and the persistence of superconductivity in high magnetic fields. Superconductivity and magnetism generally are thought to be incompatible, based on what is known about conventional superconductors. Recent results¹, however, indicate that antiferromagnetism can appear in the superconducting state of a high-T_c superconductor in the presence of an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines, each of which represents a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there². Here we report the results of a high-field nuclear-magnetic-resonance (NMR) imaging experiment^{3, 4, 5} in which we spatially resolve the electronic structure of near-optimally doped YBa₂Cu₃O_7- inside and outside vortex cores. Outside the cores, we find strong antiferromagnetic fluctuations, whereas inside we detect electronic states that are rather different from those found in conventional superconductors.