I.N. Adamenko, K.E. Nemchenko, V.A. Slipko, and A.F.G. Wyatt, Collective modes in superfluid helium when there is a relative velocity between the normal and superfluid components: Жидкий гелий, Low Temp. Phys. 34, (2008) [Fiz. Nizk. Temp. 34, 357-366, (2008)] DOI: https://doi.org/10.1063/1.2908889.