What Causes Quantum Locking in Levitation Videos?

[rshalloo]

I was just wondering if I have the right idea about what's going on in these quantum 'levitation' videos.

So the little discs that zoom around on the track are made up of YBCO film on a super thin sapphire disc. YBCO is a superconductor and so it experiences the Meissner effect, that is that it produces a current within the material to oppose the magnetic field thus essentially expelling it. While it experiences this it is important to remember that YBCO is a type two superconductor so it has thin quantums of flux passing through it (fluxoids). These are areas in which the YBCO has lost its superconducting ability and so its ability to expel a magnetic field. It's these flux tubes which cause the material to become locked in the magnetic field. Due to the Meissner effect the superconductor will create a current to counter the flux in the flux tubes. Once this has happened the material is essentially locked in place as any attempt to move it will result in an opposing force from the superconductor. It is this phenomenon which allows the YBCO to locked in any position i.e tilted, upside-down etc.

My question (aside from do I have the right idea of what's going on) is, what exactly is the role of the sapphire? Is it the sapphire or the YBCO that causes the flux tubes? I've read conflicting reports about which one causes it.

 

[f95toli]

Sapphire just happens to be a good material to grow YBCO films on (you can also use STO, MgO,LAO, LSAT, MgO and many others), mainly because the YBCO crystal is lattice matched to sapphire.
You don't need to use YBCO grown on a substrate, a piece of bulk YBCO works just as well.

Also, there is nothing "quantum" about this, this is mostly just classical physics.

 

[rshalloo]

But surely the Meissner effect and especially superconductivity are quantum effects? There's no classical explanation for them?

 

[f95toli]

But surely the Meissner effect and especially superconductivity are quantum effects? There's no classical explanation for them?

True, but then there is no classical explanation for most things in solid state physics; just about everything requires at least some QM.
The Meissner effect can be descrived reasonably well using the 2-fluid model, which is semi-classical.

 

[rshalloo]

Ah right, I see what you mean. Thanks very much :)