Superconductors: Miracle Materials - Public Lecture

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Good evening everybody. Welcome to this evening. I'm Andrew Boothroyd, I'm a professor of physics here and I'm going to tell you a little bit about some rather unremarkable looking materials but which when when you cool them down to low temperatures possess some very remarkable properties including complete loss of electrical resistance and the ability to levitate indefinitely and I'm talking of course about superconductors and what I hope to do this evening is to say a little bit about what electrical resistance is and I'll give a little bit of history of superconductivity of which Oxford - this department - has played a vital role. I'll say a little bit about what superconductors do in magnetic fields, which is important for applications, and I'll talk about quantum coherence which is something you've heard about elsewhere and say something about the applications of superconductivity. So that's the the recipe. Let me start off by talking a bit about electrical resistance. So what is electrical resistance? Well, we know that electrical current is carried by electrons, which flow through a metal and if you were paying attention in your GCSE physics you will know that the flow of current is controlled by Ohm's law, which tells you that the current is proportional to the voltage and the constant of proportionality is resistance. Our understanding of resistance, at least at some level, is that you imagine that the electrons are just passing through the metal and every time they encounter some object they they scatter they scatter randomly and they form this kind of pinball path through the material. So I like to think of it a bit - this is a bit like if you're at a party and you're trying to get from one end of the party to the bar and there's all these people dancing away and you've got to keep on bumping into people and working your way around people - this is a bit like what electric electrons do in solids - in metals. Alright, now resistance is something which changes with temperature and this graph on the right here shows the resistance of copper, a metal that's used in many wires and conductors, as a function of temperature. And you can see that the resistance just drops smoothly as you reduce the temperature. This is in units of - this an absolute units of Kelvin, so in these units room temperature is actually somewhere near to this end of the graph, and this is the absolute zero of temperature. People were actually very interested in the early part of the 20th century in what actually really happens as you approach the lowest possible temperatures down here. Notice that - so actually what I'm plotting here is not resistance but resistivity, which is a measure of the intrinsic resistance of a material and is independent of the shape of the sample that you make, so it's always the same for raw materials. The resistivity is just a measure of the intrinsic resistance. And notice - you probably can't read it, but this is in units of 10 to the minus 9 I think, so just park that number in your mind for later - 10 to the minus 9 Ohm meters is the size of resistance in copper. So people are very interested in what happens as you cool metals down to low temperatures, and some people said oh well what happens is that the the atoms stop vibrating because everything's very cold and so that will let the resistance - let the metal have a low resistance because the electrons can just travel without colliding so much. And other people said: oh but at very low temperatures the electrons won't have any energy so in fact what will happen is they'll stop moving and the resistance will just rise up at low temperatures. So there was just no understanding, and at that time the person who was pioneering refrigeration technology was this man here: Heike Kamerlingh Onnes in Leiden. This is a picture of his laboratory. It looks a little bit like what our laboratories looked like today - full of wires and pipes and tubes and things like that, but this was his refrigeration apparatus to cool down to very low temperatures. And his achievement actually was to be able to cool down to the temperature at which helium liquefies and for that he was awarded the Nobel Prize in 1913. So it's a very impressive achievement. Now one of the first things he did with his liquefier was to measure the resistance of mercury as a function of temperature and these are his original measurements that he made. So you can see these are the points, and as he cooled down - I don't know whether you can see this scale, but this says 4.4, 4.3, 4.2 - these are in units of Kelvin. As he cooled down his mercury sample in his liquefier eventually all of a sudden he found this of a sudden drop from this value to essentially something which was close to zero on his scale. Then at lower temperatures he could not measure the resistance and you can see here is written by hand 10 to the minus 5 Ohms. That was the lowest he could measure the resistance of mercury with his apparatus. So it was like an upper limit. And actually, when he first saw this he thought there must be some problem with the apparatus because actually 4.2, which is this axis temperature here, this is exactly the temperature at which helium liquefies. So he assumed that this was just a spurious effect due to the liquefying of helium. However he repeated this measurement and got the same result and then he tried with different metals and found that they also had this very sharp drop. Tin occurred at a bit low temperature. Lead occurred at a bit higher temperature. So this seemed to be a common property of the metals that he was measuring. It wasn't just an artifact of the apparatus. So in fact it was just really bad luck in a way that his first measurement produced a material that had this sharp drop at exactly the same temperature as the liquefying point of helium. And he didn't know what was causing this, but he coined the term superconductivity. obviously the resistance is very low that means that conduction is very high and so in this in the case of mercury he said that the transition temperature to this whatever was happening here was at 4.2 Kelvin Oh this is a picture of honest high key conic accounting honors and his wife Maria this is one of their house parties and he's got some of his physicist friends here and you can see that in those days what they used to do at parties is they used to talk about physics as a blackboard here whether they've been discussing physics and I don't know can anybody recognize any famous people on this on this picture yes of course we have Einstein here so so honest was he moved in you know in high circles in physics he is very well known chap right so honest realized or measured that the resistance of mercury and other metals went to very very low values but nobody really knew how low it was and there were many experiments where people tried to measure how know the resistance really was after the metal went through this sharp superconducting transition and one of the most celebrated experiments that was done I was in it was done in 60s by fire and mills and what they did was they made a ring out of a superconducting material they cooled it down to very low temperature so that became superconducting so it had a very low resistance and they passed a magnet through this ring and this will induce a current in the ring just just like the way a dynamo works you pass a magnet through a coil you get a current flowing in it so after the current had been induced the current has associated with it a magnetic field and what filer Mills did was they measured this magnetic field strength as a function of time and in fact they continued the experiment for several months and continued measuring this magnetic field strength as it decayed over time and in fact decayed extremely slowly solet's so slowly in fact that they couldn't really detect a reduction in the current over of timescale of several months and the conclusion from their work was in fact that the current in this loop would persist for 100,000 years if they let it to continue going for that long which of course not very practical so from that from that value fact they they could calculate that the resistance of their loop here was or the resistivity I should say was not greater than 10 to the minus 23 Oh meters remember I said before that the resistivity of copper is 10 to the minus 9 ohm meters around room temperature so this is 14 orders of magnitude lower resistance than the resistance of copper and we now believe in fact that the in a superconducting state there really is zero resistance so that this current here is the nearest thing we have on earth to perpetual motion very good so the next development which occurred in this field was by these chaps my snow and oxygen fell this is Meisner he was a professor in Leipzig and this is his PhD student at the time oxen felt and what they did was they did an experiment where they cooled a metal in a magnetic field and if it's just an ordinary metal like copper then as you cool it down nothing really very much happens the magnetic field is not disturbed by the the copper sample at all and it's all very uneventful but if you cool a superconductor down in a magnetic field at high temperatures it's still just behaving like a metal but at low temperatures when it goes superconducting remarkably what happens is that the superconducting material just expels excludes the flux and expels it actively from the interior of the metal so literally the flux is pushed out of the metal like this it's as if the superconductor develops an opposite pointing magnetic field that just cancels out within the volume of the superconductor this is very much remarkable and unexpected and it's a behavior that cannot be described by classical physics at the time so it's a quantum mechanical phenomenon and the story goes that when this when this happened oxygen valve was in the laboratory making the measurements rushed into my sinners office and said hey Meisner I have just discovered that the superconductor is excluding the flux and my sir said wunderbar you've just discovered the Meissner effect so I like to call it the Meisner ox and felt effect but in textbooks it's usually called the Meisner effect which i think is a bit uncharitable and at this point though there was no there was no theories of superconductivity that described superconductors in any adequate way and many people had tried and this this is one man who tried felix bloch he was a PhD student and then a postdoc working with Heisenberg and he tried for a whole year did nothing else but tried to develop a theory of superconductivity in the early sort of 1930s he was working late 1920s and he made one good theorem of superconductivity which is correct and and is still known today but he actually made a second sort of panicked tongue-in-cheek theorem of superconductivity which became known as blocks second theorem of superconductivity and he made this as a way of as a statement to describe his conclusions from this one year's futile attempts to make a theory what he said was the only theorem about superconductors that can be proved is that any theory of superconductivity can is refutable so essentially nothing works so this this was his this was frustrated mr. block all right this is a bit of an Oxford now so you can pay attention here this these are two brothers called the London brothers and this is Fritz and this is Heinz and these they were German Jewish physicists in the early 1930s who who had to escape Germany to escape the persecution of the Nazis and like many Jewish scientists at that time they moved around them they moved to other parts of the world quite a few came to the UK quite a few came to Oxford brought by Linda Linderman who was a director of the clown dinner poetry at the time and the London brothers so they were so Heinz was a PhD student here and Fritz was a postdoc here in the department and while they were here for a relatively short time they worked in a upstairs room in headington on the theory of that they tried to explain this this phenomena called the Meissner and felt effect they tried to understand and they worked on it very hard and eventually they got the theory that worked they basically the theory is a sort of a quantum mechanical equipment of Ohm's law that applies to superconductors in the magnetic field and they realized from their theory that for it to correctly describe this flux exclusion effect what must happen is that all the electrons in the superconductor must be in exactly the same state of motion yes this is a consequence this is the only consequence that can lead to this phenomena here so this was this was a real breakthrough although other time it wasn't particularly recognized as a breakthrough it has been particularly important and as is the way the money ran out and so both of them had to be let go by the department and hindsight she went to Bristol University had a successful career in Bristol Fritz went to Paris and then went to North Carolina Duke University in North Carolina and the story goes that Fritz because of his German passport he wasn't allowed to travel on the the ship that he planned to travel on he had to delay by by one sailing and as they went on the next sailing and then it turned out that the previous sailing was torpedoed by the Germans with great loss of life so he was actually a very lucky man sometimes you have to be lucky in physics so I'm still charting the historical development of superconductivity and a really really important breakthrough was made by these three gentlemen bardeen cooper and schrieffer in the late 1950s Cooper in fact realized that electrons in in Medan metals in fact attract one another which sounds a bit counterintuitive because they're both because electrons are negatively charged but actually they exist in a a background of positive charge which so essentially the whole metal is neutral and it turns out that Cooper showed Cooper was able to show that any small interaction between the electrons would cause the electrons to want to pair up to form a paired state which would have a lower energy and if the electrons remained unpaired and just randomly moving around so these are called Cooper pairs now and I like to think of it this rather like this the lattice in which the electrons move it's a little bit like a mattress and if two people sleep on a mattress close together then what happens is the mattress deforms and attracts the people together on the other hand if the two people lie on the mattress too far apart then the deformation of the mattress does not tend to attract attract the people so this is a bit like this is a bit like what causes the attraction between electrons in the superconductor there's a defamation of the lattice which is actually a dynamic defamation which causes an effect of attraction between the electrons and makes them want to pair up like this and if you want a sort of more real space picture what's going on here's the crystal lattice with the positive ions the electrons are moving through this lattice is one particular electron because it's negatively charged it attracts the atoms towards it like this then it moves on and then another atom seeing a positively charged region will actually attracted to that region and so in effect these two electrons have kind of talked to each other and in a set in effect what happens is as the electron moves through the metal it leaves behind a wake of positive charge which is like a transient charging of the metal and then another electron will also leave a wake like that and so you can see that as the electrons are attracted towards the positive charge you can see that there's a sort of net tendency for that funds to be bound together like that so this is a sort of hand waving picture for why you get these so-called Cooper pairs superconductors and in essence the BCS theory is founded upon the notion that electrons pair up in this way and a second a second thing that they do is they form what's called the macroscopic quantum coherence state so I want to just try to explain in very simple terms what a macroscopic quantum coherence state is and for this we have to appreciate that all particles according to quantum mechanics behave in some circumstances like as if they have waves like as if they're waves and we all know what waves are if you have a collection of waves like this which have got no particular relationship to one then you sort of add them all together what you end up with is kind of bunches of waves which I'd rather which oscillate for a bit and then they decay away so you have little sort of packets of waves but but with no particular regularity to the the whole so this is a bit like the picture we would have for waves on water if you look at the surface of water you can sort of see waves but after a little time they eventually decay away and become rather jumbled up again and there's no kind of regular behavior you know you know in the surface like this another another example is the light that comes from ordinary lamps like this actually consists of packets wave packets which are very very short in duration but but have no special relationship to one another so it looks a bit like this so this is situation that you this is that where we not how we normally encounter waves in real life but there's another type of situation that can arise which is where you have what's called the coherent wave and this occurs if all if you start with a whole bundle of ways all of which have the same wave length and where the maxima and the minima of the waves are all lined up like this then if you add these together they reinforce one another constructive interference and give you one big way with a large amplitude and which will extend you know over a large distance so this is this is a coherent wave and this is the kind of this is that this is a coherent wave is the kind of wave that you get in laser light so this pointer here consists of a long stream of photons or light waves which is coherent over very very long distances and times now in a superconductor we have electron waves and when the super conductor is just acting as a normal metal or it's just a normal metal then then we get this situation where we have just short waves in this so-called incoherent mixture like this whereas when the super conductor becomes superconducting when it loses its resistance then what happens is the electron waves all can organize themselves so that the phases and the wavelengths are all match to match together and and form what's known as this cuckoo this coherent microscopically coherent state like this so like the laser light and it's the reason why they do this is subtle the reason why it's favorable favorable for them to to lock all their phases together like that is rather subtle but I think by analogy in the same as my kind of party analogy where normal resistance is a bit like trying to get to the bar in a party with lots of people that are in the way the other other thing that you could do is you could all agree in the party in the room that everybody is going to step sideways together and keep on doing that and if you do that and you can see that you can get from one end of the room to the other without bumping into anybody so it requires a degree of kind of corporation organization and that's that's essentially what the electrons can do when they're going to superconducting phase so I want to talk a little bit about magnetic levitation which is the other property that superconductors have and for this I want to to demonstrate a little bit this property this is a piece of well it has a piece of black ceramic which is actually a superconducting material when we cool it down so I want to I want to just demonstrate that so I'm going to cool this superconductor down with a little bit of liquid nitrogen my super spoon and I've got another one here which is the same but it's wrapped up in arm like a foam that's just to insulate it a bit going to cool that one down as well it just takes a little time to just take all the heat out of the superconductor and you can you can tell once it's cooled down because it stops it stops bubbling vigorously so that one's nearly there this one is not okay so with this one what I want to do is I've got a little steel plate here which contains which has got some strong magnets which are just are laid on the surface like this and they create a magnetic field and what I want to do is to just show you what happens when we when we put the superconductor on on the magnet so you see that that's floating and then after rather a few seconds if it loses all of its super conductivity because it warms up above the transition temperature so I try that again oops so this rather unstable as you can see you can see it's it's floating it then it just warms up and then it goes it goes normal like that so this is an example where the superconductor is actually behaving a bit like a magnet and it's just repelling it's as if the superconductor has a magnetic pole and it's just repelling the magnetic poles here light poles repel it's just it's just floating above so this one here is a bit easier to play with because of the insulation it lasts a bit longer so what I've got here is a number of these magnets laid on a track like this and I've just cooled it down with a little bit of space between the track and the superconductor itself and you can see that now the superconductor is actually trapped onto the magnet so the magnets on this there's a run a row of three magnets north south and north like that and that the superconductor actually trapped on that magnet because the magnetic field is stopped it from moving and it's actually quite it's quite well trapped for example you can turn it sideways like this you should even be able to times upside down we're just warmed up too far then I'll just try that again so that's unusual right do you think that's unusual why is it doing this because it's behaving as if it's behaving almost as if it's both repelling the magnet and also when it's underneath it's attracting the magnet so no conventional magnet can can do that so what's going on here well what's happening is that when the when the superconductor is a normal metal just behaving like an ordinary metal the magnetic field lines just go all the way through the metal as if nothing happens and it would drop and that's what happened the first time because it wasn't sufficiently cold it just dropped but if we place the superconductor on top of the magnetic the magnet as shown here you can see the magnetic field line sort of distort and they get squashed like this and this provides an upward force which holds the superconductor up and it's doing this because the magnetic field cannot get inside the superconductor these waves are supposed to represent those coherent electron waves these these lines yet there's no magnetic field inside the superconductor now you can see that actually some of these magnetic field lines actually go above the superconductor that's because I cool as I cooled it on here some of the magnetic field lines will actually go over the top of the superconductor like that so when you turn the superconductor upside down what will happen is that these magnetic field lines will still be going around the end of the superconductor but the superconductor cannot drop because these lines cannot pass into the superconductor they're forced to stay underneath and this can provide enough force if you do it properly to hold the superconductor in place so this is why it can behave both as a like as a attractive and as a repulsive magnet and you can imagine that this this phenomenon could have applications in in real life because you could you could make a track a train track out of these magnets and you could put blocks of superconductors on the underside of the train and levitate it and because there's no friction or sound for that matter are the only resistance is the air resistance this would be a very efficient way to travel along very fast and with little energy and in fact these so-called mad left's have been in existence for a long time but the technologies up till now has been based on non superconducting magnets so it's really it requires a lot of energy then to make them work but there are demonstration mag lifts which are made precisely on this principle there's one in China and you know maybe this will be one of the technologies of the future I have to finish shortly but I just wants to say something about how this field is moving of course for this technology to be practical we don't want to have to keep on cooling things down with the liquid nitrogen we want things to work at room temperature and the evolution of superconducting materials as a function of time has had a rather erratic and unpredictable history so this was the first superconductor discovered by honors in in 1908 and this had a transition temperature of 4.2 Kelvin if you remember and then he discovered lead which is 7 Kelvin and other people discovered niobium and so on and so forth and gradually people discovered metals or alloys with higher and higher superconducting transition temperatures a big breakthrough was made in 1986 when Ed Norton Mulla discovered this material which I've been showing you today which is a ceramic based on copper and oxygen with various other elements and this actually shot the superconducting transition temperature up to well above 100 Kelvin in fact that the highest is about 150 Kelvin but you have to pressurize it to get to that that kind of temperature and there's also been another family discovered in the last 10 years based on iron which is which has reached quite high temperatures around about 50 or 60 Kelvin and even actually three years ago the this field is moving on and even three years ago a group in Germany discovered and you won't believe this they discovered a superconductor based on hydrogen sulfide yeah stink bomb where if you press pressurize it to unimaginably high pressures these are so geological pressures 150 Giga Pascal's that's a hundred and that's at that time like a million more than a million atmospheric pressures I think then what they found was they got a superconducting material which which worked at just below 200 Kelvin so that that's that's the highest that's a world record for the highest temperature that we have at the moment and on this graph there's actually two important temperatures one of them is the temperature of liquid nitrogen which is this stuff here and that is at 77 Kelvin so you can see that anything that works at higher temperatures than that can be demonstrated on that you know in a laboratory like this and this is quite cheap this this costs about the same as milk so this is actually reasonably inexpensive this technology and the other relevant temperature is this one here which is the lowest recorded temperature on earth at the Vostok Antarctic the base - 89 Celsius and in fact hydrogen sulfide is slightly above that so in fact one can argue that this actually is a room-temperature superconductor if you go to Vostok on a cold day one of the big applications of superconductors apart from transportation is and carrying current is in magnets and the advantage over ordinary magnets that have resistance of course is that you can put much higher current in them because they don't dissipate energy and so you can generate much higher magnetic fields and they also cost a lot less to run because there's no there's very little energy lost in the power this that's used to generate the currents and I'm sure you you're all aware that MRI magnets are in fact these days are entirely made of superconductors if you're ever an unfortunate enough to have to go into one of these things this is a big superconducting magnet carrying about a thousand amps and so you roll into here and yeah it's very interesting because a thousand amps is a big current and if that was to fail and you were inside it hmm unfortunately because it remains because it remains superconducting you know you just don't notice there's any current there at all these are magnets in the CERN particle accelerator superconducting magnets all the way around a 27 kilometer diameter ring and these carry current of about 12,000 amps so huge huge currents but no power dissipation so that they don't lead up at all so these are two of the applications of superconducting magnets and just to finish those of us who work ins in superconductors and there's quite a big group here in the department that do we all have the vision of a future which looks something like this now we have we have soup we have superconducting wires that carry our power that's generated by some sort of renewable source with superconducting generators superconducting storage devices that holds the current until you need it superconducting motors you know basically everything is made of superconductors even possibly quantum computers made of squid technology so there's a huge number of potential applications and the only limitation at moment is in fact that the materials have to be cooled down to low temperatures which which just has practical and cost considerations but if if somebody can find a room temperature superconductivity then this this vision will become a reality so wishes luck thank you you
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Channel: QM Oxford
Views: 2,275
Rating: 4.9333334 out of 5
Keywords: physics, oxford, superconductor, superconductivity, materials, levitation, floating, magnet, super conductor, lecture, public, public lecture, talk
Id: NzzchLXdGmE
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Length: 32min 48sec (1968 seconds)
Published: Mon Mar 19 2018
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