Nature Physics 4, 108–111 (2008); published online 20 January 2008; corrected after print 15 January 2009

In the version of this article originally published, Fig. 1c depicted a CuO plane, instead of the intended CuO2 plane. The lattice should have appeared as below. The text of the article and conclusions reached are unaffected. The error has been corrected in the HTML and PDF versions of the article.