1. Physics Department and Institute for Nanoscience and Engineering; University of Arkansas; Fayetteville AK 72701 USA
2. Electronic Materials Research Laboratory-Key Laboratory of the Ministry of Education and International Center for Dielectric Research; Xi'an Jiaotong University; Xi'an 710049 China
3. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB; 08193 Bellaterra Spain