Nature Physics 9, 480–484 (2013); published online 2 July 2013; corrected after print 19 August 2013.

In the version of this Letter originally published, the second author affiliation should have read “The Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu 980-8578, Taiwan”. This error has now been corrected in the HTML and PDF versions of the Letter.