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 $v_{\mathrm{cL}}$, proposed by Landau, to zero. We check experimentally whether stable superflow is nevertheless possible in the polar phase of $p$-wave superfluid ${}^3\mathrm{He}$, 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 $v_{\mathrm{c}}\approx 0.2\mathrm{cm}$/s, despite the zero value of the Landau critical velocity. We suggest that the stability of the superflow above $v_{\mathrm{cL}}$ but below $v_{\mathrm{c}}$ 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.

中文翻译：

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拓扑Bogoliubov Fermi曲面超过了Landau速度极限

拓扑系统的一个共同属性是出现拓扑受保护的零能量激发。在超导体或超流体中，这些状态设置了无耗散流的临界速度$v_L$，由Landau提出，为零。我们通过实验检查了是否在极地的极性阶段仍可以实现稳定的超流$p$波超流体 ${}^3他$，在Bogoliubov准粒子的能谱中具有Dirac结线。流体由整个低温恒温器的旋转驱动，并且超流破裂被视为单量子涡旋或半量子涡旋的出现。使用在流体中产生的磁振子的玻色-爱因斯坦冷凝物的弛豫率来检测涡旋。发现在有限的临界速度下，极性相中的超流是稳定的$v_{\mathrm{C}}\approx 0.2\mathrm{厘米}$/ s，尽管Landau临界速度为零。我们建议上面的超流的稳定性$v_L$ 但低于 $v_{\mathrm{C}}$流动引起的准粒子在动量空间中以Bogoliubov Fermi曲面为边界的口袋中的堆积提供了动力。在极性阶段，此表面具有非平凡的拓扑结构，其中包括两个伪魏尔点。在临界速度之上形成的涡流被牢固地固定在约束矩阵中，用于稳定极性相位，因此即使将外部驱动器调零，也可以保持稳定的宏观超流量。