1. 2.6 Electrical Quantum Metrology, Physikalisch-Technische Bundesanstalt (PTB), 38116 Braunschweig, Germany
2. Information Materials and Intelligent Sensing Laboratory of Anhui Province, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, China