Experiments on quantum vortices in a pure superfluid condensate, 3 - - PowerPoint PPT Presentation

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Experiments on quantum vortices in a pure superfluid condensate, 3 - - PowerPoint PPT Presentation

Mar 08, 2023 365 likes •945 views

Experiments on quantum vortices in a pure superfluid condensate, 3 He-B at ultralow temperatures. S.N.Fisher Lancaster Quantum Fluids Ian Bradley Pam Crookston Viktor Efimov Matt Fear Shaun Fisher George Foulds Deepak Garg Mark Giltrow

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Experiments on quantum vortices in a pure superfluid condensate,

3He-B at ultralow temperatures.

S.N.Fisher

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Lancaster Quantum Fluids Ian Bradley Pam Crookston Viktor Efimov Matt Fear Shaun Fisher George Foulds Deepak Garg Mark Giltrow Andrei Ganshyn Tony Guénault Richard Haley Matthew Holmes Martin Jackson Oleg Kolosov Chris Lawson Peter McClintock Ian Miller Alan Stokes Samantha O’Sullivan Roch Schanen David Potts George Pickett Viktor Tsepelin Nikolai Vasilev Martin Ward Paul Williams Louise Wheatland Peter Skyba (Kosice) Joe Vinen (Birmingham)

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~1 in 104000 @ T~80μK

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Quantum Vortices (3He-B and 4He)

core size:

4He : ξ0 ~ 0.1 nm 3He : ξ0 ~ 65 - 15 nm (pressure dep.)

2π phase change around core Gives circulating superfluid flow,

vS = κ/2πr

circulation :

κ4 = h/m4 κ3 = h/2m3

Classical Vortices (eddies)

can have a wide range of shapes and sizes.

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Vortices can end on cell walls Form self propagating Rings Or form a tangle (Quantum Turbulence)

Leonardo da Vinci 1515 d~5μm ⇒ u~10mm s-1 Inter-vortex spacing l Line Density L=1/l2 (line length per unit volume)

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Vortices produced by a rapid phase transition

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Cosmological Analogue: Phase Transitions after the Big Bang

Maybe Cosmic Stings produced here ? (Kibble Mechanism)

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neutron

n + 3He p + 3H + 764keV

The “Big Bang”

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Hot Expanding Universe

(normal 3He)

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The Phase Transition

(to superfluid 3He)

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Ordering produces domains, limited by causality (fast transition gives small domains)

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The order parameter smoothes, leaving defects (Cosmic strings / vortices)

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Line defects form a random tangle

(Quantum Turbulence)

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The tangle may evolve very slowly (and may store a lot of energy)

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The detector is calibrated (using the heater wire to input a known energy) which then allows us to determine the energies of individual events.

6.5 6.0 5.5 5.0 4.5 Thermometer damping (Hz) 13:00 13:15 13:30 13:45 14:00 Time (hours:mins) The damping of the thermometer wire in the box with an external neutron source.

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Good agreement with the `Cosmological’ model (Kibble-Zurek mechanism) Energy deficit measures the amount of vortices produced

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Vortices produced by annihilation of phase boundaries

(analogous to Brane-collisions in cosmology, which may have triggered inflation)

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First with B-phase only (with a magnetic field JUST below what is needed to create the A-phase slice.)

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Now with the A-phase slice present Extra impedance from the A-phase and AB-BA phase boundaries

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After annihilation, we do NOT go back to the

  • riginal state.
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Vortex Production by a vibrating Grid in 3He-B Intrinsic damping (approx. linear) `Turbulent’ damping F∝ v2

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The excitation dispersion curve is tilted by superflow (energies are shifted by pF.v). Liquid static Liquid moving

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~10μm

Andreev Scattering by vortex lines

The flow around a vortex, Andreev reflects excitations (particles on one side and holes on the other side).

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Vibrating Wire Resonator

a loop of superconducting wire is placed in a magnetic field and set into motion by passing an ac current through it .

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B

Vibrating Wire Resonator

a loop of superconducting wire is placed in a magnetic field and set into motion by passing an ac current through it .

Vo exp(iωt) Io exp(iωt)

Damped by Quasiparticles

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B

Vibrating Wire Resonator

a loop of superconducting wire is placed in a magnetic field and set into motion by passing an ac current through it .

Vo exp(iωt) Io exp(iωt)

Quasiparticle damping reduced by surrounding vortices

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The wire damping is suppressed when the grid is oscillated. This is our `vortex signal’.

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Note the noisy signal – we see fluctuations in the vortex configuration.

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First, focus on the decay

  • f the vorticity after

we switch off the grid.

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Based on simulations by Makoto Tsubota’s group. The grid frequency, 1300Hz, predominantly excites 5μm diameter loops.

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Simulations by Makoto Tsubota’s group. At low ring production rates, the rings are ballistic and travel with their self-induced velocity with almost no interaction.

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Simulations by Makoto Tsubota’s group. At higher ring production rates, the rings collide to produce a vortex tangle (quantum turbulence).

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Simulations by Makoto Tsubota’s group. The quantum turbulence then decays relatively slowly.

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Arrows give ange of 5 micron rings based on mutual friction measurements by Bevan et. al. JLTP 109, 243 (1997).

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Decay of Pure Quantum Turbulence

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Richardson cascade - Kolmogorov spectrum

E(k)=Cε2/3 k -5/3

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Viscous Dissipation UR/ν~1

Classical cascade model (assuming ω=κL) predicts: Vortex Line Density at late times, L=D/2πκ (27C/ν)1/2 t-3/2 ν = `effective’ kinematic viscosity. Reynolds Number, Re=UR/ν >> 1

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Decay of Pure Quantum Turbulence

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Turbulent Fluctuations

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Power spectrum of turbulent fluctuations

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Power spectrum of turbulent fluctuations

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Cross-correlation of turbulent fluctuations

Peak at Δt ∼ − 2s, suggests the tangle has a net outward velocity

  • f ~0.5mm/s.
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Turbulent fluctuations observed in the most recent experiment

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Cross-correlation due to ballistic rings (grid V=1.8mm/s)

Cross-correlation of vortex ring signal (grid v=1.8mm/s)

Peak at Δt ∼ − 0.15s for 2mm separation, implies a ring velocity of ∼ 13mm/s.

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Cross-correlation due to ballistic rings (grid V=2.6mm/s)

Cross-correlation of vortex signal (grid v=2.6mm/s)

Long tail develops as vortex tangle starts to form.

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Cross-correlation due to ballistic rings (grid V=3.3mm/s)

Tangle almost fully developed (note the time axis is 10 times longer).

Cross-correlation of vortex signal (grid v=3.3mm/s)

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Cross-correlation due to ballistic rings (grid V=5.7mm/s)

Cross-correlation of vortex tangle signal (grid v=5.7mm/s)

Peak at Δt ∼ − 2s, for 1mm separtation, suggests the tangle has a net outward velocity

  • f ~0.5mm/s.
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Quasiparticle Imaging

Heated Radiator Box produces a beam of ballistic quasiparticles

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Quasiparticle Imaging

Array of detectors (e.g. tuning forks) produce an image of the excitation beam flux.

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Quasiparticle Imaging

We can then image the quasiparticle shadows cast by vortices or other superfluid structures. Can anything like this be done in superconductors ?