Exceeding the Landau speed limit with topological Bogoliubov Fermi surfaces
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 v_{cL}, proposed by Landau, to zero. We check experimentally whether stable superflow is nevertheless possible in the polar phase of p wave superfluid ^{3}He, 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 v_{c}≈0.2 cm /s, despite the zero value of the Landau critical velocity. We suggest that the stability of the superflow above v_{cL} but below v_{c} 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.
 Publication:

Physical Review Research
 Pub Date:
 July 2020
 DOI:
 10.1103/PhysRevResearch.2.033013
 arXiv:
 arXiv:2002.11492
 Bibcode:
 2020PhRvR...2c3013A
 Keywords:

 Condensed Matter  Other Condensed Matter;
 Condensed Matter  Superconductivity
 EPrint:
 8 pages, 6 figures, version accepted to Phys. Rev. Research