1. National Laboratory of Solid State Microstructures and
Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China
2. Key Laboratory
of Modern Acoustics of MOE and Institute of Acoustics, Department
of Physics, Nanjing University, Nanjing 210093, People’s Republic of China
3. Department
of Physics, Ningbo University, Ningbo 315211, People’s Republic of China