Author contributions
Yann Hautier and Andy Hector conceived and designed the experiment at the University of Zurich and Pengfei Zhang, Xiaolong Zhou and Guozhen Du at Lanzhou University. Yann Hautier performed the experiment at the University of Zurich and Pengfei Zhang, Xiaolong Zhou and Zhi Guo at Lanzhou University. Pengfei Zhang and Yann Hautier analyzed the data with input from all authors. Pengfei Zhang and Yann Hautier wrote the manuscript with inputs from all authors.