1. Department of Physics, University of Warwick, Coventry, West Midlands CV4 7AL, United Kingdom
2. Diamond Light Source Ltd., Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 ODE, United Kingdom
3. Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
4. Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
5. Institute FEMTO-ST, Université Bourgogne Franche-Comté, 26 rue de l'Epitaphe, 25030 Besançon, France