Fundamental dissipation due to bound fermions in the zero-temperature limit

Autti, Samuli and Ahlstrom, Sean and Haley, Richard and Jennings, Ash and Pickett, George and Poole, Malcolm and Schanen, Roch and Soldatov, A. A. and Tsepelin, Viktor and Vonka, Jakub and Wilcox, Tom and Woods, Andrew and Zmeev, Dmitry (2020) Fundamental dissipation due to bound fermions in the zero-temperature limit. Nature Communications, 11: 4742. ISSN 2041-1723

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Abstract

The ground state of a fermionic condensate is well protected against perturbations in the presence of an isotropic gap. Regions of gap suppression, surfaces and vortex cores which host Andreev-bound states, seemingly lift that strict protection. Here we show that in superfluid 3He the role of bound states is more subtle: when a macroscopic object moves in the superfluid at velocities exceeding the Landau critical velocity, little to no bulk pair breaking takes place, while the damping observed originates from the bound states covering the moving object. We identify two separate timescales that govern the bound state dynamics, one of them much longer than theoretically anticipated, and show that the bound states do not interact with bulk excitations.