Nature Communications 7: Article numbber:10699 (2015); Published: 7 March 2016; Updated: 19 July 2016.

Technical support for this Article was not fully acknowledged. The Acknowledgements should have included the following: We are also grateful to Dr Alexei Barinov for his kind help during our beamtime at the spectramicroscopy beamline of the Elettra synchrotron.