1. Oak Ridge National Laboratory, PO Box 2008 MS 6430, Oak Ridge, Tennessee 37831-6430, USA
2. Department of Condensed Matter Physics, University of Oxford, Clarendon Laboratory Parks Road, Oxford OX1 3PU, UK
3. CEA (MDN/SPSMS/DRFMC), 17 Ave. des Martyrs, 38054 Grenoble cedex 9, France
4. Materials Research Department, Risø National Laboratory, 4000 Roskilde, Denmark
5. Ørsted Laboratory, Niels Bohr Institute for APG, Universitetsparken 5, DK 2100, Copenhagen, Denmark
6. NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540-6634, USA
7. BENSC, Hahn-Meitner Institut, Glienicker Strasse 100, 14109 Berlin, Germany
8. Department of Advanced Materials Science, Graduate School of Frontier Sciences, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
9. Experimental Facilities Division, Spallation Neutron Source, 701 Scarboro Road, Oak Ridge, Tennessee 37830, USA

Correspondence to: B. Lake^1,2 Correspondence and requests for materials should be addressed to B.L. (e-mail: Email: bella.lake@physics.ox.ac.uk).

Abstract

One view of the high-transition-temperature (high-T_c) copper oxide superconductors is that they are conventional superconductors where the pairing occurs between weakly interacting quasiparticles (corresponding to the electrons in ordinary metals), although the theory has to be pushed to its limit¹. An alternative view is that the electrons organize into collective textures (for example, charge and spin stripes) which cannot be 'mapped' onto the electrons in ordinary metals. Understanding the properties of the material would then need quantum field theories of objects such as textures and strings, rather than point-like electrons^{2, 3, 4, 5, 6}. In an external magnetic field, magnetic flux penetrates type II superconductors via vortices, each carrying one flux quantum⁷. The vortices form lattices of resistive material embedded in the non-resistive superconductor, and can reveal the nature of the ground state—for example, a conventional metal or an ordered, striped phase—which would have appeared had superconductivity not intervened, and which provides the best starting point for a pairing theory. Here we report that for one high-T_c superconductor, the applied field that imposes the vortex lattice also induces 'striped' antiferromagnetic order. Ordinary quasiparticle models can account for neither the strength of the order nor the nearly field-independent antiferromagnetic transition temperature observed in our measurements.