1. Institut für Physik, Humboldt-Universität zu Berlin, Newtonstraße 15, 12489 Berlin, Germany
2. Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany
3. Frontier Institute of Science and Technology and State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Yanxiang Road 99, 710054 Xi'an, People's Republic of China