If there are more than six names listed, use et al. after the first author.
[11] F. Gao et al., “Modal analysis of a PMSG-based dc electrical power system in the more electric aircraft using eigenvalues sensitivity,” IEEE Trans. Transport. Electrific. Vol. 1, no. 1, pp. 65-76, Jun 2015. [Online]. doi:10.1109/TTE.2015.2427312.
[12] T. Seppalainen, "Directed random growth models on the plane", in Analysis and stochastics of growth processes and interface models, P. Morters, et al., Eds. Oxford: Oxford University Press, 2008, pp. 9-38. [Online] Available: Oxford Scholarship Online.