Episode 18: That Superconductor Episode with Ryan North

[bn]

It's Podcast Time
 C'mon grab your friends
we'll go to a poorly explained land
with darren the guy, and Fiona the human
the physics will never end
it's podcast time!

Listen People. Did you know that there are NO GOOD JOKES ABOUT SUPERCONDUCTORS? it's true.

Today's episode is on superconductors.
and Ryan North is back!
He's done lots of fun things recently, including the new Adventure time Comic book! you should buy them.

anyway, today's episode is super fun. It tells you HOW SUPERCONDUCTORS WORK.
i never really understood correctly before i did this episode.
do you know why I never understood?
because they are CRAZY DIFFICULT TO EXPLAIN.

we do a good job though, thanks to Fiona and Darren.


click me to listen

for an awesome demonstration of the effects of magnetic pinning see the following TED talk demonstration: click me to watch

[thehugo]

Hey!!
(First on the board, so... no pressure!)

Now... I can use the board for questions on the topic at hand, right?
Because I'm listening to the podcast right now and I'd like a clarification on something.

I just listened to the rule of thumb explanation about the opposing fields - the electromagnetic field of a superconductor as a result of an external magnetic field. I get that as the resistance in the superconductor approaches zero, the field that the circling electrons are creating approaches (in strength) the field that got them circling. And the two fields are equally strong if the superconductor if perfect. So far so good.

The issue is as follows - this process defines the superconductor as a... quantitatively* different from a normal conductor. Is there something that separates them qualitatively* or there's just a theoretical borderline that separated the definitions of the two?

* sorry for those words, I'm not playing smar;, English is not my first language and I've got to rely on dictionaries sometimes...

P.S. Great job, great show! Keep up!

[bn]

yaaaaay! Welcome to the jungle.

 

The issue is as follows - this process defines the superconductor as a... quantitatively* different from a normal conductor. Is there something that separates them qualitatively* or there's just a theoretical borderline that separated the definitions of the two?

yeah, this is a neat question, because the answer is crazy and historic and we alluded to it a little in the show, but we forgot to talk about it explicitly.

so 100 years ago  Heike Kamerlingh Onnes was testing the conductivity properties of mercury. he would lower the temperature a bit, and then the resistance would drop by a little bit.

and then there was a threshold. 4.2 Kelvin, where all of a sudden *BOOM* it went to zero. it turned into a superconductor. as far as I understand it (and you'd be surprised the number of times in this episode where my experts corrected me and made me re-state things) superconductors are fundamentally different than regular conductors, the same way solids are different from gasses. there's a sudden transition between the two.

essentially what happens is that above 4.2 kelvin, all the electrons are regular electrons. as the heat in the metal decreases, they start to stack on the "bottom" of the bathtub.

and as darren kept saying, the electrons who live "below" the water line of the bath tub can't really do much. they're stuck in place (well. not quite in space. but you can't change their momentum)... (because the bath tub is a metaphor for the momentum space. )

but then at 4.2 kelvin, BOOM, all the "surface" electrons pair up and start acting like bosons. this effectively freezes ALL of the electrons to not change what they're doing. all of the electrons "below" the surface are stuck in place, as we've said. all of the electrons *on* the surface are stuck because they've been paired up.

um. so yeah. they are characteristically and fundamentally different than regular metals. it's not just like when... I dunno... a student graduates from university. technically, you could have pulled them out of class 2 weeks before the end of term, and they'd be pretty much the same.

instead they're more like the hulk, when they change, you get something super different.
*bows to rampant applause that occurs whenever the hulk is used in a metaphor*

[thehugo]

Well... I obviously wasn't paying enough attention. :/
I'm going through the show again and it's all there.
Sorry! Ashamed...

And thanks.

[bn]

what?
don't be ashamed!
when i was editing the show i was thinking, the whole time "Man, we forgot to talk about this sudden phase transition! it's kind of important". also, there was no overall summary.

but never be ashamed!
you're a ti-phyter. stand tall.

[thehugo]

OK then, just a bit embarrassed

Can I ask a follow-up question? (this time a listened to the specific part in the podcast a couple of times to be sure what is said; it's about 42 min into the show)

About the flux pinning - there's that: "...you'll form a grid of these [magnetic flux], they will get trapped if you have impurities...".
So if the lock that is caused by the electromagnetic field piercing the superconductor is allowed by microdefects and impurities in the superconductor, does that mean that a in perfect superconductor there will be no vortices and so the quantum levitation won't happen? If so, that's... cool - a very promising technology enabled by material imperfection In this tube movie (Quantum Levitation) they also use "defects" to explain the superconductor-magnet lock.

And - a follow-up question to the follow-up question - if these are not defects but predictable/reproducible features of the superconductor, can their... arrangement in space or number or any other characteristic be used to control the parameters of the lock? I mean - creating the superconductor with specifically designed imperfections in which case they would be just features.

P.S. I hope I'm not getting annoying, but I've never had a specialist willing to explain something like that as "for dummies". It's irresistible!

[dpeets]

For the first couple decades after the discovery of superconductivity, scientists debated whether superconductivity was some completely new phase, or just the resistivity going to zero for some reason.  What settled it is the Meissner effect.  It's not just that a magnet brought up next to a superconductor will levitate (or vice versa) -- any perfect conductor would do that, because it would resist changes to the magnetic field.  But if a material that superconducts (Type-I) is cooled through its transition temperature into the superconducting state with a magnet sitting on top of it, that magnet spontaneously levitates, because the magnetic field that was inside the superconductor gets ejected.  It's easier (lower energy) for the magnet and superconductor to fling themselves apart than for the superconductor to have to deal with that magnetic field. 

You can levitate a perfect conductor without flux pinning if your magnetic field is bowl-like (minimum in the middle to trap it).  If you see a superconductor running around above a magnetic track, you'll probably find that the track has three strips of magnets.  They're arranged north-south-north or south-north-south, so that there are minima in the field.  However, there's a trick where you can hang the superconductor under the track, and that absolutely requires that the flux be pinned.

In a pure superconductor, the magnetic vortices form some regular grid (usually triangular/hexagonal, square, or something in between), where the spacing of vortices in the grid is set by how strong the magnetic field is.  If there's a lot of dirt in there, they stick to the dirt, because superconductivity's already weak there and it's easier to poke a hole through.  This is done deliberately for power applications, because keeping the vortices stuck in position increases the amount of current you can run through the superconductor before it goes "normal" (the "critical current" that suppresses superconductivity).  It's also done if the objective is levitation.  The dirt is in there randomly because that's far easier than the alternative.

[thehugo]

Right, got it!
Thanks a lot!

[bn]

yaaay

[Nick]

What's with the bit at the biggining of this episode about time travel using superconductors and the government recently stifling it?
I assume it was a reference to a piece of news I missed in the past.. ?

[bn]

i think there was some fun speculation along that line
when CERN kept breaking as they were turning the LHC on.

like the baguette in the transformer story.

and there was an april fools joke about a time traveller  sabotaging it.

[Ed Lolington]

CERN is the recipient of so much cool April Fools' Day love.

[Nick]

Ah yes, the bells are ringing now.  The time traveler sabotaged it to save the future of humanity.

[bn]

correct.