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Physical Review Letters

Beyond the two-fluid model: Transition from linear behavior to a velocity-independent force on a moving object in B3

S. N. Fisher, A. M. Guénault, C. J. Kennedy, G. R. Pickett

Published Dec 4, 1989• 163 citations
Publisher

Abstract

Using a simple one-dimensional model, we show that the existence of the energy gap for excitations in an isotropic BCS superfluid leads to strongly nonlinear mechanical behavior of the liquid in the ballistic quasiparticle limit. The nonlinear damping of a vibrating wire in $^{3}\mathit{B}$ below 200 \ensuremath{\mu}K is explained, both in its velocity dependence and magnitude. At modest velocities (\ensuremath{\upsilon}>kT/${\mathrm{p}}_{\mathrm{F}}$), the damping force on an object moving through the superfluid becomes independent of velocity, an unexpected result with several interesting implications.

DOI
10.1103/physrevlett.63.2566

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Using a simple one-dimensional model, we show that the existence of the energy gap for excitations in an isotropic BCS superfluid leads to strongly nonlinear mechanical behavior of the liquid in the ballistic quasiparticle limit. The nonlinear damping of a vibrating wire in 3B^{3}\mathit{B}3B below 200 \ensuremath{\mu}K is explained, both in its velocity dependence and magnitude. At modest velocities (\ensuremath{\upsilon}>kT/pF{\mathrm{p}}_{\mathrm{F}}pF​), the damping force on an object moving through the superfluid becomes independent of velocity, an unexpected result with several interesting implications.
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The resistance to motion of a wire in superfluid 3B^{3}\mathit{B}3B has three contributions: that due to the internal friction of the resonator; that due to pair breaking; and that due to the quasiparticle gas. We have been able to separate quasiparticle contribution and find that it is highly nonlinear with velocity on the scale of the Landau velocity, \ensuremath{\Delta}/pF{\mathrm{p}}_{\mathrm{F}}pF​, and falls with increasing velocity. This indicates that the quasiparticle resistance is also governed by the condensate, almost certainly through Andreev reflection in the flow field around the wire.
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