Nature Communications 4: Article number: 1536 (2013); Published: 26 February 2013; Updated: 17 October 2013.

The funding for this Article was not fully acknowledged. The Acknowledgements should have read:

This work was funded by DOE, Office of Basics Energy Sciences through DE SC 0004889, and by the National Science Foundation through MRSEC DMR-1119826. We thank Sindee Simon and Dan Miracle for useful discussions.