1. Electronic Materials Research LaboratoryKey Laboratory of the Ministry of Education & International Center for Dielectric ResearchXi'an Jiaotong University Xi'an 710049 P. R. China
2. Shaanxi Engineering Research Center of Advanced Energy Materials & DevicesXi'an Jiaotong University Xi'an 710049 P. R. China
3. Department of Nuclear Science and Engineering and Department of Materials Science and EngineeringMassachusetts Institute of Technology Cambridge MA 02139 USA