1. Department of Physics and Astronomy and Petersen Institute for NanoScience and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
2. Donostia International Physics Center (DIPC), San Sebastián, Donostia, Spain, Centro de Física de Materiales (CFM-MPC), CSIC-UPV/EHU, San Sebastián 20018, Spain, and Departamento de Física de Materiales, UPV/EHU, Facultad de Quimica, San Sebastián, Spain
3. Physics Department, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui, China.