3 ms·

Type-II super conductors may exhibit "flux pinning". Type-I super conductors do not.

alecst 3y ago
Cool thanks. Gonna read up a little on that. Edit: yea it's interesting. Believe it or not, I studied L-G theory in grad school, taught a lab about (type-II) superconductors, but had no idea that type-I superconductors didn't flux pin. Just leaving this here from Wikipedia: > The superconductor must be a type-II superconductor because type-I superconductors cannot be penetrated by magnetic fields. Some type-I superconductors can experience the effects of flux pinning if they are thin enough. If the material's thickness is comparable to the London penetration depth, the magnetic field can pass through the material. https://en.wikipedia.org/wiki/Flux_pinning https://en.wikipedia.org/wiki/Flux_pinning
- adastra22 3y ago
  Also to follow up on your original point, this is a purported example of a linear superconductor. There are parallel columnar lines of superconductivity inside the mineral, like a bundle of wires. No such thing has ever been demonstrated before, and it is unlikely to have macro properties like those you are familiar with. For example, flux pinning is (IIRC) due to circular currents induced in the superconductor. But how do you induce a circular current into a straight-line conductive wire with ~zero cross section?
- someplaceguy 3y ago
  > If the material's thickness is comparable to the London penetration depth, the magnetic field can pass through the material. Indeed. A girl I was seeing told me the same once, but obviously things didn't work out between us...
zarzavat 3y ago
As far as I understand it (not an expert on these things), flux pinning is caused by microscopic defects that allow the magnetic field to penetrate at certain points. An idealized superconductor that is perfectly uniform expels the magnetic field at all points and so would not display the effect, it would simply be diamagnetic. So it’s mistaken and somewhat perverse to view the absence of flux pinning as proof that something is not a superconductor. In the case of LK99, the claim is that it does not show flux pinning because the sample is impure and not uniformly superconductive, i.e. it is not expelling the magnetic field enough.
- jamesmaniscalco 3y ago
  No defects needed for flux penetration in a type-II superconductor. When the conference length is smaller than the penetration depth (up to a factor of sqrt(2)), flux vortices can nucleate as soon as the surface magnetic field gets above the lower critical field Bc1.
  - jamesmaniscalco 3y ago
    Sorry, that should say “coherence length”.
- jacquesm 3y ago
  That also explains - assuming it is all true - the lack of current through the sample.
asdfman123 3y ago
> Type-II super conductors may exhibit "flux pinning" Type-II diamagnetism?
radioactivist 3y ago
Even for type-II superconductors one may need single crystals if the material is anisotropic enough (e.g. if the flux pinning is confined mostly to 2D layers then randomly oriented grains in a powder aren't going to trap flux well)

Cool thanks. Gonna read up a little on that. Edit: yea it's interesting. Believe it or not, I studied L-G theory in grad school, taught a lab about (type-II) superconductors, but had no idea that type-I superconductors didn't flux pin. Just leaving this here from Wikipedia: > The superconductor must be a type-II superconductor because type-I superconductors cannot be penetrated by magnetic fields. Some type-I superconductors can experience the effects of flux pinning if they are thin enough. If the material's thickness is comparable to the London penetration depth, the magnetic field can pass through the material. https://en.wikipedia.org/wiki/Flux_pinning https://en.wikipedia.org/wiki/Flux_pinning

adastra22 3y ago
Also to follow up on your original point, this is a purported example of a linear superconductor. There are parallel columnar lines of superconductivity inside the mineral, like a bundle of wires. No such thing has ever been demonstrated before, and it is unlikely to have macro properties like those you are familiar with. For example, flux pinning is (IIRC) due to circular currents induced in the superconductor. But how do you induce a circular current into a straight-line conductive wire with ~zero cross section?
someplaceguy 3y ago
> If the material's thickness is comparable to the London penetration depth, the magnetic field can pass through the material. Indeed. A girl I was seeing told me the same once, but obviously things didn't work out between us...

zarzavat 3y ago

As far as I understand it (not an expert on these things), flux pinning is caused by microscopic defects that allow the magnetic field to penetrate at certain points. An idealized superconductor that is perfectly uniform expels the magnetic field at all points and so would not display the effect, it would simply be diamagnetic. So it’s mistaken and somewhat perverse to view the absence of flux pinning as proof that something is not a superconductor. In the case of LK99, the claim is that it does not show flux pinning because the sample is impure and not uniformly superconductive, i.e. it is not expelling the magnetic field enough.

jamesmaniscalco 3y ago
No defects needed for flux penetration in a type-II superconductor. When the conference length is smaller than the penetration depth (up to a factor of sqrt(2)), flux vortices can nucleate as soon as the surface magnetic field gets above the lower critical field Bc1.
- jamesmaniscalco 3y ago
  Sorry, that should say “coherence length”.
jacquesm 3y ago
That also explains - assuming it is all true - the lack of current through the sample.

asdfman123 3y ago

> Type-II super conductors may exhibit "flux pinning" Type-II diamagnetism?

radioactivist 3y ago

Even for type-II superconductors one may need single crystals if the material is anisotropic enough (e.g. if the flux pinning is confined mostly to 2D layers then randomly oriented grains in a powder aren't going to trap flux well)