Correction to: Nature Physics, published online 10 February 2013.

In the version of this Article originally published, a normalization factor was missing in the function used to create the right side of the plot in Fig. 2b (where β > 2); that is, \({\mathrm{Re}}\lambda _ + = - \beta {\mathrm{/}}2 + \sqrt {\beta ^2 - 4}\) was used, but it should have been \({\mathrm{Re}}\lambda _ + = - \beta /2 + \sqrt {\beta ^2 - 4} {\mathrm{/}}2\) (as indicated in the text). As Fig. 2b was only used as an illustration, the conclusions in the Article do not change. The original and corrected Fig. 2b are shown below.

figure 2

Fig. 2b