1 00:00:01,450 --> 00:00:06,220 080. 2 00:00:11,020 --> 00:00:14,470 Good evening, everybody, and welcome to this evening. 3 00:00:15,580 --> 00:00:24,700 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. 4 00:00:25,420 --> 00:00:29,590 But when you reach when you cool them down to low temperatures, possess some very remarkable properties, 5 00:00:29,590 --> 00:00:35,820 including complete loss of electrical resistance and the ability to levitate indefinitely. 6 00:00:35,830 --> 00:00:43,900 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, 7 00:00:45,310 --> 00:00:49,150 and I'll give you a little bit of history of superconductivity, which of which Oxford. 8 00:00:49,480 --> 00:00:53,820 This department has played a vital role. Outside. 9 00:00:53,830 --> 00:00:57,180 It's a bit about what superconductors do in magnetic fields. 10 00:00:58,210 --> 00:01:00,400 Which is important for applications. 11 00:01:01,090 --> 00:01:09,190 And I'll talk about quantum coherence, which is something you've heard about elsewhere and exciting about the applications of superconductivity. 12 00:01:10,570 --> 00:01:15,880 So that's the recipe. And let me start off by talking a little bit about electrical resistance. 13 00:01:18,310 --> 00:01:25,480 So what is electrical resistance? Well, we know that electrical current is carried by electrons which flow through a metal. 14 00:01:26,440 --> 00:01:35,320 And if you were paying attention in your GCSE physics, you will know that the flow of current is controlled by Ohm's Law, 15 00:01:35,980 --> 00:01:42,100 which tells you that the current is proportional to the voltage and the constant of proportional functionality is its resistance. 16 00:01:42,640 --> 00:01:45,610 And our understanding of resistance, at least at some level, 17 00:01:46,720 --> 00:01:53,110 is that you imagine that the electrons are just passing through the metal and every time they encounter some object they they scatter, 18 00:01:53,860 --> 00:01:58,580 they scatter randomly and they form this kind of pinball path through the material. 19 00:01:58,600 --> 00:02:05,829 So I like to think of that 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 20 00:02:05,830 --> 00:02:10,900 all these people dancing away and you've got to keep pumping into people and working your way round people. 21 00:02:10,930 --> 00:02:14,860 This is a bit like what electric electrons do in solids in metals. 22 00:02:16,420 --> 00:02:22,389 All right. Now, resistance is something which changes with temperature. 23 00:02:22,390 --> 00:02:29,950 And this graph on the right here shows the resistance of copper metal that's used in many wires and conductors as a function of temperature. 24 00:02:30,430 --> 00:02:34,839 And you can see that the resistance just drops smoothly as you reduce the temperature. 25 00:02:34,840 --> 00:02:37,330 This is in units of this in absolute units of Kelvin. 26 00:02:38,320 --> 00:02:44,140 So in these units, room temperature is actually somewhere near to this end of the graph and this is the absolute zero of temperature. 27 00:02:45,130 --> 00:02:50,740 And people were actually very interested in the early part of the 20th century and what 28 00:02:50,740 --> 00:02:55,450 actually really happens as you approach the lowest possible temperatures down here. 29 00:02:56,600 --> 00:03:00,889 And I noticed that. So actually what I'm plotting here is not resistance, but resistivity, 30 00:03:00,890 --> 00:03:06,980 which is a measure of the intrinsic resistance of material and independent of the shape of the sample that you make. 31 00:03:06,980 --> 00:03:11,510 So it's always the same for raw materials. The resistivity is just a measure of the intrinsic resistance, 32 00:03:11,510 --> 00:03:16,879 and notice you probably can't read it, but this is in units of ten to the minus nine, I think. 33 00:03:16,880 --> 00:03:24,740 So this is just part of that number in your mind for later, ten to the minus nine oh metres is the size of resistance in copper. 34 00:03:25,550 --> 00:03:30,379 So people are very interested in what happened as you cool metals down to low temperatures and some 35 00:03:30,380 --> 00:03:35,810 people said oh well what happens is that the the atom stopped vibrating because everything's very cold. 36 00:03:36,020 --> 00:03:44,000 And so that will let the resistance let the metal have a low resistance because the electrons can just travel without colliding so much. 37 00:03:44,480 --> 00:03:48,320 And other people said, oh, but at very low temperatures, the electrons won't have any energy. 38 00:03:48,320 --> 00:03:53,059 So in fact what will happen is they will stop moving and the resistance will just rise up at low temperatures. 39 00:03:53,060 --> 00:03:55,340 So there is just no understanding. 40 00:03:56,210 --> 00:04:06,320 And at that time, the person who was pioneering refrigeration technology was this man here, Hideki, coming on as in Leiden. 41 00:04:06,740 --> 00:04:08,390 This is a picture of his laboratory. 42 00:04:09,020 --> 00:04:15,200 It looks a little bit like what our laboratories look like today, full of wires and pipes and tubes and things like that. 43 00:04:15,200 --> 00:04:18,440 But this was his refrigeration apparatus to cool down to very low temperatures. 44 00:04:19,010 --> 00:04:23,090 And his achievement actually was to be able to cool down to the temperature at which helium liquefies. 45 00:04:24,610 --> 00:04:28,230 And for that, he was awarded the Nobel Prize in 1913. 46 00:04:28,240 --> 00:04:30,010 So it's a very impressive achievement. 47 00:04:30,850 --> 00:04:38,410 Now, one of the first things he did with his liquid fire was to measure the resistance of mercury as a function of temperature. 48 00:04:39,280 --> 00:04:43,870 And these are his original measurements that he made. So you can see these the points. 49 00:04:44,990 --> 00:04:52,170 And as he cool down and if you don't but you can see the scale but this is 4.4, 4.3, 4.2. 50 00:04:52,190 --> 00:04:56,840 These are units of Kelvin as he cooled down his mercury sample and his liquid fire. 51 00:04:57,200 --> 00:05:04,760 Eventually, all of a sudden, he found this a sudden drop from this value to essentially something which was close to zero on his scale. 52 00:05:05,510 --> 00:05:09,739 And then from at lower temperatures, he could not measure the resistance. 53 00:05:09,740 --> 00:05:13,639 And you can see here is written by hand, ten to the minus five ohms. 54 00:05:13,640 --> 00:05:17,990 That was the lowest you could measure the resistance of mercury with his apparatus. 55 00:05:18,770 --> 00:05:19,730 So it was like an upper limit. 56 00:05:20,480 --> 00:05:26,450 And actually when he first saw this, he thought there must be some problem with the apparatus because actually 4.2, which is this. 57 00:05:28,070 --> 00:05:32,750 Access temperature here. This is exactly the temperature which helium liquefies. 58 00:05:33,470 --> 00:05:39,140 So he assumed that this was just a spurious effect due to the liquid liquefying of helium. 59 00:05:40,050 --> 00:05:43,290 But however, he repeated this measurement and got the same result. 60 00:05:43,290 --> 00:05:49,330 And then he tried with different metals and found that they also had this very sharp drop ten code, 61 00:05:49,440 --> 00:05:55,490 a bit lower temperature led occurred in a bit higher temperature. So this seemed to be a common property of the metals that he was measuring. 62 00:05:55,500 --> 00:06:02,549 It wasn't just an artefact of the apparatus, so in fact it was just really bad luck in a way that his his first measurement produced a material 63 00:06:02,550 --> 00:06:06,750 that had this sharp drop at exactly the same temperature as the liquefying point of helium. 64 00:06:08,570 --> 00:06:12,590 And he didn't know what was causing this, but he coined the term superconductivity. 65 00:06:13,700 --> 00:06:16,940 Obviously, the resistance is very low. That means that conduction is very high. 66 00:06:17,630 --> 00:06:24,920 And so in this in the case of Mercury, he said that the transition temperature to this, whatever is happening here was at 4.2 Kelvin. 67 00:06:29,100 --> 00:06:35,520 Oh. So this is a picture of an ice hockey coach coming on, isn't his wife Maria. 68 00:06:36,000 --> 00:06:39,630 This is one of the house parties and he's got some of his physicist friends here. 69 00:06:40,260 --> 00:06:41,790 And you can see that in those days, 70 00:06:41,790 --> 00:06:46,680 what they used to do at parties is they used to talk about physics of the blackboard here with they've been discussing physics. 71 00:06:47,610 --> 00:06:53,310 And I don't know, can anybody recognise any famous people on this on this picture artist? 72 00:06:53,640 --> 00:07:00,210 Yes, of course. We have Einstein here. So. So honest was he moved in, you know, in high circles in physics. 73 00:07:01,200 --> 00:07:04,259 He's very well known, champ. Right. 74 00:07:04,260 --> 00:07:12,630 So on this realised of measured that the resistance of mercury and other metals went to very, very low values, but nobody really knew how low it was. 75 00:07:13,170 --> 00:07:17,370 And there were many experiments where people tried to measure how low the resistance 76 00:07:17,370 --> 00:07:20,699 really was after the metal went through this sharp superconducting transition. 77 00:07:20,700 --> 00:07:27,640 And one of the most celebrated experiments that was done was it was done in the sixties by Finland mills. 78 00:07:28,230 --> 00:07:31,980 And what they did was they made a ring out of a superconducting material. 79 00:07:32,340 --> 00:07:35,430 They cooled it down to very low temperature. So that became superconducting. 80 00:07:35,430 --> 00:07:41,999 So it had a very low resistance. Then they passed the magnet through this ring and this will induce a current in the ring. 81 00:07:42,000 --> 00:07:47,160 Just just like the way a dynamo works, you pass a magnet through, you get a current flowing in it. 82 00:07:48,330 --> 00:07:53,820 So after the current had been induced, the current has associated with it a magnetic field. 83 00:07:54,180 --> 00:07:58,890 And what file of mills did was they measured this magnetic field strength as a function of time. 84 00:07:59,490 --> 00:07:59,910 And in fact, 85 00:07:59,910 --> 00:08:07,080 they continued the experiment for several months and continued measuring this magnetic field strength as it decayed over time and in fact, 86 00:08:07,080 --> 00:08:15,720 decayed extremely slowly. So, so slowly, in fact, that they couldn't really detect a reduction in the current over a timescale of several months. 87 00:08:16,350 --> 00:08:19,559 And the conclusion from the work was, in fact, 88 00:08:19,560 --> 00:08:28,710 that the current in this loop would persist for 100,000 years if they let it continue going for that long, which of course, not very practical. 89 00:08:29,670 --> 00:08:31,830 So from that from that value, 90 00:08:32,070 --> 00:08:42,080 they they could calculate that the resistance of their loop here was the resistivity I should say was not greater than ten to the -23 oh metres. 91 00:08:42,760 --> 00:08:48,089 Remember I said before that the resistivity of copper is ten to the minus nine oh metres around room temperature. 92 00:08:48,090 --> 00:08:53,700 So this is 14 orders of magnitude lower resistance than the resistance of copper. 93 00:08:54,620 --> 00:08:58,999 And we now believe in fact that the in the superconducting state there really is zero resistance 94 00:08:59,000 --> 00:09:03,620 so that this current here is the nearest thing we have on earth to perpetual motion. 95 00:09:07,120 --> 00:09:13,580 Very good. So the next development which occurred in this field was by these chaps myself. 96 00:09:13,600 --> 00:09:16,990 An auction felt this is Meisner. He was a professor in. 97 00:09:18,360 --> 00:09:23,220 Leipzig and this is his PhD student at the time Oxford Felt. 98 00:09:23,760 --> 00:09:29,100 And what they did was they did an experiment where they cooled a metal in a magnetic field. 99 00:09:29,910 --> 00:09:35,729 And if it's just an ordinary metal like copper, then as you cool it down, nothing very, very much happens. 100 00:09:35,730 --> 00:09:41,040 The magnetic field is not disturbed by the copper sample at all, and it's all very uneventful. 101 00:09:41,580 --> 00:09:47,040 But if you cool the superconductor down in the magnetic field at high temperatures, it's still just behaving like a metal. 102 00:09:47,700 --> 00:09:54,839 But at low temperatures when it goes superconducting, remarkably what happens is that the superconducting material just expels, 103 00:09:54,840 --> 00:09:58,740 excludes the flux and expels it actively from the interior of the metal. 104 00:09:59,190 --> 00:10:06,380 So literally the flux is pushed out of the metal like this. It's as if the superconductor develops an opposite pointing magnetic field. 105 00:10:06,390 --> 00:10:09,030 It just cancels out within the volume of the superconductor. 106 00:10:09,630 --> 00:10:15,750 This is very remarkable and unexpected and it's a behaviour that cannot be described by classical physics at the time. 107 00:10:15,750 --> 00:10:17,940 So it's a quantum mechanical phenomenon. 108 00:10:18,720 --> 00:10:24,480 And the story goes that when this when this happened, oxygen fell within the laboratory, making the measurements, 109 00:10:24,990 --> 00:10:33,180 rushed into my office and said, Hey, Meisner, I have just discovered that the superconductor is excluding the flux. 110 00:10:33,570 --> 00:10:37,890 And Meissner said, Wunderbar, you've just discovered the Meissner effect. 111 00:10:41,100 --> 00:10:43,139 So I like to call it the Meissner Ox and wealth effect. 112 00:10:43,140 --> 00:10:46,590 But in the textbooks it's usually called the Meissner effect, which I think is a bit uncharitable. 113 00:10:48,810 --> 00:10:57,870 And at this point, though, there was no there was no theories of superconductivity that described superconductors in any adequate way. 114 00:10:58,590 --> 00:11:02,460 And many people had tried. And this this is one man who tried, Felix Bloch. 115 00:11:03,690 --> 00:11:08,340 He was a PhD student and then a postdoc working with Heisenberg. 116 00:11:09,210 --> 00:11:14,730 And he tried for a whole year, did nothing else, but tried to develop a theory of superconductivity. 117 00:11:15,480 --> 00:11:18,000 In the early sort of 1930s. 118 00:11:18,000 --> 00:11:26,310 He was working late 1920s and he he made one good theorem of superconductivity, which is correct and and is still known today. 119 00:11:26,760 --> 00:11:31,860 But he actually made a second sort of technique tongue in cheek theorem of superconductivity, 120 00:11:31,860 --> 00:11:35,780 which became known as Bloch Second Theorem of Superconductivity. 121 00:11:35,790 --> 00:11:43,560 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. 122 00:11:43,980 --> 00:11:51,300 What he said was, The only theorem about superconductors that can be proved is that any theory of superconductivity is refutable. 123 00:11:52,680 --> 00:11:57,180 So essentially nothing works. So this this was his. 124 00:11:57,180 --> 00:12:00,329 This is frustrated, Mr. Bloch. All right. 125 00:12:00,330 --> 00:12:07,380 This is a bit about Oxford now, so you can pay attention here. This these are two brothers called the London Brothers. 126 00:12:07,620 --> 00:12:10,020 This is Fritz and this is Heinz. 127 00:12:10,050 --> 00:12:19,680 And these they were German Jewish physicists in the early 1930s who were who had to escape Germany to escape the persecution of the Nazis. 128 00:12:19,770 --> 00:12:23,040 And like many Jewish scientists of that time, they moved around. 129 00:12:23,250 --> 00:12:24,540 They moved to other parts of the world. 130 00:12:24,930 --> 00:12:32,430 Quite a few came to the U.K. quite a few came to Oxford, brought by Linda Lindemann, who is a director of the kind in the poetry at the time. 131 00:12:33,330 --> 00:12:40,890 And the London Brothers. So they were so Heintz was a Ph.D. student here, and Fritz was a post-doc here in the department. 132 00:12:41,760 --> 00:12:46,410 And while they were here for a relatively short time, they worked in an upstairs room in Headington. 133 00:12:47,400 --> 00:12:53,430 On a theory of that, they tried to explain this. This phenomenon called the mice knocks and felt the fact they tried to understand 134 00:12:53,430 --> 00:12:57,510 this and they worked on it very hard and eventually they got the theory that worked. 135 00:12:58,140 --> 00:13:05,640 Basically, the theory is a sort of a quantum mechanical equivalent of Ohm's Law that applies to superconductors in the magnetic field. 136 00:13:06,450 --> 00:13:12,360 And they realised from their theory that for it to correctly describe this flux exclusion effect, 137 00:13:12,810 --> 00:13:18,420 what must happen is that all the electrons in the superconductor must be in exactly the same state of motion. 138 00:13:19,110 --> 00:13:23,520 This is a consequence. This is the only consequence that can lead to this phenomenon here. 139 00:13:25,630 --> 00:13:31,690 So this was this was a real breakthrough, although at the time it wasn't particularly recognised as a breakthrough, 140 00:13:31,690 --> 00:13:36,460 it was being particularly important and as is the way the money ran out. 141 00:13:36,820 --> 00:13:39,940 And so both of them had to be let go by the department. 142 00:13:40,510 --> 00:13:45,280 And hindsight, she went to Bristol University, had a successful career in Bristol for it, 143 00:13:45,370 --> 00:13:50,560 went to Paris and then went to North Carolina, Duke University in North Carolina. 144 00:13:50,950 --> 00:13:59,769 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. 145 00:13:59,770 --> 00:14:03,790 He had to delay by by one sailing. And and so he went on the next sailing. 146 00:14:03,790 --> 00:14:08,290 And then it turned out that the previous sailing was torpedoed by the Germans with great loss of life. 147 00:14:08,770 --> 00:14:13,750 So he was actually a very lucky man. Sometimes you have to be lucky in physics. 148 00:14:16,820 --> 00:14:21,070 So I'm still charting the historical development of superconductivity. 149 00:14:21,070 --> 00:14:29,470 And really, really important breakthrough was made by these three gentlemen, Bardeen, Cooper and Schrieffer in the late 1950s. 150 00:14:31,150 --> 00:14:36,790 Cooper, in fact, realised that electrons in a metal in metals, in fact attract one another, 151 00:14:37,330 --> 00:14:41,379 which sounds a bit counterintuitive because they're both because electrons are negatively charged, 152 00:14:41,380 --> 00:14:46,150 but actually they exist in a a background, a positive charge, which so essentially the whole metal is neutral. 153 00:14:46,720 --> 00:14:53,950 And it turns out that Cooper showed Cooper was able to show that any small interaction 154 00:14:53,950 --> 00:14:58,989 between the electrons would cause the electrons to want to pair up to form a repaired state, 155 00:14:58,990 --> 00:15:03,520 which would have a lower energy and if the electrons remained unpaired and just randomly moving around. 156 00:15:04,000 --> 00:15:09,930 So these are called Cooper pairs now. And I like to think of it this rather like this. 157 00:15:11,030 --> 00:15:18,439 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, 158 00:15:18,440 --> 00:15:22,010 then what happens is the mattress deforms and attracts the people together. 159 00:15:23,390 --> 00:15:30,170 On the other hand, if the two people lie on the mattress too far apart and the defamation of the mattress does not tend to attract the people. 160 00:15:30,170 --> 00:15:35,149 So this is a bit like this is a bit like what causes the attraction between electrons in the superconductor? 161 00:15:35,150 --> 00:15:36,770 There is a defamation of the lattice. 162 00:15:38,470 --> 00:15:44,560 Which is actually a dynamic defamation, which causes an effect of attraction between the electrons that makes them want to pair up like this. 163 00:15:46,480 --> 00:15:52,600 And if you want to sort of more real space picture of what's going on, here's the crystal lattice with the positive ions. 164 00:15:52,600 --> 00:15:54,520 The electrons are moving through this lattice. 165 00:15:54,940 --> 00:16:01,040 Here's one particular electron because it's negatively charged, it attracts the atoms towards it like this. 166 00:16:01,070 --> 00:16:07,180 Then it moves on and then another atom, seeing a positively charged region will actually be attracted to that region. 167 00:16:07,420 --> 00:16:11,020 And so, in effect, these two electrons of kind of talk to each other. 168 00:16:13,730 --> 00:16:17,209 And in a set in effect, what happens is as the electron moves through the metal, 169 00:16:17,210 --> 00:16:23,630 it leaves behind a wake of positive charge, which is like a transient charging of the metal. 170 00:16:24,110 --> 00:16:30,739 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, 171 00:16:30,740 --> 00:16:34,910 you can see that there's a sort of a net tendency for that wants to be bound together like that. 172 00:16:35,600 --> 00:16:41,780 So this is a sort of hand-waving picture for why you get these so-called Cooper Pairs superconductors. 173 00:16:43,480 --> 00:16:49,300 And in essence, the BCS theory is founded upon the notion that electrons pair up in this way. 174 00:16:49,930 --> 00:16:55,090 And the second thing that they do is they form what's called the macroscopic quantum coherence state. 175 00:16:55,690 --> 00:17:01,150 So I want to just try to explain in very simple terms what a macroscopic quantum coherence state is. 176 00:17:02,670 --> 00:17:07,240 And for this, we have to appreciate that all particles, according to quantum mechanics, 177 00:17:07,480 --> 00:17:11,980 behave in some circumstances, like as if they have waves, like as if they were waves. 178 00:17:12,850 --> 00:17:15,460 And we all know what waves are. 179 00:17:16,810 --> 00:17:22,180 If you have a collection of waves like this, which have got no particular relationship to one another and you sort of add them all together, 180 00:17:22,720 --> 00:17:29,120 what you end up with is kind of bunches of waves which are rather which oscillate for a bit, and then they decay away. 181 00:17:29,140 --> 00:17:34,360 So you have little sort of packets of waves, but with no particular regularity to the whole. 182 00:17:35,550 --> 00:17:41,910 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. 183 00:17:42,240 --> 00:17:46,950 But after a little time, they eventually decay away and become rather jumbled up again. 184 00:17:46,950 --> 00:17:50,730 And there's no kind of regular behaviour in the surface like this. 185 00:17:51,350 --> 00:17:58,980 Another another example is the light that comes from ordinary lamps like this actually consists of packets, 186 00:17:59,070 --> 00:18:05,670 wave packets, which are very, very short in duration, but have no special relationship to one another. 187 00:18:05,670 --> 00:18:12,990 So it looks a bit like this. So this is a situation that you this is how we normally encounter waves in real life. 188 00:18:13,620 --> 00:18:19,100 But there's another type of situation that can arise, which is where you have what's called a coherent wave. 189 00:18:19,110 --> 00:18:23,940 And this occurs if all if you start with a whole bundle of waves, all of which have the same wavelength, 190 00:18:24,420 --> 00:18:27,960 and where the maxima and the minima of the waves are all lined up like this, 191 00:18:28,470 --> 00:18:33,510 then if you add these together, they reinforce one another, constructive interference, 192 00:18:33,870 --> 00:18:39,600 and give you one big wave with a large amplitude and which will extend over a large distance. 193 00:18:39,810 --> 00:18:48,330 So this is this is a coherent wave. And this is the kind of this is this is a coherent wave is the kind of wave that you get in laser light. 194 00:18:48,360 --> 00:18:57,480 So this pointer here consists of a long stream of photons, of light waves, which is coherent over very, very long distances and times. 195 00:18:59,050 --> 00:19:02,590 Now in a superconductor, we have electron waves. 196 00:19:03,100 --> 00:19:07,839 And when the superconductor is just acting as a normal metal or is just a normal metal, 197 00:19:07,840 --> 00:19:14,880 then then we get the situation where we have just short waves in this liquid incoherent mixture like this. 198 00:19:16,100 --> 00:19:20,780 Whereas when the superconductor becomes superconducting, when it loses its resistance, 199 00:19:21,110 --> 00:19:26,569 then what happens is the electron waves all kind of organise themselves so that the phases and 200 00:19:26,570 --> 00:19:32,480 the wavelengths are all matched to match together and form what's known as is this coherent, 201 00:19:32,750 --> 00:19:36,110 microscopically coherent state like this. So like the laser light. 202 00:19:37,290 --> 00:19:42,929 And it's the reason why they do this is subtle. 203 00:19:42,930 --> 00:19:48,330 The reason why it's favourable, favourable for them to to lock all their phases together like that is rather subtle. 204 00:19:49,560 --> 00:19:56,520 But I think by analogy the same is my kind of party analogy where normal resistance is 205 00:19:56,520 --> 00:20:00,090 a bit like trying to get to the bar in a party with lots of people that are in the way. 206 00:20:01,080 --> 00:20:04,530 The other thing that you could do is you could all agree in the party, in the room, 207 00:20:04,530 --> 00:20:08,640 that everybody is going to step sideways together and keep on doing that. 208 00:20:09,210 --> 00:20:13,590 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, 209 00:20:14,040 --> 00:20:16,740 that requires a degree of kind of cooperation organisation. 210 00:20:16,750 --> 00:20:22,110 And that's that's essentially what the electrons can do when they go into a superconducting phase. 211 00:20:23,640 --> 00:20:30,150 So I want to talk a little bit about magnetic levitation, which is the other property that superconductors have. 212 00:20:30,930 --> 00:20:35,310 And for this, I want to to demonstrate a little bit this property. 213 00:20:35,640 --> 00:20:43,500 This is a piece of one piece of black ceramic, which is actually a superconducting material when we cool it down. 214 00:20:44,160 --> 00:20:56,700 So I want to I want to just demonstrate that stuff to get the needle. 215 00:21:03,500 --> 00:21:11,540 So I'm going to cool the superconductor down with a little bit of liquid nitrogen, a soup spoon, 216 00:21:17,570 --> 00:21:24,740 and I've got another one here, which is the same, but it's wrapped up in like a foam that's just to insulate it a bit. 217 00:21:27,300 --> 00:21:54,629 I'm going to cool that one down as well. It just takes a little bit of time to just take all the heat out of the superconductor when 218 00:21:54,630 --> 00:21:59,610 you can you can tell once it's cooled down because it stops sort of bubbling vigorously. 219 00:22:00,660 --> 00:22:17,520 So that one's nearly there. This one is not. Okay. 220 00:22:17,520 --> 00:22:18,419 So with this one, 221 00:22:18,420 --> 00:22:27,959 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 laid on the surface like this, 222 00:22:27,960 --> 00:22:34,930 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 the magnet. 223 00:22:36,220 --> 00:22:40,230 So. But. Look. 224 00:22:43,990 --> 00:22:45,190 And you see that that's floating. 225 00:22:45,190 --> 00:22:53,230 And then after a rather a few seconds, it loses all of its superconductivity because it warms up above the transition temperature. 226 00:23:01,150 --> 00:23:07,180 So I tried that again. Oops. So this rather unstable, as you can see. 227 00:23:10,420 --> 00:23:14,500 You can see it. It's floating. And then it just warms up. And then it goes it goes normal like that. 228 00:23:15,370 --> 00:23:20,200 So this is an example where the superconductor is actually behaving a bit like a magnet and it's just repelling. 229 00:23:20,850 --> 00:23:26,139 It's as if the superconductor has a magnetic pole and it's just repelling the magnetic poles here. 230 00:23:26,140 --> 00:23:34,450 Light poles repel. It's just it's just floating about. So this one here is a bit easier to play with because of the insulation. 231 00:23:34,450 --> 00:23:39,640 It lasts a bit longer. So what I've got here is a number of these magnets laid on a track like this, 232 00:23:40,420 --> 00:23:45,920 and I've just cooled it down with a little bit of space between the track and the superconductor itself. 233 00:23:47,050 --> 00:23:51,760 And you can see that now the superconductor is actually trapped onto the magnets. 234 00:23:51,970 --> 00:23:58,360 So the magnets and the there's a row of three magnets north, south and north like that. 235 00:23:58,360 --> 00:24:04,030 And that the superconductor actually trapped on that magnet because the magnetic field has stopped it from moving. 236 00:24:04,030 --> 00:24:08,470 And it's actually quite it's quite well tracked. For example, you can turn it sideways like this. 237 00:24:10,390 --> 00:24:19,630 You should even be able to turn it upside down. But it just warmed up too far and. Let's just try that again. 238 00:24:48,410 --> 00:24:55,430 So that's unusual, right? Do you think that's unusual? Why is it doing this? 239 00:24:55,940 --> 00:25:00,469 Because it's behaving as if it's behaving almost as if it's both repelling the magnet. 240 00:25:00,470 --> 00:25:05,880 And also when it's underneath, it is attracting the magnet. So so no conventional magnet can can do that. 241 00:25:05,900 --> 00:25:10,400 So what's going on here? Well. 242 00:25:11,640 --> 00:25:16,710 What's happening is that when the when the superconductor is a normal metal just behaving like an ordinary metal, 243 00:25:17,190 --> 00:25:21,680 the magnetic field lines just go all the way through the metal as if nothing happens and it would drop. 244 00:25:21,690 --> 00:25:26,520 And that's what happened the first time because it wasn't sufficiently cold, it just dropped. 245 00:25:28,380 --> 00:25:33,540 But if we place the superconductor on top of the magnetic, the magnet, as shown here, 246 00:25:33,540 --> 00:25:37,020 you see the magnetic field lines sort of distort and they get squashed like this. 247 00:25:37,200 --> 00:25:40,170 And this provides an upward force which holds the superconductor up. 248 00:25:40,770 --> 00:25:44,430 And it's doing this because the magnetic field cannot get inside the superconductor. 249 00:25:44,670 --> 00:25:51,630 These waves are supposed to represent those coherent electron waves. These these lines here, there's no magnetic field inside the superconductor. 250 00:25:52,410 --> 00:25:56,460 Now, you can see that actually some of these magnetic field lines actually go above the superconductor. 251 00:25:56,670 --> 00:26:04,140 That's because I as I cooled it on here, some of the magnetic field lines would actually go over the top of a superconductor like that. 252 00:26:04,710 --> 00:26:07,290 So when you turn the superconductor up upside down, 253 00:26:08,190 --> 00:26:12,990 what will happen is that these magnetic field lines will still be going around the end of the superconductor, 254 00:26:13,350 --> 00:26:17,969 but the superconductor cannot drop because these lines cannot pass into the superconductor. 255 00:26:17,970 --> 00:26:24,630 They're forced to stay underneath. And this can provide enough force if you do it properly to hold the superconductor in place. 256 00:26:24,780 --> 00:26:29,910 So this is why it can behave both as a like as attractive and as a repulsive magnet. 257 00:26:33,440 --> 00:26:41,360 And you can imagine that this this phenomenon could have applications in real life because you could you could make a trek, 258 00:26:42,170 --> 00:26:50,149 a train track out of these magnets, and you could put blocks of superconductors on the underside of the train and levitated. 259 00:26:50,150 --> 00:26:56,680 And because there's no friction or sound, for that matter, the only resistance is the air resistance. 260 00:26:56,690 --> 00:27:02,600 This would be a very efficient way to travel along very fast and with little energy. 261 00:27:02,990 --> 00:27:07,670 And in fact, these so-called mark-ups have been in existence for a long time, 262 00:27:07,940 --> 00:27:12,680 but the technologies up till now has been based on non superconducting magnets. 263 00:27:12,950 --> 00:27:16,160 So it requires a lot of energy then to to make them work. 264 00:27:17,030 --> 00:27:22,490 But there are demonstration maglev which are made precisely on this principle. 265 00:27:23,450 --> 00:27:28,250 There's one in China and now maybe this will be one of the technologies in the future. 266 00:27:30,970 --> 00:27:34,270 I have to finish shortly, but I just want to say something about how this field is moving. 267 00:27:34,960 --> 00:27:41,800 Of course, for this technology to be practical, we don't want to have to keep on cooling things down with liquid nitrogen. 268 00:27:41,800 --> 00:27:43,510 We want things to work at room temperature. 269 00:27:44,020 --> 00:27:54,460 And the evolution of superconducting materials as a function of time has had a rather erratic and unpredictable history. 270 00:27:54,580 --> 00:28:04,270 So this was the first superconductor discovered by honours in 1908, and this had a transmission temperature of 4.2 Kelvin, if you remember. 271 00:28:04,690 --> 00:28:09,969 And then he discovered lead, which is seven Kelvin. And then other people discovered niobium and so on and so forth. 272 00:28:09,970 --> 00:28:15,700 And gradually people discovered metals or alloys with higher and higher superconducting transition temperatures. 273 00:28:16,420 --> 00:28:24,069 A big breakthrough was made in 1986 when Bad Notes and Müller discovered this material, 274 00:28:24,070 --> 00:28:29,590 which I've been showing you today, which is a ceramic based on copper and oxygen with various other elements. 275 00:28:29,860 --> 00:28:35,530 And this actually shot the superconducting transition temperature up to well above 100 Kelvin. 276 00:28:36,220 --> 00:28:39,490 In fact, the highest is about 150 Kelvin. 277 00:28:39,490 --> 00:28:42,550 But you have to pressurise it to get to that kind of temperature. 278 00:28:42,940 --> 00:28:46,840 And there's also been another family discovered in the last ten years based on iron, 279 00:28:47,110 --> 00:28:52,060 which is which has reached quite high temperatures around about 50 or 60 Kelvin. 280 00:28:52,750 --> 00:28:55,870 And even actually three years ago, this this field is moving on. 281 00:28:55,870 --> 00:29:03,190 And even three years ago, a group in Germany discovered and you won't believe this discovered a superconductor based on hydrogen sulphide. 282 00:29:03,550 --> 00:29:11,379 Yeah. Stink bomb where if you pressure pressurised it to unimaginably high pressures. 283 00:29:11,380 --> 00:29:14,950 These are geological pressures 150 gigapascals. 284 00:29:14,950 --> 00:29:22,600 That's a hundred and that's that's like a million more than a million atmospheric pressures. 285 00:29:23,560 --> 00:29:29,860 I think then what they found was they got a superconducting material which which worked at just below 200 Kelvin. 286 00:29:29,860 --> 00:29:34,240 So that's that's the highest that's a world record for the highest temperature that we have at the moment. 287 00:29:35,930 --> 00:29:38,570 And on this graph, there's actually two important temperatures. 288 00:29:38,960 --> 00:29:44,240 One of them is the temperature of liquid nitrogen, which is this stuff here, and that is at 77, Kelvin. 289 00:29:44,630 --> 00:29:52,190 So you can see that anything that works up at higher temperatures than that can be demonstrated on, you know, in a laboratory like this. 290 00:29:52,730 --> 00:29:56,000 And this is quite cheap. This this costs about the same as milk. 291 00:29:56,600 --> 00:29:59,390 So this is actually reasonably inexpensive, this technology. 292 00:30:00,020 --> 00:30:11,810 And the other relevant temperature is this one here, which is the lowest recorded temperature on Earth at the Vostok Antarctic base, -89 Celsius. 293 00:30:12,530 --> 00:30:18,350 And that hydrogen sulphide is slightly above that. So, in fact, one can argue that this actually is a room temperature superconductor. 294 00:30:18,590 --> 00:30:31,180 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. 295 00:30:31,190 --> 00:30:35,749 And the advantage over ordinary magnets that have resistance, of course, 296 00:30:35,750 --> 00:30:42,440 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. 297 00:30:42,950 --> 00:30:51,920 And they also cost a lot less to run because there's no there's very little energy lost in the power that's that's used to generate the currents. 298 00:30:52,190 --> 00:30:58,670 And I'm sure you're well aware that MRI magnets are, in fact, these days are entirely made of superconductors. 299 00:30:59,090 --> 00:31:06,710 If you're ever unfortunate enough to have to go into one of these things, this is a big superconducting magnet carrying about a thousand amps. 300 00:31:07,430 --> 00:31:14,720 And so you roll into here. And yeah, it's interesting because a thousand amps is a big current and if that was to fail and you were inside it. 301 00:31:16,430 --> 00:31:22,400 But fortunately, because it remains because it remained superconducting, you just don't notice any current there at all. 302 00:31:23,450 --> 00:31:26,210 These are magnets in the sun, particle accelerator, 303 00:31:27,020 --> 00:31:34,040 superconducting magnets all the way around a 27 kilometre diameter ring and these carry of about 12,000 amps. 304 00:31:34,160 --> 00:31:38,060 So huge, huge currents, but no power dissipation. So they don't heat up at all. 305 00:31:39,550 --> 00:31:43,030 So these are two of the applications of superconducting magnets. 306 00:31:43,690 --> 00:31:49,299 And just to finish. Those of us who work in superconductors and there's quite a big group here in the 307 00:31:49,300 --> 00:31:54,550 department that do we all have the vision of a future which looks something like this? 308 00:31:55,450 --> 00:32:05,290 We have we have we have superconducting wires that carry power that's generated by some sort of renewable source with superconducting generators, 309 00:32:05,290 --> 00:32:08,950 superconducting storage devices that holds the current until you need it. 310 00:32:09,880 --> 00:32:19,330 Superconducting motors know basically everything is made of superconductors, even possibly quantum computers made of squid technology. 311 00:32:19,960 --> 00:32:22,450 So there's a huge number of potential applications. 312 00:32:22,450 --> 00:32:27,909 And the only limitation at the moment is in the fact that the materials have to be cooled down to low temperatures, 313 00:32:27,910 --> 00:32:30,700 which which just has practical and cost considerations. 314 00:32:31,360 --> 00:32:36,790 But if if somebody can find a room temperature, superconductivity, then this this vision will become a reality. 315 00:32:36,820 --> 00:32:39,160 So wish us luck. Thank you.