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To Infinity and Beyond! The Physics of Superconductivity

By StephenFuqua in Technology
Mon Sep 02, 2002 at 08:46:03 AM EST
Tags: Science (all tags)
Science

Whilst its real-world applications are yet difficult to find, superconductivity may well prove to be one of the greatest discoveries of the 20th century. Herein we examine the physics behind this phenomenon, touch upon some of the applications we have developed thus far, and give some indication of the future development. Intended for the literate yet not necessarily technical audience, with no pictures or equations but plenty of links.


The phenomenon of superconductivity is a fascinating one that, like many other important discoveries in the 20th century, was not expected when first detected (c.f. penicillin, electron diffraction). At the time of its discovery in 1911, it was known that the conductivity of a metal--the ease with which electrical current passes through the substance--increases as it cools down. However, as Dutch physicist Heike Kamerlingh Onnes was investigating this property of metals, he stumbled across a never-before seen state of perfect conductivity in mercury when he cooled it with liquid helium (to 4.2 kelvin, or -268.80 C). Thus his experiments were the first to show that certain elements can have perfect--infinite!--conductivity at the right temperature, implying a loss of all resistance to electron flow (resistivity).

Electrical conductivity is what makes our microwave popcorn, and the television set we eat it in front of, possible: strands of metallic wire (generally copper) carry electric current from the power plant to our homes. The better the conductivity in the wire, the better it will be able to carry current to your home and supply the electricity needed to run your appliances. Certain physical properties of metals make them good conductors, while non-metals are generally considered insulators (that is, they will not pass a current through them, similar to your garden-variety Thermos).

But let us get a better understanding of conduction before we move on. Metals (conductors) allow small charged particles called electrons to flow through them, much like a tube through which water flows. Naturally, current--be it electrons or water--can flow back-wards or forwards through its conduit. The direction of flow is simply a matter of where you start it from.

Resistivity is the opposite of conductivity. Resistivity, as its name implies, is a literal blocking of flow--like a boulder in a stream or sandpaper that resists movement across a wooden block. And when you finally do get that sandpaper to move across the block, it requires a greater output of energy (or power, which is energy per unit of time) from you than just moving a regular piece of paper. Resistivity operates the same way: the more resistance a metal has, the more power is lost. This means that one must continually add energy--or, in the case of power lines, electricity--to the system in order to have a “constant” current flow. Without adding more energy, all of the electricity would eventually become lost power, adding unwanted dollars to your electric bill.

The resistance itself arises from intramolecular collisions. As the electrons flow through the metal, they often run into atoms. These atoms are then like the rocks in the creek that obstruct flow: resistance. Resistivity! Electrons, furthermore, belong to a class of particles (fermions) whose general behavior encourages collisions.

Fermions are not alone in the world of particles; there also exists a class of elementary particles called bosons. Primarily, two factors differentiate the two types: spin and group behavior. Spin is a quantum mechanical term for an intrinsic property of particles', it can be in whole units (-1,-0,1,2, and so on) or in halves (-1/2,1/2,3/2, etc.). Bosons are those particles that have whole member spins, while fermions are all of those (including electrons) with half member spins. Aside from the spin, these two classes exhibit opposite behavior while in groups: while bosons like to act together, no two fermions can be in the same state at the same time (the Pauli Exclusion Principle). This is a quantum mechanical rule, a law of nature if you will. For fermions, this means that two electrons (or any other fermion particle) cannot both be doing the same thing in the same place at the same time. For instance, I once heard a joke that illustrates the matter well:

Two electrons walk into a bar. The first asks for a gin and tonic. The second slams his hand on the table and says "Damn, that's what I wanted!"

Because they cannot travel together, in the same state, there is much chaos in an electron's life. And, as we all know, chaos leads to trouble. In this case, the chaos of energetic electrons traveling in such a haphazard way leads to many collisions, much like cattle stampeding every which way and bumping into each other.

Bosons are not like this, however. For bosons actually prefer to be in groups together, if a particle can prefer anything. Indeed, it turns out that the more bosons there are traveling together, the more likely it becomes that all of the bosons in a system will be in the same state. Thus it is not long before all of the bosons are traveling together under the same conditions.

When you cool down a material, you are doing the same thing as telling a child to "cool down." That is, you are telling the child to have less energy and be less agitated. When you cool down the material, the electrons have less energy and are less agitated. With less energy, fewer collisions occur and the resistivity is lowered. Life becomes more ordered. Experience, however, had always shown that the resistivity would bottom out at a certain positive value different for each material. This bottoming out factor is due to the impurities in the substance, which will always cause some small amount of disorder.

Onnes's discovery showed once again that experience fails to reveal the full story of reality. As he cooled the mercury down, he found that it exhibited the same tendency to decrease in resistivity as all other metals had. However, in 1908 Onnes had developed a system to liquefy helium (for which he won the Nobel Prize), which is much colder than the liquid nitrogen that was typically used in these kinds of experiments. Utilizing his new invention, he cooled the mercury down to 4.2 kelvin and noticed that all resistivity disappeared at this critical temperature. (The critical temperature is usually denoted Tc). He dubbed the new attribute superconductivity.

Though most good conductors (such as copper and silver) do not have superconductor characteristics, many other elements and even complex materials achieve zero resistance at critical temperatures. When Onnes performed his experiments, and indeed for many years to follow, there was no understanding of the mechanisms whereby superconductivity occurs. In 1957 John Bardeen (inventor of the transistor), Leon Cooper, and J. Robert Schrieffer formulated a theory to explain superconductivity, leading to their own Nobel.

BCS theory, as it is called for its inventors, postulates that electrons can actually form pairs which act as bosons instead of fermions (Cooper pairs). Though electrons normally repel each other due to their like charges, at very low temperatures the lattice structure of the atoms in a superconductor material becomes distorted by the passing of an electron. From this distortion arises a weak force that actually attracts a second electron to the first. Though the two are not physically bound together, it is as if they were in a three-legged race and forced to act cooperatively. Thus the two spin one half’s become one spin one and the Cooper pair acts like a boson instead of two separate fermions.

As bosons, the Cooper pairs naturally draw each other into cooperative states. At Tc, the pairs move together in the same direction and all with the same status: we have organization and no resistance. Instead of having a chaotic stampede, the cattle are now herded in an orderly manner. But as soon as you introduce even small amounts of energy, the cattle get agitated and start to move around. Pretty soon they are all scared and the order is completely destroyed. Back to resistivity we go.

Thus superconductors achieve zero resistivity when they reach Tc. And zero resistivity of course means that we now have perfect conductivity--zero power loss and a persistent current. In fact, some superconductors have been tested which are so good at conducting current that a current could last in them without power loss for 100,000 years.

Not only do these materials have perfect conductivity, but they also have an additional defining factor, the Meissner effect. The Meissner effect refers to the ability of superconductors to keep magnetic fields from passing through them. Instead the magnetic field flows around the superconductor. In a normal material, the magnetic field passes through the interior uniformly. In the superconductors state however, the Meissner effect kicks in and the flow of magnetic field around the material causes the material to repel permanent magnets. Combined, these two properties create a wealth of potential applications such as trains that levitate on superconductors magnets, efficient switches, amplifiers and other circuit components, power lines, and motors and other propulsion systems. 

Unfortunately, the practical use of liquid helium is prohibitively expensive. Thankfully J. Georg Benorz and K. Alex Müller made a remarkable discovery in 1986: lanthanum-barium-copper-oxide (La2-xBaxCu4) achieves superconductivity at a critical temperature of 30 kelvin. Not only was this the highest Tc ever found, but the discovery also led to research in a whole new class of materials: ceramics. Later that same year, the ceramic YBa2Cu3O7 was discovered to exhibit zero resistance at 92 kelvin. For the first time, superconductivity was attainable at liquid nitrogen temperatures. Continuing research has brought that temperature up to 130 kelvin, still quite cold by human standards (-143 C), but easily attainable with off-the-shelf components. Strangely, BCS theory does not seem a good fit for high Tc  superconductivity, and a new theory yet to be settled upon.

High temperature superconductivity gives new life to all of the applications that physicists once dreamed of, but never thought fiscally possible. While there is much work and research still to come, superconductors are beginning to show up in applied technologies, such as the Yamanashi Maglev Test Line (rail) in Japan, electric power transmission lines in Detroit, and enhanced Magnetic Resonance Imaging for medical testing. The cutting edge of experimental research is finding superconductivity in fullerenes, ferromagnets, and other materials, while the theoretical focuses on finding that new theory and determining an upper limit for any TC. The limits to superconductivity and its applications... well, they just may not exist. And thus it may be on the back of infinite conductivity that we ride to our Jetsonian future.

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To Infinity and Beyond! The Physics of Superconductivity | 39 comments (30 topical, 9 editorial, 0 hidden)
Levitation (4.00 / 1) (#3)
by Rasman on Mon Sep 02, 2002 at 02:00:21 AM EST

I saw a documentary on television where physicists were suspending small, not-particularly-magnetic objects in the air, as you could with a regular magnet, using superconductive (or was it semiconductive?) technology. I seem to recall some mention of ceramics. They even had a live cockroach suspended.

Can somebody help me here?

Anyway, it seemed like levitation of non-magnetic objects might be pretty useful. Friction is at least as much of an enemy as resistivity, for all the same reasons.

---
Brave. Daring. Fearless. Clippy - The Clothes Pin Stuntman
IIRC... (5.00 / 2) (#5)
by Work on Mon Sep 02, 2002 at 02:17:03 AM EST

some ceramics are a pretty popular superconductivity choice (the article mentions that complex structures are often more superconductive than normal materials such as copper)

I was watching a nifty program the other day about magnetism on the history channel which displayed a small frog being levitated by a 40-gauss supermagnet. Everything in nature is magnetic, in some way. Water especially has a tendency to repel at high gauss levels. Superconductors are used to generate these ultra-intense magnetic levels so thats probably what you've seen.

Unfortunately even supermagnets can only levitate very lightweight objects which are considered normally nonmagnetic. Though that russian fellow displayed a gravitational lessening in mass when he used a specific setup of superconductors.

I think we've only scratched the surface of what superconductors can achieve. Some very basic evidence has shown that gravity can be counteracted in part by some kind of previously unknown antigravity that superconductors in certain setups can generate. Sort of like how electricity and magnetism are complementary.

[ Parent ]

Yep. That's it! Frogs too. [nt] (none / 0) (#7)
by Rasman on Mon Sep 02, 2002 at 03:28:16 AM EST



---
Brave. Daring. Fearless. Clippy - The Clothes Pin Stuntman
[ Parent ]
Check the numbers. (4.50 / 2) (#17)
by bunsen on Mon Sep 02, 2002 at 12:03:05 PM EST

I think it would have been a 40 Tesla magnet. 40 Gauss (=.004 T) is easily achievable by refrigerator magnets, IIRC.

The frog levitation is simply a demonstration of the diamagnetism of water. Most materials that we consider nonmagnetic are, in fact, very weakly diamagnetic (they are repelled by magnetic fields). The effect is too small to notice in everyday circumstances, but extremely large magnetic fields can do things like levitate frogs and cockroaches.

The reason we haven't seen humans floating inside big magnets is the problem of scale. The frog was floating in a hole in the middle of the magnet, maybe 2 cm across (or something like that). It has so far been impossible to build a magnet that produces a field that strong in a volume anywhere near large enough to contain a human.

---
Do you want your possessions identified? [ynq] (n)
[ Parent ]

Very powerful magnets (none / 0) (#35)
by Shren on Wed Sep 04, 2002 at 11:50:05 AM EST

It has so far been impossible to build a magnet that produces a field that strong in a volume anywhere near large enough to contain a human.

But if you find a way, the DoD has a contract for you. Imagine taking down all the hard drives in a city.

[ Parent ]

Yup (none / 0) (#14)
by MotorMachineMercenary on Mon Sep 02, 2002 at 07:53:37 AM EST

I saw this, too, a while ago. The levitating frog must be one of the funniest clips on the 'net.

Here is a clip of the poor bastard in suspended free fall. There are a couple of other clips with the same theme.

Now, when do we get this to work on humans?

--
Would you like an anti-radar detector -detector to go with the anti anti-missile-missile missile?


[ Parent ]
Caution (5.00 / 2) (#16)
by Herring on Mon Sep 02, 2002 at 09:12:24 AM EST

Remember to remove your Prince Albert first.


Say lol what again motherfucker, say lol what again, I dare you, no I double dare you
[ Parent ]
Anti-gravity actually :-) (none / 0) (#21)
by drquick on Mon Sep 02, 2002 at 12:43:34 PM EST

Some weak shielding of the gravitational force seems possible actually. Here are some links. In experiments some 2% of the gravitational force has been shielded.

[ Parent ]
thats different. (none / 0) (#23)
by Work on Mon Sep 02, 2002 at 01:13:57 PM EST

thats a still somewhat unproven mystery. What he's talking about is the diamagnetic repulsion of water (frogs are 90% water, thus supermagnetic fields can cause them to 'levitate')

[ Parent ]
Bzzzzzzzzt! (3.00 / 2) (#12)
by pyramid termite on Mon Sep 02, 2002 at 06:53:36 AM EST

Whilst its real-world applications are yet difficult to find, superconductivity may well prove to be one of the greatest discoveries of the 20th century.

It'll revolutionize the joy buzzer industry for sure.

"I forget, in a certain way, everything I write, doubtless also, in another way, what I read." - Jacques Derrida
Resistivity and conduction (5.00 / 1) (#15)
by epepke on Mon Sep 02, 2002 at 08:06:20 AM EST

It is important to note that, while superconductors have zero resisitivity, they still exhibit one of the problems that resistivity causes in a different way: a limitation on the amount of current that can be passed through a conductor of a certain width. That's one of the major drawbacks of the high Tc superconductors--they can carry very little current indeed.

Now, in the resitive world, if you want to carry more power, you just crank up the voltage. This does two good things: it reduces the effect of resistance according to Ohm's Law, and it reduces the amount of current that you need for the same power, so the usefulness of the conductor goes up by the voltage squared (roughly, pedants will note this is a first order approximation). That's why long-distance power lines are high-voltage. But with superconductors, you're pretty much stuck until someone comes up with a better material.


The truth may be out there, but lies are inside your head.--Terry Pratchett


counter emf (none / 0) (#28)
by loualbano on Tue Sep 03, 2002 at 04:41:09 AM EST

I always wondered how superconductors react to CEMF.  I always hear about whole cities being powered by a fiber that's very thin.

If they are running AC through it, won't Lenz's law kind of ruin that possiblity by generating backwards current and causing some heat to be generated?

Could someone please set me straight?

-Fran

[ Parent ]

About Bednorz and Muller (5.00 / 1) (#18)
by demi on Mon Sep 02, 2002 at 12:14:31 PM EST

While it's typical for scientists, when coming across a new discovery, to first try fitting orthodox theory to the new results, even shortly after the time of the discovery Bednorz and Muller did not believe that the 1-2-3 superconductors were BCS superconductors. The electronic structure of the ceramic copper oxides is totally different from a metal.

Another promising avenue in superconductivity research may be with compounds derived from, or similar to magnesium boride (MgB2), which was recently found to have an anomolously high Tc. And while you have cited medical MRI, superconducting magnets made of alloys like Niobium-Titanium have been commercially available in various kinds of spectrometers (like NMR) for decades.

any chance? (none / 0) (#19)
by el_guapo on Mon Sep 02, 2002 at 12:34:58 PM EST

any chance that resistivity is kinda like it's physical world (and completely unrelated, AFAIK) relative - friction - in that for a lot of things it's actually neccesary for a lot of things? think about it - no friction, no cars: your tires can't make you go, your brakes can't make you stop. if the magnetron in my microwave was superconductive, would it make microwaves? would the crt in my tv make a picture if it was superconductive?
mas cerveza, por favor mirrors, manifestos, etc.
sheesh (none / 0) (#20)
by el_guapo on Mon Sep 02, 2002 at 12:37:07 PM EST

"for a lot of things it's actually neccesary for a lot of things" - this is what you get when posting whilst watching the history channel...
mas cerveza, por favor mirrors, manifestos, etc.
[ Parent ]
Sure. (none / 0) (#22)
by Work on Mon Sep 02, 2002 at 01:09:53 PM EST

Resistors are a common part of most circuitry.

[ Parent ]
of course... (none / 0) (#24)
by joto on Mon Sep 02, 2002 at 03:35:21 PM EST

But that doesn't make it less neat. Non-friction (as in vacuum) is also a quite useful phenomenon.

My pet idea is (if we ever find a cheap way of building tunnels), to have frictionless trains/cars/whatever in tunnels going pretty deep. You just give it a little push at the start, and it accelerates downward, travelling quite fast, and decelerates when it moves back up to the surface. Much cheaper than planes (which needs to move up first (loosing potential energy).

[ Parent ]

sigh (none / 0) (#34)
by EriKZ on Tue Sep 03, 2002 at 11:12:37 PM EST

Except, ya know, for air friction.

[ Parent ]
that's why you use (none / 0) (#36)
by Wah on Wed Sep 04, 2002 at 03:19:25 PM EST

a spincter and an air pump on each end of the tunnel.  Hmmm, maybe it is a shitty idea...
--
Where'd you get your information from, huh?
[ Parent ]
Superconductive magnetron (none / 0) (#25)
by sigwinch on Mon Sep 02, 2002 at 07:35:47 PM EST

if the magnetron in my microwave was superconductive, would it make microwaves?
In theory, it would work better if you made the resonator and waveguides out of superconductor. In practice, the magnetron uses a red-hot filament and it would tend to cook the superconductor to death. (If you ever look inside a microwave you'll see that the magnetron is completely encased in a big heat sink.)

--
I don't want the world, I just want your half.
[ Parent ]

friction good and bad (none / 0) (#27)
by squidinkcalligraphy on Tue Sep 03, 2002 at 01:14:24 AM EST

Resistance is necessary for any sort of useful electrical circuitry, just as it is necessary for any sort of mechanical systems. However, for the transferral of energy, it is best not have low (or no) friction/resistance elements - friction only wastes the energy we are trying to transport. That's why power companies are investing heavily into this (I think there's one place where they put a superconductor link between a power generator and the substation a kilometer or so away).
An identity card is better that no identity at all
[ Parent ]
Energy for meissner effect (none / 0) (#26)
by jovlinger on Mon Sep 02, 2002 at 11:38:33 PM EST

Arguably, no work is being performed by the meissner effect (hence no energy need be expended), but still it _is_ creating a force. Is this force free? Or does a superconductor suspended in a magnetic field stay cooler longer than one that is not? Are there any other incident energy flows that enter into this equation?

Meissner Effect = String (none / 0) (#38)
by MyrddinE on Thu Sep 19, 2002 at 06:21:44 AM EST

Take a string, and tack it to your wall. Suspend a small magnet from it. The string is now creating an upward force on the magnet... the force is free, and will exist permanently, yet you do not question it.

Take a small cylindrical magnet about 1" thick and 1" in diameter... in fact, take two of them. Put them inside a clear plastic tube that is just larger than they are, and insert them so that the like poles are repelling.

Now tip the tube vertically. The magnet on top will slide down the tube until the repulsive magnetic force keeps it suspended an inch or two above the other magnet. This system will stay in balance forever, expending no energy.

These are relatively simple examples, but you get the point. A force that exists indefinitely is not new.

What makes the meissner effect interesting (and seemingly 'magical' is that a magnet will stay suspended above a superconductor no matter what its orientation. In addition, the magnet has no tendancy to 'slide' off the side of the superconductor, like it would if you held two magnets above each other with no clear plstic tube to restrain them.

The meissner effect creates a kind of 'bowl' shaped force area... I'm probably explaining it horrendously, but the idea is that a magnet would have to go 'uphill' as it slid toward the edge of a superconductor. The point where the magnet is at its lowest potential energy is approximately equidistant from the edges of the superconductor. So it has a tendancy to 'slide' to the center of the material it is floating above. That, combined with the fact that it is repelled no matter what orientation it has keeps it in a stable position above the center of the superconductor. It is still an ordinary force, just like a string or a magnet, repelling gravity. And just like those, it requires no fancy energy to keep working... it just works, a fact of nature.

Weird though it may look. :-)

Disclaimer: IANA Physicist. I've never even studied the meissner effect specifically, so my information could be off somewhat. Do not go designing a maglev train based on my description. Thank you. :-)

[ Parent ]

LOX fridge for cost of LN (none / 0) (#29)
by texchanchan on Tue Sep 03, 2002 at 09:39:26 AM EST

Liquid oxygen is colder than liquid nitrogen and costs more to refrigerate. You can cut your costs  this way--once the crystal-growing technology succeeds, that is.

All you need is two single-crystal high-temperature superconductors as follows: One is shaped like an ice chest. The other is shaped like the lid of the ice chest.

In a cryo lab where everything, including the ice chest, is at LOX temps, put your LOX in the ice chest and put the lid on.

Hook up the power to the ice chest so that current is flowing through the thing. Now it is nonconductive to heat. Your only heat loss is through the (ideally microscopic) crevice between the (ideally perfectly machined) lid and chest.

Now bring the whole thing into the LN temperature zone.

The LOX in the chest stays cold, although you are only refrigerating down to LN temperatures. The amount of current to make the superconductor non-heat-conductive is very small.

Disclaimer: IANA cryogenicist (but one I knew said it would work).

Superconductors & thermal energy? (none / 0) (#30)
by nowan on Tue Sep 03, 2002 at 02:20:00 PM EST

You're not the first person I've read suggest that superconductors would behave oddly wrt regular thermal energy, but I've never heard any explanation for this idea (which to me sounds really bizarre).

So is this true? Do you have any references on the subject? Can a superconducting material be used as a perfect heat barrier and/or a perfect heat conductor?

[ Parent ]

Conductivity (none / 0) (#32)
by awgsilyari on Tue Sep 03, 2002 at 04:49:48 PM EST

At least in metals, electrical and thermal conductivity are closely linked. Both phenomena involve movement of electrons within the lattice. In general, the less often the electrons interact with the lattice ions, the more electrically conductive and less thermally conductive the material is.

Consider how thermal energy is actually transmitted within the metal lattice. If there were no conduction electrons, the thermal energy would have to move entirely by interatomic collisions (one metal atom banging into another one). With a partially filled conduction band (as in metals) the electrons are free to move large distances through the lattice without interacting with the metal atoms. The electrons then become a major mode of heat flow.

However, the situation is different in semiconductors and quite different in dielectrics (insulators). There is a relationship between electrical and thermal conductivity but it is complicated.

--------
Please direct SPAM to john@neuralnw.com
[ Parent ]

Knew? (none / 0) (#31)
by awgsilyari on Tue Sep 03, 2002 at 04:41:39 PM EST

Disclaimer: IANA cryogenicist (but one I knew said it would work).

You knew him? Is he in cryostasis now or what?

--------
Please direct SPAM to john@neuralnw.com
[ Parent ]

Nice idea, not so true to reality (none / 0) (#37)
by FissionChips on Thu Sep 05, 2002 at 01:07:15 AM EST

First, from a common sense point of view, no superconductor can be a perfect insulator of heat; they need constant cooling to maintain the low temperature necesary for superconduction.

From a little googling, I found that SC's have an exponentially decreasing specific heat, that is, at lower temperatures an SC requires a *small* amount of heat to drastically increase its temperature. To relate this back to electrical and heat conduction is beyond my ability, but I'm starting a further thread on sci.physics called "Specific heat of superconductors" where I hope to get more info on this topic.

[ Parent ]
one application today: cell phone towers (none / 0) (#33)
by gps on Tue Sep 03, 2002 at 10:35:58 PM EST

modern cell phone towers reportedly have a high temperature superconducting thing within their radio circuits that boosts their interference filtering (and thus their range, number of towers, your coverage, etc.) by roughly 2x.

here are some articles on a company selling products for this.


Electronics? (none / 0) (#39)
by Dyolf Knip on Mon Sep 23, 2002 at 12:04:14 AM EST

So how do you go about using superconductors in electronics? They obviously won't do as a simple replacement for semiconducting materials. How does a superconducting transistor work?

---
If you can't learn to do something well, learn to enjoy doing it poorly.

Dyolf Knip

To Infinity and Beyond! The Physics of Superconductivity | 39 comments (30 topical, 9 editorial, 0 hidden)
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