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Abstract
A common property of topological systems is the appearance of topologically protected zeroenergy excitations. In a superconductor or superfluid, such states set the critical velocity of dissipationless flow vcL, proposed by Landau, to zero. We check experimentally whether stable superflow is nevertheless possible in the polar phase of pwave superfluid 3He, which features a Dirac node line in the energy spectrum of Bogoliubov quasiparticles. The fluid is driven by rotation of the whole cryostat, and superflow breakdown is seen as the appearance of single or halfquantum vortices. Vortices are detected using the relaxation rate of a BoseEinstein condensate of magnons, created within the fluid. The superflow in the polar phase is found to be stable up to a finite critical velocity vc≈0.2cm/s, despite the zero value of the Landau critical velocity. We suggest that the stability of the superflow above vcL but below vc is provided by the accumulation of the flowinduced quasiparticles into pockets in the momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase, this surface has nontrivial topology which includes two pseudoWeyl points. Vortices forming above the critical velocity are strongly pinned in the confining matrix, used to stabilize the polar phase, and hence stable macroscopic superflow can be maintained even when the external drive is brought to zero.
Original language  English 

Article number  033013 
Number of pages  8 
Journal  PHYSICAL REVIEW RESEARCH 
Volume  2 
Issue number  3 
DOIs  
Publication status  Published  2 Jul 2020 
MoE publication type  A1 Journal articlerefereed 
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TOPVAC: From Topological Matter to Relativistic Quantum Vacuum
Volovik, G., Nissinen, J., Eltsov, V., Rysti, J., Mäkinen, J., Rantanen, R., Zhang, K., Volard, M. & Ikäheimo, A.
01/10/2016 → 30/09/2022
Project: EU: ERC grants