Physical Review Letters

Blackbody source and detector of ballistic quasiparticles in B3 : Emission angle from a wire moving at supercritical velocity

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

Published Aug 17, 1992• 128 citations

Abstract

Using a new quasiparticle detector and source consisting of a blackbody radiator, we have made the first measurements of the divergence of a beam of quasiparticles produced by a vibrating-wire resonator moving at supercritical velocities in superfluid $^{3}\mathit{B}$ at the zero-temperature limit. At velocities just above the critical velocity of \ensuremath{\Delta}/3${\mathit{p}}_{\mathit{F}}$ we see a very narrow beam directed along the direction of motion of the wire. As the maximum wire velocity increases the beam spreads to saturate at a half-angle divergence of approximately 55\ifmmode^\circ\else\textdegree\fi{} at a maximum velocity of order \ensuremath{\Delta}/${\mathit{p}}_{\mathit{F}}$.

DOI: 10.1103/physrevlett.69.1073

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Physical Review Letters

Blackbody source and detector of ballistic quasiparticles in B3 : Emission angle from a wire moving at supercritical velocity

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

Published Aug 17, 1992• 128 citations

Publisher

Abstract

^{3}\mathit{B}

at the zero-temperature limit. At velocities just above the critical velocity of \ensuremath{\Delta}/3

{\mathit{p}}_{\mathit{F}}

we see a very narrow beam directed along the direction of motion of the wire. As the maximum wire velocity increases the beam spreads to saturate at a half-angle divergence of approximately 55\ifmmode^\circ\else\textdegree\fi{} at a maximum velocity of order \ensuremath{\Delta}/

{\mathit{p}}_{\mathit{F}}

DOI: 10.1103/physrevlett.69.1073

Related papers

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Physical Review LettersSep 2, 199161 citations

^{3}\mathit{B}

at the zero-temperature limit. At velocities just above the critical velocity of \ensuremath{\Delta}/3

{\mathit{p}}_{\mathit{F}}

{\mathit{p}}_{\mathit{F}}

We have observed that the motion of a vibrating-wire resonator in superfluid

^{3}\mathit{B}

at temperatures below

{\mathrm{T}}_{\mathrm{c}}

/8 is associated with a sharp critical velocity,

{\mathrm{v}}_{\mathrm{c}}

, above which very high dissipation sets in. We identify this velocity with the Landau critical velocity for pair breaking. That the observed value of

{\mathrm{v}}_{\mathrm{c}}

is a factor of 2 lower than the critical velocity expected from the BCS value of the gap we take as evidence that the gap is suppressed by about 50% of its bulk value near a moving boundary.

The resistance to motion of a wire in superfluid

^{3}\mathit{B}

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}/

{\mathrm{p}}_{\mathrm{F}}

, 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.

We report the first observation of quantized circulation in superfluid

^{3}\mathit{B}

. The apparatus consists of a straight vibrating wire immersed in liquid

^{3}\mathrm{He}

, which is cooled by a rotating nuclear demagnetization cryostat. The experiment is carried out at about 250 \ensuremath{\mu}K. The superfluid at this temperature is in the ballistic quasiparticle regime. Circulation around the wire is found to be stable only when it takes on the values -h/2

{\mathit{m}}_{3}

, 0, and +h/2

{\mathit{m}}_{3}

, where h is Planck's constant and

{\mathit{m}}_{3}

is the mass of the

^{3}\mathrm{He}

atom. This experiment confirms that superfluid

^{3}\mathit{B}

is a Cooper-paired superfluid with a macroscopic quantum wave function.

We have observed the distortion of the gap and associated excitation spectrum in superfluid

^{3}\mathit{B}

with increasing magnetic field up to the first-order transition to the A phase at a temperature of

{\mathit{T}}_{\mathit{c}}

/T of about 7 (\ensuremath{\le}140 \ensuremath{\mu}K at P=0). The pair-breaking critical velocity of a vibrating-wire resonator is monitored as a function of field. By plotting the critical velocity we can follow changes in the B-phase excitation spectrum with increasing magnetic field, which are in excellent agreement with the calculations of Ashida and Nagai. The sudden transition to the A phase at the critical field is clearly seen.