R.-J. Duan would like to thank Prof. Peter Markowich and Dr. Massimo Fornasier for their support during the postdoctoral studies of the year 2008-2009 in RICAM. K. Fellner's work has been supported by the KAUST Award No. KUK-I1-007-43, made by King Abdullah University of Science and Technology (KAUST). The research of C.-J. Zhu was supported by the National Natural Science Foundation of China #10625105 and The Key Laboratory of Mathematical Physics of Hubei Province.