Open Access
EPJ Appl. Metamat.
Volume 6, 2019
Article Number 20
Number of page(s) 6
Published online 07 November 2019

© Y. Liu et al., published by EDP Sciences, 2019

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.