I Flux pinning in type 2 superconducotrs

[DariusP]

I'm wondering if for example YBCO superconductor (which is considered type-II superconductor) that exhibits flux pinning effect is a bulk or some substrate with YBCO film on top of it?

I always thought that YBCO which exhibits flux pinning is a bulk in all of demonstration videos but it struck me when someone on youtube (he was not a scientist btw, so he could very well be wrong/uneducated in this field) said that only a substrate with a thin YBCO film can exhibit flux pinning and if it was an YBCO bulk then we would get Meissner effect (complete expulsion of magnetic field lines) and no pinning.

Got me confused. Anyone know how it is for sure?

 

[DariusP]

someone on youtube (he was not a scientist btw, so he could very well be wrong/uneducated in this field) said that only a substrate with a thin YBCO film can exhibit flux pinning and if it was an YBCO bulk then we would get Meissner effect (complete expulsion of magnetic field lines) and no pinning.

I think the guy got this whole thing backwards.
From here: http://shop.can-superconductors.com/index.php?id_product=10&controller=product
it says that
"The disk for Meissner Effect is prepared in a way that the trapped magnetic field is very weak and the Meissner Effect then dominates over the trapped field. Therefore this Y-Ba-Cu-O disk for Meissner Effect demonstration is made of polycrystalline material that shows low critical currents.

On the other hand, the disk for demonstration of strong levitation force (flywheel demo, etc.) is prepared in a way that allows flow of high critical currents and the trapped magnetic field is high and dominates over the Meissner Effect. This is achieved by preparing the Y-Ba-Cu-O disk as a single crystal of high critical currents."Monocrystalline means that the bulk exhibiting flux pinning is just YBCO and nothing else, right?

 

[ZapperZ]

I'm wondering if for example YBCO superconductor (which is considered type-II superconductor) that exhibits flux pinning effect is a bulk or some substrate with YBCO film on top of it?

I always thought that YBCO which exhibits flux pinning is a bulk in all of demonstration videos but it struck me when someone on youtube (he was not a scientist btw, so he could very well be wrong/uneducated in this field) said that only a substrate with a thin YBCO film can exhibit flux pinning and if it was an YBCO bulk then we would get Meissner effect (complete expulsion of magnetic field lines) and no pinning.

Got me confused. Anyone know how it is for sure?

YBCO does exists without a substrate. One can even grow "large" single-crystal YBCO using floating zone method, and then simply cut off chunks of it. Many experiments have been done on such "chunks".

I think the guy got this whole thing backwards.
From here: http://shop.can-superconductors.com/index.php?id_product=10&controller=product
it says that
"The disk for Meissner Effect is prepared in a way that the trapped magnetic field is very weak and the Meissner Effect then dominates over the trapped field. Therefore this Y-Ba-Cu-O disk for Meissner Effect demonstration is made of polycrystalline material that shows low critical currents.

On the other hand, the disk for demonstration of strong levitation force (flywheel demo, etc.) is prepared in a way that allows flow of high critical currents and the trapped magnetic field is high and dominates over the Meissner Effect. This is achieved by preparing the Y-Ba-Cu-O disk as a single crystal of high critical currents."Monocrystalline means that the bulk exhibiting flux pinning is just YBCO and nothing else, right?

No, monocrystalline or single-crystal of ANYTHING means that the object contains only a single crystal orientation throughout the entire bulk material. A polycrystalline means that it still contains just that material, but with random crystal orientation throughout the bulk material.

Both of these show different and distinct patterns when one performs, say, x-ray diffraction experiments.

Zz.

 

[DariusP]

YBCO does exists without a substrate. One can even grow "large" single-crystal YBCO using floating zone method, and then simply cut off chunks of it. Many experiments have been done on such "chunks".

Thanks for your reply, but what I wanted to know was if flux pinning exists in this YBCO "bulk" or only in YBCO thin films+substrate?

 

[f95toli]

It can (and does) exist in either.
The large pieces of YBCO used for demonstration of the Meissner effect tend to have a LOT of defects (they are typically sintered; not single crystals) so flux pinning is fairly obvious.
However, flux pinning is a general property of type II superconductors and you get it in films as well; the pinning centres can be e.g. defects/voids in the film or even corners or constrictions in patterned films.

 

[DariusP]

It can (and does) exist in either.
The large pieces of YBCO used for demonstration of the Meissner effect tend to have a LOT of defects (they are typically sintered; not single crystals) so flux pinning is fairly obvious.
However, flux pinning is a general property of type II superconductors and you get it in films as well; the pinning centres can be e.g. defects/voids in the film or even corners or constrictions in patterned films.

Could you elaborate on what exactly are these defects/voids? I'm finding these replies really helpful. A lot less confused.

 

[f95toli]

Could you elaborate on what exactly are these defects/voids? I'm finding these replies really helpful. A lot less confused.

The same thing as in any material. Basically anything that makes the crystal "imperfect". This could be impurities, voids (missing atoms), grain boundaries, changes in chemical composition etc
In real life there is no such thing as a perfect crystal; meaning all materials have pinning centres.