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Outline of my talk: First, we need a quick magic mystery tour around superconducting 3 He. PowerPoint Presentation
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Outline of my talk: First, we need a quick magic mystery tour around superconducting 3 He.

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Outline of my talk: First, we need a quick magic mystery tour around superconducting 3 He.

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Outline of my talk: First, we need a quick magic mystery tour around superconducting 3 He.

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  1. Outline of my talk: First, we need a quick magic mystery tour around superconducting 3He. A quick explanation of our (very simple) experimental tools: Two experiments: 1) Simulation of cosmic string creation. 2) Simulation of brane annihilation

  2. Outline of my talk: First, we need a quick magic mystery tour around superconducting 3He. A quick explanation of our (very simple) experimental tools: Two experiments: 1) Simulation of cosmic string creation. 2) Simulation of brane annihilation

  3. Outline of my talk: First, we need a quick magic mystery tour around superconducting 3He. A quick explanation of our (quite simple) experimental tools: Two experiments: 1) Simulation of cosmic string creation. 2) Simulation of brane annihilation

  4. Outline of my talk: First, we need a quick magic mystery tour around superconducting 3He. A quick explanation of our (quite simple) experimental tools: Two experiments: 1) Simulation of cosmic string creation (which sparked off COSLAB). 2) Simulation of brane annihilation

  5. We can cool the liquid to ~80mK This gives a purity of = ~1 in 104000

  6. The liquid us therefore absolutely pure even before we think anything about the superfluidity aspect.

  7. The superfluid state emerges as 3He atoms couple across the Fermi sphere to create the Cooper pairs. Pz Px Py

  8. The superfluid state emerges as 3He atoms couple across the Fermi sphere to create the Cooper pairs. Pz Py Px

  9. Since 3He atoms are massive, p-wave pairing is preferred, i.e. L = 1 which means S must also be 1. The ground state thus has S = 1 and L = 1 making the Cooper pairs like small dimers (and easier to visualise than the s-wave pairs in superconductors).

  10. With S = L = 1 we have a lot of free parameters and the superfluid can exist in several phases.

  11. With S = L = 1 we have a lot of free parameters and the superfluid can exist in several phases (principally the A- and B-phases) . Let us start with the A phase which has only equal spin pairs. The directions of the S and L vectors are global properties of the liquid as all pairs are in the same state (this is the “texture” of the liquid). However, that causes problems for the pairs.

  12. Assume the global L vector lies in the z-direction -

  13. Assume the global L vector lies in the z-direction - We can easily have pairs like this:- L-vector That is fine as the constituent 3He fermion states can simply orbit the “equator” of the Fermi sphere:

  14. However, if we try to couple pairs across the “poles” of the Fermi sphere there is no orbit that these pairs can make which gives a vertical L. Thus the liquid is a good superfluid in the equatorial plane and lousy at the poles – this is reflected in the A-phase energy gap:-

  15. D The A-phase gap:- large round the equator, zero at the poles. (because there are only equal spin pairs).

  16. Thus the equal-spin pairs form a torus around the equator in momentum space, and there are no pairs at the poles. L-vector pairs The A phase is thus highly anisotropic. Also very odd excitation gas.

  17. In the B phase we can also have opposite spin pairs (the L- and S-vectors couple to give J = 0) This now allows us to have Lz = Sz = 0 pairs which can fill in the hole left at the poles by the A phase, giving an “isotropic” gap:

  18. D The B-phase gap:- equal in all directions. (because all spin-pair species allowed).

  19. pairs pairs pairs pairs The equatorial equal-spin pairs torus is still there but along with the Lz = Sz = 0 pairs which now fill the gap at the poles. L-vector

  20. pairs pairs pairs pairs The equatorial equal-spin pairs torus is still there but along with the Sz = 0 pairs which now fill the gap at the poles. L-vector pairs (which add up to a spherically symmetric total)

  21. The A phase has a higher susceptibility than the B phase (because all pairs are ßß or ÝÝ no non-magnetic Ýß components). Thus by applying a magnetic field we can stabilise the A phase. The A phase is the preferred phase at T = 0 when the magnetic field reaches 340 mT.

  22. Having made the five minute trip around the superfluid the context for what follows is: We can cool superfluid 3He to temperatures where there is essentially no normal fluid (1 in ~108 unpaired 3He atoms). We can cool and manipulate both phases to these temperatures by profiled magnetic fields. That means we can create a phase boundary between two coherent condensates, itself a coherent structure, at essentially T = 0.

  23. The main interest in this system is that we know in principle just about everything about the fundamentals of the pairing mechanism and the condensate wavefunction. In other words: “The superfluid 3He condensate is the most complex system for which we already have THE THEORY OF EVERYTHING.” And also know what our “vacuum” actually is. It is the zero-temperature ensemble of our input 3He fermion liquid states. That in a sense is our Planck scale, but we know what that is physically.

  24. Before we look at a typical experimental set-up we first introduce our workhorse microkelvin tool which does a large fraction of all our measurements for us.

  25. Before we look at a typical experimental set-up we first introduce our workhorse microkelvin tool which does a large fraction of all our measurements for us. The vibrating wire resonator (VWR).

  26. This consists of a “croquet hoop” shaped length of superconducting wire which is placed in a magnetic field and set into motion by passing an ac current through it . 7 30 136

  27. This consists of a “croquet hoop” shaped length of superconducting wire which is placed in a magnetic field and set into motion by passing an ac current through it . B Io exp(iωt) 7 30 136

  28. How can we use a mechanical resonator to probe a pretty good vacuum? It’s a trick of the dispersion curve! 7 30 136

  29. Liquid static Here we have the excitation dispersion curve – standard BCS form.

  30. Liquid static Liquid moving If the liquid is in motion then we see the dispersion curve in a moving frame of reference. Excitations approaching will have higher energies and those receding lower energies.

  31. The flow field provides a Maxwell demon which allows only quasiparticles to strike the front of the wire and only quasiholes to strike the rear – implication? Anyway it provides a very sensitive thermometer or quasiparticle number probe. 7 30 136

  32. Here’s our calibration 7 30 136

  33. First a quick look at some of the hardware. We use a dilution refrigerator to cool the experiment to a few millikelvin and finally use the adiabatic demagnetization of copper (nuclei) to cool to T < 100µK.

  34. This is a typical instrument package launched into the seriously hostile sub-100 mK environment.

  35. Neutron Detection

  36. The Quasiparticle Scintillator (10-7 eV “photons”)

  37. At 100 mK 1 cm3 has a total enthalpy of ~ 100keV. • Suggested long ago as a possible dark-matter detector PRL 75, 1887 (1995).

  38. Absorption of a neutron by a 3He nucleus • Capture process: • n + 3He++→ p+ + T+ • +764 keV

  39. Phase changes by 2p round the loop

  40. . • This is the Kibble-Zurek mechanism for the generation of vortices by a rapid crossing of the superfluid transition driven by temperature fluctuations. • (And similar to the mechanism for creating cosmic strings during comparable symmetry-breaking transitions in the early Universe.)

  41. Now let’s think about branes - • and also the justification of using superfluid 3He as a model “Universe”.