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Meissner effect

  1. Jul 6, 2008 #1
    Check this out: http://www.youtube.com/watch?v=c3asSdngzLs

    Why is this so stable? Is i due to the Meissner effect ? I mean you can't do this with two normal magnets. (normal: Tc = 300K+)

    I was thinking something like since the semiconductor, or what ever it is, becomes a superconducter the system can minimize its energy by creating surface currents and thats why the magnet stays on top and does'nt fall out to one of the sides..
     
    Last edited by a moderator: Jul 6, 2008
  2. jcsd
  3. Jul 6, 2008 #2
    This is a GREAT experiment to sort out.

    I couldn't hear any audio out of the video clip, but I agree that this must be a case of superconductivity because in the first part the 2 object do not attract/repel each other, and they start doing that only after the "fixed" object (F) is cooled.

    When cooled by the mysterious fluid, F enters the superconductivity state. I am not sure but I think that the superconductor F itself is not a magnet. If you'd put an iron object nearby M, it would not be attracted/repelled because F does not "produce" a magnetic field.

    The "moveable" object (M) must then be a small magnet by itself, so it does have a magnetic field around. When M approaches F, it inducts currents within F but due to the Meissner effect, the force lines of the magnetic field of M going through F are expelled as if the material is perfectly able to compensate for the inducing magnetic field by rearranging its internal microcurrents (i.e. orbiting electrons) -> perfect diamagnetism (something akin to a conductor being able to guarantee that E=0 inside it).

    However, does this internal rearrangement cause an external magnetic field as well or not?
    Is it now F behaving like a permanent magnet as well?
    If so, since F "internally rearranges" itself at every minimum change in the position of M*, can this really prevent M from sliding down to the side?

    *but note that a rotation of M has no effects
     
  4. Jul 7, 2008 #3

    f95toli

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    The shiny object is a strong permanent magnet, the black thing it is levitating above is a piece of YBCO which is a high temperature superconductor (Tc 92K).
    YBCO is -like all cuprates- a type II superconductor which means that it is possible to create vortices by applying a magnetic field when the superconductor is close to Tc.
    A vortex is somewhat like a "magnetic flux tube" with a normal core surrounded by a "tube" or circulating supercurrent.
    While vortices can move around they tend to "stick" to defects, impurities etc known as pinning sites (this is why pinning sites are introduced on purpose in superconducting cables, without them the vortices will be dragged along by the current which causes all sorts of problems).

    Now, when the field "freezes" (i.e. the YBCO becomes superconducting) the vortices will be arranged so that they "align" themselves with the magnetic field (hence minimizing the energy) meaning the magnet will levitate in one position; once the superconductor is cold it is still possible to force the magnet to move around but it now -as you can see in the video- require you to overcome some "friction", it feels a bit like dragging the magnet through yello. However, if enough force is used it is possible to "drag" the vortices to a new position and the magnet will come to rest in a new place.

    Hence, the reason for why it is stable is the presence of vortices. It is MUCH harder to levitate a magnet on top of a type I superconductor (the magnet and superconductor needs to have a certain shape etc)
     
  5. Jul 7, 2008 #4
    Why do vortices form?

    Isn't it so that a Type II superconductor has a range Tc1-Tc2 rather than a single Tc, and therefore a gradual transition (in terms of temperature drop) from conductor to superconductor rather than an abrupt one?

    Would the vortices disappear if it was cooled below Tc1?
     
  6. Jul 7, 2008 #5

    ZapperZ

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    Er.. not Tc1 and Tc2, but rather Hc1 and Hc2, which are the critical magnetic fields.

    By definition, Hc1 is the critical field below which no fields penetrate the Type II superconductor.

    Zz.
     
  7. Jul 7, 2008 #6

    f95toli

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    No, the only thing that happens is that they stop moving around. It is always possible to "trap flux" (i.e. create a vortex) in real samples, regardless of the temperature; it is just less likely to happen at very low temperatures. Unintentional flux trapping is a major problem when working with superconducting devices and is e.g. a major source of noise in SQUIDs, the only way to avoid it is to use a LOT of magnetic shielding and be very careful when cooling through Tc.
     
  8. Jul 7, 2008 #7
    Ah, I forgot everything... and I even got a 90% on my solid state physics exam :P

    So the mixed state between Hc1 and Hc2 has partial but not total penetration of H.

    Maybe the T1 and T2 I was remembering are temperatures at which a certain H becomes Hc1 and Hc2?

    Anyway... if the experiment works only on Type II superconductors, does it mean that it's because they have the mixed state, that Type I don't have? Or is there another property? If it's because of the mixed state, then I guess the trick works only if the H of the magnet is exactly between Hc1 and Hc2?
     
  9. Jul 7, 2008 #8

    ZapperZ

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    When a flux line has penetrated into the bulk of the material, it tends to resist being twisted and bent, just like the force you feel when you try to twist a spinning wheel. So while it isn't good that magnetic flux lines have penetrated the material, the off-shoot of it is that it also creates stability to the levitated object, because it tends to hold it in place.

    Zz
     
  10. Jul 10, 2008 #9
    Nice video. Brings back memories of when I did solid state research back in the mid-80s. We would make our own YBaCuO samples, and do this same type of test to verify superconductivity.

    I agree with everyones comments about the flux lines through the sample stabilizing the magnet.
     
  11. Jul 11, 2008 #10
    ...but it does need the (mixed) superconductor state right? If the magnet above was strong enough to destroy superconductivity and therefore H penetrated the material completely, would the magnet above fall instead of float?
     
  12. Jul 11, 2008 #11

    ZapperZ

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    If the superconductor becomes normal, you won't have the same magnetic field strength for levitation.

    Zz.
     
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