Correction to: Nature Communications https://doi.org/10.1038/ncomms6404, published online 20 November 2014

This Article contains an error in Equation 2 in that the denominator is inverted. The correct form of Equation 2 is: \(D^ \ast = \frac{{\left( {{\rm{A}}f} \right)^{1/2}}}{{\left( {i_n{\mathrm{/}}R} \right)}}\). The error has not been fixed in the PDF or HTML versions of the Article.