1. Center
for Condensed Matter and Materials Physics, Department of Physics, Beihang University, Beijing 100191, China
2. NIST
Center for Neutron Research, NIST, Gaithersburg, Maryland 20899-6102, United States
3. School
of Science, Beijing Jiaotong University, Beijing 100044, China
4. Department
of Nuclear Physics, Chinese Institute of Atomic Energy, Beijing 102413, China