19. Y. Zhang, K. He, C. Z. Chang, C. L. Song, L. L. Wang, X. Chen, J. F. Jia, Z. Fang, X. Dai, W. Y. Shan, S. Q. Shen, Q. Niu, X. L. Qi, S. C. Zhang, X. C. Ma, and Q. K. Xue, Crossover of the three-dimensional topological insulator Bi2Se3 to the two-dimensional limit, NATURE PHYSICS 6, 584 (2010)