Exceeding the Landau speed limit with topological Bogoliubov Fermi surfaces

Abstract

A common property of topological systems is the appearance of topologically protected zero-energy 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 half-quantum vortices. Vortices are detected using the relaxation rate of a Bose-Einstein 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 flow-induced quasiparticles into pockets in the momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase, this surface has nontrivial topology which includes two pseudo-Weyl 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.

Non-vanishing zero-temperature normal density in holographic superfluids

“…In such cases, it would be natural to expect that these fermionic-like, uncondensed degrees of freedom lead to a non-vanishing normal density at zero temperature, as in superfluid systems where Bogoliubov Fermi surfaces are formed, see e.g. [55].…”

“…In [32], superfluid flows in a top-down Type IIb embedding were considered and similar results obtained: above a certain value of the superfluid velocity, the infrared geometry ceases to be another copy of Anti de Sitter spacetime. It would be interesting to further study these systems and determine the relation, if any, to the formation of Bogoliubov Fermi surfaces in weaklycoupled superfluids at large superfluid velocities, [55].…”

$$^3$$He Universe 2020

“…In the moving polar phase of 3 He, the Bogoliubov Fermi surface has an exotic shape: it consists of two Fermi pockets which touch each other at two pseudo-Weyl points [34]. In cuprate superconductors, the local Bogoliubov Fermi surfaces caused by supercurrents around Abrikosov vortices lead to the √ H field dependence of the electronic density of states in the vortex state of the superconductor [35].…”

Suppressing the Kibble-Zurek Mechanism by a Symmetry-Violating Bias

“…Measuring the initial density of KZ defects has traditionally been difficult due to the fast annihilation of nonequilibrium defects at temperatures close to the phase transition [3,4,10,43]. In our experiments the confining strands pin vortices in place [9,30,41], providing the observer a frozen window to the out-of-equilibrium physics of the phase transition and a direct measurement of the KZ vortex density. We calibrate the size of the satellite peak by preparing a state by a very slow cooldown through the critical temperature T c at H ¼ 0 while the sample is in rotation.…”

“…Now the homotopy group is π 1 ðG=ΥÞ ¼ Z, which means that only SQVs remain stable, since they are not influenced by the spin-orbit interaction. Spin vortices and HQVs become termination lines of topological solitons [9,30,[40][41][42], as illustrated in Fig. 1 The presence of the solitons can be detected and their total volume in the sample measured using the nuclear magnetic resonance (NMR) techniques.…”