Lin, Y., Zhang, L., Mao, H.-K., Chow, P., Xiao, Y., Baldini, M., Shu, J. & Mao, W. L.
Phys. Rev. Lett
Zeng, Z., Yang, L., Zeng, Q., Lou, H., Sheng, H., Wen, J., Miller, D.J., Meng, Y., Yang, W., Mao, W.L. & Mao, H.-K. Nature Communication (in press).
 Electron microscopy was performed at the Center for Nanoscale Materials at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. The authors acknowledge the financial support of the National Natural Science Foundation of China (grant U1530402).