Exceeding the Landau speed limit with topological Bogoliubov Fermi surfaces

The authors show that topological superfluid 3He can flow without friction in a phase which possesses a line of zero energy in the excitation spectrum, although the Landau's limit for superflow is zero. The flow expands the node line to a Fermi surface for Bogoliubov quasipartices, which is usually absent in Cooper-paired systems, but may appear in unconventional superconductors and superfluids with certain broken symmetries.