Funding for this research was provided by:
National Science Foundation (DMS-1118971)
U.S. Department of Energy (SC0013838)
National Natural Science Foundation of China (11371220, 11671005)
Article History
Received: 15 January 2017
Revised: 14 March 2017
Accepted: 18 March 2017
First Online: 9 April 2017