Hello and welcome to this video on type 2
superconductors
This short video explains how vortex pinning influences a levitating superconductor.
This will explain the quantum levitation or quantum locking videos on the internet.
I'll begin with the most basic Meissner effect demonstration.
Here, the lecturer takes a magnet
and places a superconductor that hovers
above it.
This is explained as Meissner effect.
the superconductor is a perfect
diamagnetic material.
Aside from a very and outer layer, it expels all magnetic fields
causing it to levitate over the magnet
If you're having trouble visualizing
this, I'll draw the magnetic field lines
out for you.
For future reference, this is type 1
superconductor behavior.
However, there is a serious
problem with this demonstration.
It requires several components: a permanent magnet,
an SC puck,
and some cryogen to cool the puck.
To demonstrate a type 1 superconductor,
I'd want liquid helium, which is at 4 Kelvin...
Unfortunately, liquid helium is way too
expensive for a demonstration,
forcing us to use liquid nitrogen at a balmy 77 Kelvin.
This forces us to use high TC superconductors.
Most people use YBCO, a type two
superconductor.
Often, they use a fairly thin film of YBCO
that has been deposited onto the puck.
Since YBCO is  a type II SC, the fields start to penetrate
above a critical field.
I'll draw the field lines.
However, the fields do not penetrate uniformly.
They penetrate in narrow columns called vortices, each holding a quantum of flux.
tThey are called vortices because each penetrating quantum of flux
has a superconducting current around it.
To understand how this affects
levitation, here is an example.
Let's say I have a wide piece of YBCO floating over a small magnet.
The vortices will form above the magnet.
What will happen when we move the magnet?
Will the vortices move with the YBCO?
To answer this, we switch to a top view
and move the magnet with respect to
the YBCO.
If the vortices move with the YBCO,
the areas with highest vortex density no longer correspond to areas with the highest applied magnetic field.
Hence, the energy would increase.
If the vortices move with the magnet, the energy is going to stay the same
and we can move the YBCO freely.
Since the lower energy state is more favorable,
the naïve answer is that vortices move with the magnet
and don't lock the YBCO to the magnet.
This assumes that the Type 2 superconductor is perfect.
They have defects and vortices get pinned to them.
Here is a quick diagram.
This means that, unless vortex pinning is broken, the vortices move with the superconducting puck.
What consequences does this have? moving the
superconductor with respect to the
Moving the superconductor with respect to the magnet
will increase the energy of the system.
In short, the superconductor is locked to the magnet.
In this particular configuration, side to side motion is restricted...
So is up and down motion.
If I have a wider magnet, the puck is free to move side to side.
This is because magnetic field is more or less constant along the length of the magnet.
Hopefully, you learned something about low temperature physics.
If you liked this video, please check out some of my others.
If you have questions or comments, please leave them below.
Thank you for watching.
