1 00:00:00,430 --> 00:00:18,900 I sort of. Thank you very much. 2 00:00:18,900 --> 00:00:22,590 Well, thank you for that warm welcome, and thank you, Katherine, for the introduction. 3 00:00:22,590 --> 00:00:27,510 Before I get began, I'm going to do a commercial in return for the Katharine's. 4 00:00:27,510 --> 00:00:33,540 So she has pioneered the quantum materials colouring book, which is freely available. 5 00:00:33,540 --> 00:00:41,280 You can download it from the website, either Google Quantum Materials Colouring Book or use that very short web address 6 00:00:41,280 --> 00:00:46,050 and you have a you have a free Christmas present to give it to a young relative, 7 00:00:46,050 --> 00:00:52,890 so I highly recommend it. OK, well, today what we're going to be talking about is the many universities of quantum materials. 8 00:00:52,890 --> 00:00:57,060 So this is the universe we actually live in or part of it. 9 00:00:57,060 --> 00:01:03,750 And what physicists do is they stare at the night sky or look at some of the phenomena they try and work out what's going on. 10 00:01:03,750 --> 00:01:10,170 So this is this is a galaxy, and scientists look at this and try and work out what's going on. 11 00:01:10,170 --> 00:01:16,800 Of course, it's constrained by physical constants, so things like the gravitational constant is a fixed constant. 12 00:01:16,800 --> 00:01:23,790 So if you want to have some explanation of this, then you need to use those constraints. 13 00:01:23,790 --> 00:01:31,260 The speed of light is another fixed constants, so any physical theory has to be constrained by those rules. 14 00:01:31,260 --> 00:01:33,360 And of course, we don't only look out for the night sky. 15 00:01:33,360 --> 00:01:41,760 We also look to the smallest parts of matter for which you have to dig up the Swiss French border and build the Large Hadron Collider. 16 00:01:41,760 --> 00:01:47,280 So this is here are some pictures of some of the detectors in the big ring around the border. 17 00:01:47,280 --> 00:01:56,370 So these very large scale experiments are used for looking at the smallest things and trying to work out what the laws of the universe are. 18 00:01:56,370 --> 00:02:02,310 So one of the questions we might ask and scientists have asked for a long time is what is the world made of? 19 00:02:02,310 --> 00:02:05,090 And various ideas have have come up from this. 20 00:02:05,090 --> 00:02:15,870 So in the very early days, some of the early pre Socratic thought that everything was made of water or possibly air or fire. 21 00:02:15,870 --> 00:02:20,520 And then finally, we came to the view that maybe it's some combination. 22 00:02:20,520 --> 00:02:25,470 In other words, it's not a simple answer. Maybe it's a complex answer. It's a mixture of things. 23 00:02:25,470 --> 00:02:30,000 So this is this is the first pleurisy substantial list. 24 00:02:30,000 --> 00:02:37,200 Somebody who thought that there was more than one thing that made up everything in the world fire, air and water. 25 00:02:37,200 --> 00:02:41,370 Of course, we have no idea what any of these people look like. So these are all representations. 26 00:02:41,370 --> 00:02:51,060 So given that the one on the right is in fact a Sicilian, I prefer to think that actually this is a better representation of what he looked like. 27 00:02:51,060 --> 00:02:57,750 Who knows? I think this is probably the best guess. OK, so we have different ideas of what the world is made of. 28 00:02:57,750 --> 00:03:03,430 But in fact, what we now would say is probably everything is made of certain elementary particles. 29 00:03:03,430 --> 00:03:09,750 And what is a particle? Well, it's something which throughout the universe, it doesn't exist, but there's one place where it exists. 30 00:03:09,750 --> 00:03:15,660 So it's something localised. So if I want to show that graphically, I can use this kind of function, it's nowhere. 31 00:03:15,660 --> 00:03:21,180 And then suddenly it appears and then it's not there anymore. That's what we mean by a particle. 32 00:03:21,180 --> 00:03:28,620 And of course, we know that there are certain fundamental particles. So you might think of, you know, electrons and protons and things like that. 33 00:03:28,620 --> 00:03:32,430 But there are also waves. There are other things in the universe as well. 34 00:03:32,430 --> 00:03:37,440 And the thing about a wave is rather than being localised, a wave is completely spread out. 35 00:03:37,440 --> 00:03:42,030 So again, we can realise that the universe must be populated by both particles and waves. 36 00:03:42,030 --> 00:03:49,680 And so examples of particles are things like the electron and the proton and the neutron and the things that make those up the corks. 37 00:03:49,680 --> 00:03:55,830 These are what you might imagine as being localised particles and things that are waves of things like light. 38 00:03:55,830 --> 00:03:58,320 That's a very obviously a wave. 39 00:03:58,320 --> 00:04:06,240 But if in fact, of course, is probably you will know in the early 20th century, we began to doubt this separation between these two types of things. 40 00:04:06,240 --> 00:04:13,050 Waves and particles, because experiments were done that show that in fact, light behaved very much like a particle. 41 00:04:13,050 --> 00:04:21,440 In fact, we sometimes call it a photon. And moreover, the electron, the proton, the neutron, we discovered they have wave like properties. 42 00:04:21,440 --> 00:04:29,090 So this separation between something localised where you know where it is and something spread out doesn't seem to be so clear. 43 00:04:29,090 --> 00:04:39,260 So how do you describe it? Well, we have a new description which is basically drawn here in the centre so we can get rid of the these other pictures. 44 00:04:39,260 --> 00:04:43,280 So you can imagine we've got something with a little knob and we can dial this up between 45 00:04:43,280 --> 00:04:48,950 particle and wave and when it's when it's a wave like it looks very light wave like. 46 00:04:48,950 --> 00:04:54,470 But then you can dial it back again and it can become localised and a particle. 47 00:04:54,470 --> 00:05:01,160 And so in fact, we now believe that all of these things are some kind of combination of a wave particle thing that, 48 00:05:01,160 --> 00:05:09,440 depending on how you look at it, can have both characteristics. So in fact, if you want a more modern view of what makes up the world, 49 00:05:09,440 --> 00:05:17,030 we would say that the world is permeated by quantum fields and particles are just excitations in those 50 00:05:17,030 --> 00:05:23,240 fields that regions where the field is suddenly decided to conglomerate and produce an electron. 51 00:05:23,240 --> 00:05:28,730 But the universe is permeated by all these different types of quantum fields, some of them electron quantum fields, 52 00:05:28,730 --> 00:05:36,500 some of them electromagnetic fields, and they spread throughout the universe and where they conglomerate, they produce particles. 53 00:05:36,500 --> 00:05:43,520 So that's maybe a modern picture. And so a lot of our physical theories are based on these quantum fields. 54 00:05:43,520 --> 00:05:49,280 Now, of course, those particles I was talking about the electrons and the protons and neutrons, they make up all of this stuff. 55 00:05:49,280 --> 00:05:58,730 This is the periodic table. And since in fact, this year is the 150th anniversary of Mandela's discovery of the periodic table, 56 00:05:58,730 --> 00:06:03,740 it's maybe just worth sort of stepping back a bit and having a look at this. So again, we see complexity. 57 00:06:03,740 --> 00:06:09,800 A chemist would say that the world is made up of all of these different elements. 58 00:06:09,800 --> 00:06:16,610 And Mandela didn't come up with a periodic table that looks exactly like this because he's had holes in and looking 59 00:06:16,610 --> 00:06:23,270 at the patterns of the elements he was able to predict certain elements might exist and fill in those gaps. 60 00:06:23,270 --> 00:06:29,190 So, for example, example germanium and gallium were elements that were not known before his time. 61 00:06:29,190 --> 00:06:33,450 But they fit fitted into little holes in his series. 62 00:06:33,450 --> 00:06:40,980 Now, one interesting part of the history of science, if we just focus on a little bit of the the periodic table, so I've just blown a section up. 63 00:06:40,980 --> 00:06:47,700 And if you focus on these smaller numbers that are just above these particular elements, these were the ones that he was working on. 64 00:06:47,700 --> 00:06:52,080 These are the mass numbers, the the big numbers 47, 48, 49, 50. 65 00:06:52,080 --> 00:06:55,770 Those are the ones where you know the answer, so you can just number them in order. 66 00:06:55,770 --> 00:07:00,510 But he was working with these masses. And what he realised was that in the periodic table, 67 00:07:00,510 --> 00:07:08,100 you went all the way from the low mass atoms like hydrogen, all the way up to the very heavy ones like uranium. 68 00:07:08,100 --> 00:07:14,250 And as you can see on this series, the numbers just steadily increase and they get bigger and bigger. 69 00:07:14,250 --> 00:07:20,200 Except those of you with sharp eyes might notice that there's a problem. So one of these tellurium? 70 00:07:20,200 --> 00:07:25,260 It doesn't actually work. You go one hundred and twenty one hundred and twenty seven. 71 00:07:25,260 --> 00:07:30,510 One hundred and twenty six point nine. So tellurium seems to be wrong. 72 00:07:30,510 --> 00:07:35,700 And mentally, I've noticed this. And he came to an obvious conclusion, which was that the experiment was wrong. 73 00:07:35,700 --> 00:07:40,860 So he told other chemists to go and redo the measurements and measure the mass of tellurium. 74 00:07:40,860 --> 00:07:43,860 And they did, and it came out to be pretty much the same. 75 00:07:43,860 --> 00:07:49,770 So he then thought, well, maybe iodine was wrong and got people to try and we measure that and it remained an anomaly. 76 00:07:49,770 --> 00:07:55,140 And in fact, this was not really properly sorted out until the work of Henry Moseley here in 77 00:07:55,140 --> 00:08:01,020 Oxford and Henry Moseley sadly killed shortly afterwards in the First World War. 78 00:08:01,020 --> 00:08:06,600 Henry mostly did experiments with X-rays to work out the energies of different atoms. 79 00:08:06,600 --> 00:08:12,360 What he realised was that in fact, ordering the periodic table by mass was the wrong way to do it. 80 00:08:12,360 --> 00:08:18,450 You had to order it by charge, and he had essentially measured the charge on the different atoms. 81 00:08:18,450 --> 00:08:22,590 And so these big numbers here are the atomic charge on the atoms. 82 00:08:22,590 --> 00:08:30,180 And this, in fact, is the right order. This was years, of course, after Mandela had done his work, but it wasn't mass. 83 00:08:30,180 --> 00:08:33,590 It was charged. And that tells you something very fundamental about the universe. 84 00:08:33,590 --> 00:08:40,430 Electromagnetism is really important and determines the structure of chemistry. 85 00:08:40,430 --> 00:08:46,430 OK, so what's the world made of particle physicist would ask the question which particles exist 86 00:08:46,430 --> 00:08:50,870 and they smash things together and try and work out which kind of particles are there? 87 00:08:50,870 --> 00:08:55,850 And they discovered the Higgs boson famously a few years ago. 88 00:08:55,850 --> 00:09:05,030 But that's the kind of question that they ask. One of the things that you find when you look at particles is that electricity is very important, 89 00:09:05,030 --> 00:09:09,530 so positive charges produce electric fields that come out. 90 00:09:09,530 --> 00:09:13,250 Negative charges have electric fields that go in. 91 00:09:13,250 --> 00:09:20,720 But since the 19th century, in the work of Maxwell, we have realised that magnetic fields behave very differently. 92 00:09:20,720 --> 00:09:23,960 The magnetic field lines just go round and round in loops. 93 00:09:23,960 --> 00:09:34,730 In other words, they never sort of originate from a charge or they never land on a charge to use the terminology, but there were no divergences. 94 00:09:34,730 --> 00:09:42,710 And so what we have realised is that there are no magnetic charges or magnetic monopoles to use the jargon. 95 00:09:42,710 --> 00:09:47,360 Now, in fact, there are various reasons why we might like magnetic monopoles to exist. 96 00:09:47,360 --> 00:09:49,790 And so therefore, over the last 100 years, 97 00:09:49,790 --> 00:09:59,000 physicists have intensively looked for magnetic monopoles and there have been detailed searches using very sophisticated, time consuming experiments. 98 00:09:59,000 --> 00:10:03,290 And so far, they have all failed to find any of these magnetic monopoles. 99 00:10:03,290 --> 00:10:10,240 So what we certainly know is that they don't exist in any abundance and possibly they don't exist at all. 100 00:10:10,240 --> 00:10:14,920 Now, that's an important thing to take away for what I'm going to say later. 101 00:10:14,920 --> 00:10:23,140 There's a big but in all of this, we are stuck when we do physics experiments with the fixed rules of the universe. 102 00:10:23,140 --> 00:10:27,850 In other words, we're stuck with a fixed set of particles, the ones that we've discovered so far. 103 00:10:27,850 --> 00:10:31,960 Maybe there are a few others yet to discover, but they probably don't live very long. 104 00:10:31,960 --> 00:10:38,560 And there's a fixed set of parameters, the fundamental constants that sit at the back of our physics textbooks. 105 00:10:38,560 --> 00:10:42,850 Things like the speed of light. And they're set at fixed values. 106 00:10:42,850 --> 00:10:50,320 And that constrains the way the universe is, that's a very good thing because it means that life can exist in various other properties can exist. 107 00:10:50,320 --> 00:10:54,940 But one of the questions we might like to ask is what if the rules were different? 108 00:10:54,940 --> 00:11:02,110 What if we set up the universe a different way? Now, people speculate about this, and they wonder about the multiverse, 109 00:11:02,110 --> 00:11:06,520 for which there is no experimental evidence, so I'm not going to talk about that at all today. 110 00:11:06,520 --> 00:11:11,590 I'm going to talk about things for which there are experimentally evidence. 111 00:11:11,590 --> 00:11:17,350 Essentially, is there a way to explore ways in which the universe could be set up with a different set of rules, 112 00:11:17,350 --> 00:11:22,420 which might then produce different sets of particles and different constraints? 113 00:11:22,420 --> 00:11:27,940 And it turns out that there are a way to change the rules, and that is through quantum materials. 114 00:11:27,940 --> 00:11:35,770 Unsurprisingly, given this is the Quantum Materials Symposium. The thing about a quantum material, it's a crystal. 115 00:11:35,770 --> 00:11:39,640 It may not necessarily be a crystal, but that's all I'm going to be talking about today. 116 00:11:39,640 --> 00:11:45,370 It's a crystal like this with atoms moving over a very large distance or fixed in a lattice, 117 00:11:45,370 --> 00:11:49,960 which is over an enormous distance on the scale of an atom. 118 00:11:49,960 --> 00:11:53,110 That may be something that you can actually hold in your hand. 119 00:11:53,110 --> 00:12:02,830 And each one essentially behaves like a new universe with its own set of rules, its own set of fields and its own set of particles. 120 00:12:02,830 --> 00:12:08,860 So what we have in every single different crystal that we work with is a different universe to play with. 121 00:12:08,860 --> 00:12:17,530 And that's where the excitement is now. Many of these quantum materials have rather complicated chemical formulae. 122 00:12:17,530 --> 00:12:23,860 And if you didn't like chemistry at school, you'll see these chemical formulae and think, No, this is not what I want to really be thinking about. 123 00:12:23,860 --> 00:12:27,340 I've come to a physics lecture. I don't want to see these things. 124 00:12:27,340 --> 00:12:32,950 But the great thing is about these crystals, they have to be made of atoms, and all we have is the periodic table. 125 00:12:32,950 --> 00:12:36,160 So we have to have various different combinations of atoms. 126 00:12:36,160 --> 00:12:41,530 But the great thing about the periodic table is there are lots of atoms in it, so there are lots of combinations. 127 00:12:41,530 --> 00:12:44,890 So there are lots of things that we can do that these are actually rather 128 00:12:44,890 --> 00:12:49,180 simple compounds here or some other ones that actually I've worked on recently. 129 00:12:49,180 --> 00:12:53,560 And one of them, as you can see, the the chemical formula just goes on forever and ever. 130 00:12:53,560 --> 00:12:56,560 So some of these things are chemically quite complicated. 131 00:12:56,560 --> 00:13:02,920 But as physicists, why we're interested interested is because they can sometimes show incredibly simple, 132 00:13:02,920 --> 00:13:11,110 beautiful properties, which don't really depend on the complexity of the chemistry that lies behind them. 133 00:13:11,110 --> 00:13:16,690 OK. How do you make quantum materials? Well, what you really need? This is an example of a quantum material. 134 00:13:16,690 --> 00:13:24,110 This is a crystal of copper near bait, and to make it, you need a very specialised piece of apparatus. 135 00:13:24,110 --> 00:13:28,450 This is a mirror furnace and we have one upstairs here in the cloud inventory. 136 00:13:28,450 --> 00:13:29,980 But you don't just need the material. 137 00:13:29,980 --> 00:13:37,390 You need somebody very clever who has spent their career optimising this, and we have Dr. Prabhakaran here who does this. 138 00:13:37,390 --> 00:13:40,930 Now, that's not the only way you can do it. This is how physicists make quantum materials. 139 00:13:40,930 --> 00:13:47,650 We tend to like crystals. But in fact, if you want to make some completely new material, what you really need is clever chemists. 140 00:13:47,650 --> 00:13:53,710 And here's to that. I work with who are based here in Oxford. But there are many, many others, and it's a very, 141 00:13:53,710 --> 00:14:01,630 very large community of people who have to make innovative choices in terms of designing new materials and the 142 00:14:01,630 --> 00:14:09,100 clever thing that these two people do in different ways is trying to find systems that are made out of equilibrium. 143 00:14:09,100 --> 00:14:17,880 So rather than just finding the ground state, they find clever ways of tricking nature into producing very unusual materials. 144 00:14:17,880 --> 00:14:22,020 OK, so we have these quantum materials. 145 00:14:22,020 --> 00:14:23,490 Each one is a new universe. 146 00:14:23,490 --> 00:14:30,360 Each one is set up with these new rules, new fields and new particles, and that's what I'm going to be telling you about today. 147 00:14:30,360 --> 00:14:35,010 So let me, first of all, start with a really, really simple example. So this is almost trivial. 148 00:14:35,010 --> 00:14:41,700 But if you take something like diamond, diamond is essentially carbon arranged in a particular lattice, 149 00:14:41,700 --> 00:14:45,840 then it has a rather unusual property, which is that light. 150 00:14:45,840 --> 00:14:53,490 We can actually change the speed of light in diamonds. Now this is something you will know and has been known for centuries that materials 151 00:14:53,490 --> 00:14:57,570 have a thing called a refractive index and that changes the speed of light. 152 00:14:57,570 --> 00:15:03,290 It's quite a dramatic change in diamond. Two point four times slower is a big effect. 153 00:15:03,290 --> 00:15:11,910 Why does it occur? It occurs because when you fire light into diamonds, all of the atoms of carbon are surrounded by charge, 154 00:15:11,910 --> 00:15:17,550 and the light interacts with that charge in such a complicated way, it gets absorbed and irradiated. 155 00:15:17,550 --> 00:15:24,330 And that complexity can be just summarised in a simple number two point four, the refractive index. 156 00:15:24,330 --> 00:15:25,650 And that's what gives diamond. 157 00:15:25,650 --> 00:15:34,800 It's rather shiny properties because you get a lot of total internal reflection when you have a lot of high refractive index interfaces with air, 158 00:15:34,800 --> 00:15:39,780 which has a low refractive index. So that's one simple example we can change the speed of light. 159 00:15:39,780 --> 00:15:44,040 It's almost trivial. Slightly less trivial is this. 160 00:15:44,040 --> 00:15:47,700 This is calcite, a different material. 161 00:15:47,700 --> 00:15:53,940 And the interesting thing about this is that it has a speed of light, which is different along different directions. 162 00:15:53,940 --> 00:16:00,870 So I took this photograph earlier today. If you just move the crystal down over where I'd written quantum materials. 163 00:16:00,870 --> 00:16:09,240 You get a double image. And this is because light with different polarisations goes through the calcite crystal with a different speed of light. 164 00:16:09,240 --> 00:16:13,200 And so you get a double image of so-called by refrigerant effect. 165 00:16:13,200 --> 00:16:20,700 So another very, very simple case these these two are almost trivial and you're probably relatively familiar with those. 166 00:16:20,700 --> 00:16:27,040 So let's maybe go to something more complicated. Let's look to see if we can make new particles. 167 00:16:27,040 --> 00:16:33,970 So this, again, is a simple example that some of you may have heard of and a semiconductor, 168 00:16:33,970 --> 00:16:38,920 you have what's known as a valence band, which is largely full of electrons. 169 00:16:38,920 --> 00:16:43,240 Then you have a band gap and then you have a conduction band, which is largely empty. 170 00:16:43,240 --> 00:16:50,750 This is the kind of structure that you have in something like silicon inside silicon chips. 171 00:16:50,750 --> 00:16:56,060 Now, the valence band is pretty much full of electrons. The conduction band is almost empty. 172 00:16:56,060 --> 00:16:59,840 But what can happen if you're not at zero temperature is that some of the electrons 173 00:16:59,840 --> 00:17:04,640 in the violence band can move up into the conduction band and they can wander around. 174 00:17:04,640 --> 00:17:14,110 And what they leave in the valence band are holes. Now the holes themselves can move around and they behave like independent particles. 175 00:17:14,110 --> 00:17:19,930 Now, you might say. And physics students often do, but hold on a whole doesn't really exist. 176 00:17:19,930 --> 00:17:25,160 Whole is just an absence. So how can a hole really exist? 177 00:17:25,160 --> 00:17:31,880 If the hole moved one place to the right, it's really because an electron moves one place to the left. 178 00:17:31,880 --> 00:17:35,810 So why are you going for this complicated description of talking about holes? 179 00:17:35,810 --> 00:17:42,260 But the reason because physicists like it is we always focus on the simple thing where there's very few of them. 180 00:17:42,260 --> 00:17:48,440 And that's the way human brains work. So we focus on the holes. And there's a there's a sort of similar analogy with this. 181 00:17:48,440 --> 00:17:52,250 If later on after this lecture, you need some refreshments. 182 00:17:52,250 --> 00:17:59,130 Then as you stare into your your beer, you will probably notice absences of beer floating up to the surface. 183 00:17:59,130 --> 00:18:05,720 Now, of course, we call these bubbles. Yes, they've got carbon dioxide in them, but they're basically regions where there's no beer. 184 00:18:05,720 --> 00:18:12,590 So really, what you should be saying is the beer is falling down, but in fact, you focus on the on the bubbles going up. 185 00:18:12,590 --> 00:18:15,890 And the reason you focus on the bubbles is because there's few of them. 186 00:18:15,890 --> 00:18:24,250 If there's lots and lots of bubbles, if your beer glass is absolutely full of bubbles and no beer, it's time to get another beer. 187 00:18:24,250 --> 00:18:29,920 OK. That's one way in which you can make new particles just by removing old particles. 188 00:18:29,920 --> 00:18:36,100 But here's a rather subtle way. So if you take something that's definitely a wave and oscillating mass on a spring, 189 00:18:36,100 --> 00:18:42,040 the kind of thing that we torture first, geophysicists, physics students here working out. 190 00:18:42,040 --> 00:18:46,960 So the spring goes, the mass goes up and down and the spring compresses. 191 00:18:46,960 --> 00:18:52,030 Now one thing you can ask is what happens if I take energy out of this system? 192 00:18:52,030 --> 00:18:56,020 So here is a mass on a spring with less energy in it. 193 00:18:56,020 --> 00:18:59,830 And here now is one with even less energy in it. 194 00:18:59,830 --> 00:19:05,140 So as you can see, the frequency stays the same, but the amplitude goes down. 195 00:19:05,140 --> 00:19:10,900 Now what happens if I keep on taking energy out? Well, eventually the mass will be stationary. 196 00:19:10,900 --> 00:19:16,330 I didn't draw that because I thought you could imagine it. When it's completely stationary, though, 197 00:19:16,330 --> 00:19:22,900 there's a problem because Heisenberg's uncertainty principle say says that if you know that it's stationary, you know that it isn't moving. 198 00:19:22,900 --> 00:19:27,820 You don't know where it is. And the consequence of that is you can't take all of the energy out. 199 00:19:27,820 --> 00:19:34,330 There has to be a little bit left. So eventually, the mass on the spring has to be vibrating very, very slightly. 200 00:19:34,330 --> 00:19:39,040 Not so much that you can see with a real big one kilogram mass on a spring. 201 00:19:39,040 --> 00:19:45,220 But if it's an atom vibrating on a bond, then this so-called zero point energy is a very real thing. 202 00:19:45,220 --> 00:19:52,820 And another consequence of that, it turns out, is that when you add the energy back in, you can only add it in lumps. 203 00:19:52,820 --> 00:19:54,830 And those lumps are cancer, 204 00:19:54,830 --> 00:20:00,830 so it's a little bit like a vending machine that will only allow you to enter your your money in multiples of 20p or something, 205 00:20:00,830 --> 00:20:05,750 it won't accept 10 days. So there's a basic unit that you have to add energy. 206 00:20:05,750 --> 00:20:13,880 And what that essentially means is waves make particles because of this fact that you can only add energy in lumps. 207 00:20:13,880 --> 00:20:19,550 It means that there is a natural quantum nature to the way you add energy. 208 00:20:19,550 --> 00:20:25,220 So any wave like system has associated with it particles of energy. 209 00:20:25,220 --> 00:20:30,560 And this is a rather fundamental feature. Of course, it's rather simple for mass on a spring. 210 00:20:30,560 --> 00:20:35,090 But when we deal with the quantum material, what we typically have is a huge number of atoms in our crystal. 211 00:20:35,090 --> 00:20:40,550 I'm just showing you a plane here with a rather interesting normal mode, 212 00:20:40,550 --> 00:20:47,960 but you have lots and lots of different ways in which the crystal can vibrate, and each one of those will correspond to a particular particle. 213 00:20:47,960 --> 00:20:55,320 These are known as photons, and they behave very much like real particles in the system. 214 00:20:55,320 --> 00:21:01,710 Now, it turns out we can do the same with magnetism, so here is a whole lot of spins which are all aligned. 215 00:21:01,710 --> 00:21:05,190 These are magnetic moments. Each one is a magnetic atom. 216 00:21:05,190 --> 00:21:11,650 And what I've done is I've set up a wave motion in them and this is a self-sustaining wave motion. 217 00:21:11,650 --> 00:21:15,950 And because essentially it's behaving like a wave. There are particles associated with it. 218 00:21:15,950 --> 00:21:20,370 We call those magnums. Here's another example of. 219 00:21:20,370 --> 00:21:30,120 And now in two dimensions of a whole lot of magnetic moments or processing in a rather complicated dance, that's one of the normal modes. 220 00:21:30,120 --> 00:21:32,190 Here's another one. Slightly different. 221 00:21:32,190 --> 00:21:39,390 So if I go back to the previous one, you can probably see lines of these magnetic moments or vibrating together. 222 00:21:39,390 --> 00:21:44,070 If I move to the next one, you can see the lines now go in a different direction. 223 00:21:44,070 --> 00:21:47,910 So there are lots of these different normal modes that you can play with. 224 00:21:47,910 --> 00:21:54,120 And if you stare at that long enough, you will soon be hypnotised. So I should probably move on from this. 225 00:21:54,120 --> 00:22:00,380 But each one of these normal modes are associated with particles. Now, how can we really detect that those particles are there? 226 00:22:00,380 --> 00:22:06,030 Well, one of the ways we can do it is we can go to somewhere that will scatter things off those particles. 227 00:22:06,030 --> 00:22:07,670 And this is very close to us here. 228 00:22:07,670 --> 00:22:17,690 This is the Rutherford Appleton laboratory, about 15 miles south of Oxford, so we can hear use very high energy photons, X-rays. 229 00:22:17,690 --> 00:22:21,680 We can use neutrons. We could also use muons. 230 00:22:21,680 --> 00:22:25,880 For those of you that are local, that's the A34 going down there. 231 00:22:25,880 --> 00:22:30,620 Actually, this is an old photograph, so you can see the power station, which no longer exists. 232 00:22:30,620 --> 00:22:34,880 So I've rubbed it out. 233 00:22:34,880 --> 00:22:41,390 But what happens in these types of measurements, particularly the neutrons and the X-rays, is we do something which is very much like this. 234 00:22:41,390 --> 00:22:43,400 It's essentially snooker. 235 00:22:43,400 --> 00:22:52,940 So a neutron or an X-ray is fired into the sample and it will bounce off one of these particles one of these magnums, or phonons. 236 00:22:52,940 --> 00:22:58,000 And so we can see these particle like excitations, we really know that they're there. 237 00:22:58,000 --> 00:23:04,220 There are various people in the department, my colleagues here, Andrew, Radu, Paolo and Roger, 238 00:23:04,220 --> 00:23:10,670 who do these kinds of measurements using these techniques to study these types of materials. 239 00:23:10,670 --> 00:23:17,830 So I'm also involved as well with the muons, as you heard earlier. Other types of new particles. 240 00:23:17,830 --> 00:23:26,410 Now this is my one equation in this talk. This is a public lecture, so this is kinetic energy equals a half the squared. 241 00:23:26,410 --> 00:23:30,160 I'm looking at a couple of people here right now know this very good. 242 00:23:30,160 --> 00:23:37,900 So if you have the energy as a half mass times, the velocity squared and you plot it as the graph, you have a parabola. 243 00:23:37,900 --> 00:23:42,950 That's a very simple thing that people learn for their GCSE physics. 244 00:23:42,950 --> 00:23:48,970 And if you look inside a quantum material, what's the equivalent? Well, it looks a little bit like this. 245 00:23:48,970 --> 00:23:54,400 So this is for Strontium Erudite, which was discussed in our symposium earlier today. 246 00:23:54,400 --> 00:23:59,470 So I'm putting energy against essentially velocity its momentum, but it's essentially the same thing. 247 00:23:59,470 --> 00:24:05,410 And so what you see is something that looks rather complicated, so it looks a little bit like this. 248 00:24:05,410 --> 00:24:10,300 Lots and lots of these energy bands. So what can we say that's going on? 249 00:24:10,300 --> 00:24:12,010 Why is it so complicated? 250 00:24:12,010 --> 00:24:20,460 Well, it's partly so complicated because we're dealing with electrons moving through a complicated periodic system with lots of atoms that. 251 00:24:20,460 --> 00:24:27,480 The electronic states are all made out of the electronic states around atoms and atoms have lots of energy levels, 252 00:24:27,480 --> 00:24:30,240 so that means lots of lines of spaghetti. 253 00:24:30,240 --> 00:24:38,830 But the interesting thing that you might notice is at the bottom of some of these bands, they look approximately parabolic. 254 00:24:38,830 --> 00:24:44,050 And that means we can say that they behave a little bit like an electron in a vacuum, 255 00:24:44,050 --> 00:24:50,470 except the curvature is different, and looking at this equation, you can see that means the mass must be different. 256 00:24:50,470 --> 00:24:55,360 So the interesting thing about many of these materials is the electron takes on a different mass. 257 00:24:55,360 --> 00:24:58,030 So one of our fundamental constants in the universe, 258 00:24:58,030 --> 00:25:03,220 we're able to adjust in these different materials because we can have the mass being anything we like. 259 00:25:03,220 --> 00:25:08,710 In fact, it can even sometimes be negative because we can be we can have something that looks like an empty parabola. 260 00:25:08,710 --> 00:25:17,600 So the design changes. So this is a very interesting thing, and it also turns out because of interactions between electrons and other electrons. 261 00:25:17,600 --> 00:25:22,070 We sometimes end up having strange enhancements of the mass. 262 00:25:22,070 --> 00:25:24,840 And this is something very interesting for physicists to understand. 263 00:25:24,840 --> 00:25:29,210 It's a little bit like the effect that you might have is if you go to a cocktail party 264 00:25:29,210 --> 00:25:35,180 and there are lots of people standing around and you want to get to the end of the room. And the problem is, there's lots of people in your way. 265 00:25:35,180 --> 00:25:38,660 So it takes you a long time to get from one end of the room to the other. 266 00:25:38,660 --> 00:25:45,050 It's like your mass is enhanced, and this kind of effect also occurs in these quantum materials. 267 00:25:45,050 --> 00:25:55,690 So one of the people who studies that here in Oxford is Amalia Kildare, who does very elegant experiments to explore these kinds of band structure. 268 00:25:55,690 --> 00:26:00,400 OK, so lots of new particles in these materials, some of them are rather complicated, 269 00:26:00,400 --> 00:26:06,220 so I'll just will quite quickly through some of the more exotic things that have been seen. 270 00:26:06,220 --> 00:26:11,470 So in graphene, the wonder wonder material made out of carbon sheets. 271 00:26:11,470 --> 00:26:19,180 In fact, you can make graphene if you have a pencil. What comes off your pencil is lumps of graphene, sometimes graphene stacks. 272 00:26:19,180 --> 00:26:26,470 And it turns out that when you do the similar kind of analysis for the electrons in graphene, 273 00:26:26,470 --> 00:26:32,110 you end up having electrons that behave not like quadratic x, but a linear behaviour. 274 00:26:32,110 --> 00:26:38,680 And this looks very much like a particle, like a photon, except the speed of light is different. 275 00:26:38,680 --> 00:26:46,240 So the electrons in graphene behave very much like light, but with a speed 300 times slower. 276 00:26:46,240 --> 00:26:50,070 So that's a very interesting phenomenon. 277 00:26:50,070 --> 00:26:59,790 Another type of particle that is studied in some of these materials, tantalum arsenide is one that shows this a so-called vhile fermions. 278 00:26:59,790 --> 00:27:04,800 Herman Vail was a physicist who worked in the first half of the 20th century, 279 00:27:04,800 --> 00:27:11,280 and he worked on various problems of what are known as massless chiral fermions. 280 00:27:11,280 --> 00:27:19,200 This was an exotic particle theory that at some point looked like might describe neutrinos, so particle physicist became very interested in it. 281 00:27:19,200 --> 00:27:26,250 In the last 10 years or so, quantum materials physicists have become really interested in these these types 282 00:27:26,250 --> 00:27:31,680 of particles because it's clear that they show up in certain compounds and, 283 00:27:31,680 --> 00:27:38,340 in other cases, Majorana fermions. These have been found in various different materials. 284 00:27:38,340 --> 00:27:47,760 These are very strange fermions, which are their own antiparticle and again have been posited as something that might be important in neutrinos. 285 00:27:47,760 --> 00:27:56,370 But again, it's not clear that that's the case. The interesting thing about Maiorana is that these Majorana fermions, 286 00:27:56,370 --> 00:28:03,510 they were proposed by this physicist atory Maiorana in 1937, a brilliant Italian physicist. 287 00:28:03,510 --> 00:28:07,530 He then went missing in 1938 and nobody knows what happened to him. 288 00:28:07,530 --> 00:28:11,160 So there are two mysteries about Maiorana. First of all, what happened to him? 289 00:28:11,160 --> 00:28:17,610 And secondly, what's really going on with his fermions? So we're really at the moment able to work on the second problem. 290 00:28:17,610 --> 00:28:26,130 And my colleague Julian Chen here in Oxford works a lot on determining the properties of these systems. 291 00:28:26,130 --> 00:28:29,370 Not only are the new particles, but there are also new properties. 292 00:28:29,370 --> 00:28:34,050 And one of those properties actually discovered more than 100 years ago in superconductivity. 293 00:28:34,050 --> 00:28:40,130 So here is a superconductor levitating above the magnet. Now, in fact, 294 00:28:40,130 --> 00:28:46,700 there's a big Oxford connexion with superconductivity because of Fritz London Fritz London was one of the German 295 00:28:46,700 --> 00:28:54,920 emigre scientists who came over in the 1930s fleeing Nazi Germany and Oxford gave him a home for a period. 296 00:28:54,920 --> 00:29:00,830 While he was here in Oxford, he came up with a crucial theory about superconductivity, 297 00:29:00,830 --> 00:29:07,700 realising that electrical current can be conducted by a super conductor forever and ever with 298 00:29:07,700 --> 00:29:14,540 no resistance in exactly the same way that electrons go around an atom forever and ever. 299 00:29:14,540 --> 00:29:18,230 And the reason is because they are in a quantum coherent state, 300 00:29:18,230 --> 00:29:25,280 unlike electrons travelling along the usual copper wire, which scatter and quickly lose their coherence. 301 00:29:25,280 --> 00:29:35,720 The electrons in a superconductor have that kind of aethereal beauty that you get with electrons going round and round an atom with no battery needed. 302 00:29:35,720 --> 00:29:46,790 And so that was a crucial insight to the theory of superconductivity, which both later this is the structure of a high temperature superconductor. 303 00:29:46,790 --> 00:29:51,440 One of the one of the compounds that was discovered in the 1980s, 304 00:29:51,440 --> 00:30:00,230 another quantum material which shows superconductivity at very high temperature, just quickly mention about the history of superconductivity. 305 00:30:00,230 --> 00:30:09,260 This is a graph of the transcendent transition temperature below which superconductivity can be observed against year of discovery. 306 00:30:09,260 --> 00:30:14,810 So superconductivity was discovered in Mercury, and it was then found in lead also found in aluminium. 307 00:30:14,810 --> 00:30:21,770 So your aluminium saucepan will be superconducting if you get it below one degree above absolute zero. 308 00:30:21,770 --> 00:30:28,850 So something you can try at home. Niobium is another very good material. 309 00:30:28,850 --> 00:30:36,830 But the real breakthroughs took place after the Second World War, when various alloys of niobium pushed the transition temperature. 310 00:30:36,830 --> 00:30:42,470 Still, a long way from room temperature, we're up here. But these materials are incredibly useful. 311 00:30:42,470 --> 00:30:51,740 And so these led to the development of MRI scanners, which all contain superconducting wire and that coils and also the coils inside. 312 00:30:51,740 --> 00:30:59,720 So one of the big companies that took advantage of this, this sort of period was Oxford Instruments, 313 00:30:59,720 --> 00:31:05,720 founded by Martin Wood, now Sir Martin Wood, and you're all here in the Sir Martin Wood lecture theatre. 314 00:31:05,720 --> 00:31:11,390 So we're actually in a lecture theatre which comes off the profits of this discovery. 315 00:31:11,390 --> 00:31:20,330 And the real breakthrough took place in the late 1980s with the discovery of discovery of the high temperature superconductors, 316 00:31:20,330 --> 00:31:30,210 and you can see the transition temperature. It's now getting almost halfway to room temperature, so there's the high temperature superconductor again. 317 00:31:30,210 --> 00:31:38,340 More recently, the iron based superconductors were discovered and something that a number of us have been studying quite intensively 318 00:31:38,340 --> 00:31:45,870 here in Oxford very recently to new discoveries of pushed the transition temperature very close to room temperature. 319 00:31:45,870 --> 00:31:52,110 The only problem with these two materials is that they are only superconducting under extraordinarily high pressure, 320 00:31:52,110 --> 00:31:57,960 almost half the pressure at the centre of the Earth. So these are not going to be very practical materials. 321 00:31:57,960 --> 00:32:03,540 But what it does show is that there's nothing unfeasible about having superconductivity at room temperature. 322 00:32:03,540 --> 00:32:10,950 We just haven't done it yet. So there's a lot of understanding, a lot of hard work that has to be done to understand these, 323 00:32:10,950 --> 00:32:16,620 these superconductors, and this is something for which there is a lot of work going on worldwide. 324 00:32:16,620 --> 00:32:25,110 Superconductivity is all to do with the pairing of electrons, electrons essentially doing this complicated dance to get into this coherent state. 325 00:32:25,110 --> 00:32:29,520 But superconductivity isn't the only type of dance that electrons can do. 326 00:32:29,520 --> 00:32:36,540 And that's actually something that we're starting to do in quantum materials is to understand the subtle choreography of electrons. 327 00:32:36,540 --> 00:32:41,400 So if you think of this analogy of dance as the dance, this could all be information. 328 00:32:41,400 --> 00:32:48,060 This is what we call ferromagnetic order, whether the the magnetic moments all are aligned. 329 00:32:48,060 --> 00:32:54,630 We can also have stripe order. This is the fractional quantum hall liquid. 330 00:32:54,630 --> 00:32:58,890 And here we have a so-called spin liquid or a string liquid. 331 00:32:58,890 --> 00:33:03,870 So you can see this is a rather recent article trying to use trying to find the right analogies 332 00:33:03,870 --> 00:33:11,060 to describe these rather complicated ways in which electrons do a rather subtle dance. 333 00:33:11,060 --> 00:33:19,880 Now, I just want to finish with one example, which is rather a deep example, but I think exemplifies a lot of these ideas about new universes. 334 00:33:19,880 --> 00:33:27,470 Let me take you back to a very familiar piece of physics. So if you have ball magnets that you may have played around with as a child, 335 00:33:27,470 --> 00:33:34,580 you probably know that north and south attract and south and south repel and north and north repel. 336 00:33:34,580 --> 00:33:37,190 So that's a basic property of magnets. 337 00:33:37,190 --> 00:33:44,810 So another thing you might know about magnets is that and this is an experiment that was done, I think, first about 700 years ago. 338 00:33:44,810 --> 00:33:48,170 If you take a magnet and you cut it. 339 00:33:48,170 --> 00:33:55,100 You make two new magnets, almost like one of these snakes, if you cut it and you form a new head or a new tail in mythology, 340 00:33:55,100 --> 00:34:00,200 this was how William Gilbert described it in sixteen hundred. But this certainly happens. 341 00:34:00,200 --> 00:34:07,130 You make a new South Pole in the new North Pole. So one problem is we don't seem to be able to separate the North and South Pole. 342 00:34:07,130 --> 00:34:10,490 And that's another way of saying what I said right at the beginning of the lecture. 343 00:34:10,490 --> 00:34:16,550 We can't make magnetic monopoles in our universe. What about in a quantum materials universe? 344 00:34:16,550 --> 00:34:19,730 Is there a way of doing it? Well, there's a little experiment. 345 00:34:19,730 --> 00:34:27,170 You can kind of do a little thought experiment that might show you how this works if you take a periodic line of magnets, 346 00:34:27,170 --> 00:34:31,930 and let's just pick the one in the middle and let's rotate it. 347 00:34:31,930 --> 00:34:34,030 Now, if we rotate it, it will not like this, 348 00:34:34,030 --> 00:34:40,060 this will cost energy because what we've done is we put two south poles together and we put two north poles together. 349 00:34:40,060 --> 00:34:45,100 So this will cost a lot of energy. So this isn't a very good thing to do. 350 00:34:45,100 --> 00:34:53,090 But having done that, there's another interesting thing that can happen if I take the next Margaret and I rotate that round. 351 00:34:53,090 --> 00:34:58,730 That won't cost me any extra energy, because all I've done is move the pain from one place to the other. 352 00:34:58,730 --> 00:35:02,990 But what I have now done is I've got a double south and a double north, and I've separated them. 353 00:35:02,990 --> 00:35:07,790 And of course, I could now rotate this magnet magnets and move this along. 354 00:35:07,790 --> 00:35:14,390 So this whole business of being able to just move the pain from one place to another means that what 355 00:35:14,390 --> 00:35:21,950 I've done is I've made two exhortations that can separate to use the jargon they fractionalised. 356 00:35:21,950 --> 00:35:29,600 We've taken a single flip. We've sold it in half and move the two parts away from each other. 357 00:35:29,600 --> 00:35:33,920 So just another kind of picture of the same thing. Here's a line of ball magnets. 358 00:35:33,920 --> 00:35:39,290 Then I've just rotated one, and then I've separated the two by a whole series of spin flips. 359 00:35:39,290 --> 00:35:45,800 And you can see what I've now done is I've made something that looks like a monopole here and a monopole there. 360 00:35:45,800 --> 00:35:52,640 Well, this is clearly just a little game in one dimension, but it turns out in quantum materials you can find a material. 361 00:35:52,640 --> 00:35:56,660 It's called dysprosium tie tonight. That does essentially the same thing. 362 00:35:56,660 --> 00:36:03,260 This looks rather complicated, but these magnetic moments have what's known as the two in two out rule. 363 00:36:03,260 --> 00:36:10,820 So two of the spins points in two of them point out, If I take a spin and I flip it. 364 00:36:10,820 --> 00:36:18,560 What I do is I do just as before. I make a defect here and a defect here that cost me some energy. 365 00:36:18,560 --> 00:36:23,420 But then with no further energy, I can just move the defects apart. 366 00:36:23,420 --> 00:36:31,910 And so what I've done is I've made things that behave very much like magnetic monopoles, and I've separated the north and the south poles. 367 00:36:31,910 --> 00:36:37,730 Interestingly, this can be done in this material dysprosium tie tonight, and here's how it looks. 368 00:36:37,730 --> 00:36:42,200 So this is one of Dr. Background's crystals of Dysprosium Titan eight. 369 00:36:42,200 --> 00:36:48,410 You can hold it between your thumb and forefinger, and it does contain magnetic monopoles. 370 00:36:48,410 --> 00:36:52,850 There a special type of magnetic monopoles, they're not breaking any fundamental laws, 371 00:36:52,850 --> 00:37:00,200 but they are proper emergent monopoles that have one overall squared force between them. 372 00:37:00,200 --> 00:37:10,730 And so now there's a lot of work to study these magnetic monopoles. Very recently with my my colleague Seamus Davis, who's just moved from Cornell. 373 00:37:10,730 --> 00:37:16,190 We've been thinking a lot about how to do this, and this is the sound of magnetic monopoles. 374 00:37:16,190 --> 00:37:21,920 So this is measurements of dysprosium tie tonight in his crystal. 375 00:37:21,920 --> 00:37:26,960 And this is with the sample in the squid. 376 00:37:26,960 --> 00:37:29,540 And this is with it out so you can tell the difference. 377 00:37:29,540 --> 00:37:41,810 It turns out the noise of the fluctuating monopoles are such that the fluctuations are in the kilohertz regime, which means you can hear them. 378 00:37:41,810 --> 00:37:47,660 So although we normally plot graphs for this particular case, we thought it was rather fun to actually listen to the data. 379 00:37:47,660 --> 00:37:51,920 So you've just heard the sound of magnetic monopoles. 380 00:37:51,920 --> 00:38:00,530 OK, I've talked a lot about these monopoles holes in semiconductors, magnums, phonons, vile fermions, magnetic monopoles. 381 00:38:00,530 --> 00:38:04,100 So one question you might ask is, are these real? 382 00:38:04,100 --> 00:38:11,060 After all, isn't the fundamental thing the things that you measure in particle physics, those are the things in our real universe. 383 00:38:11,060 --> 00:38:16,760 These are things that are just existing in crystals. They're somehow not as fundamental. 384 00:38:16,760 --> 00:38:23,420 Well, in fact, even the real particles in our universe are just excitations in a quantum field, and that's our current best theory. 385 00:38:23,420 --> 00:38:28,130 And when we do the descriptions of these crystals, we're using exactly the same kind of mathematics, 386 00:38:28,130 --> 00:38:35,090 quantum field theory and these monopoles and these various other things are just excitations in the quantum field. 387 00:38:35,090 --> 00:38:41,750 So I think from a philosophical point of view, it's just as valid. I'm not claiming that the particle physics is a wasting their time. 388 00:38:41,750 --> 00:38:44,540 No, that's a very important avenue of research. 389 00:38:44,540 --> 00:38:51,080 But what I'm really trying to make the case for is that in quantum materials, we have a richness that is really extraordinary. 390 00:38:51,080 --> 00:38:59,800 It's limited only by the combinatorial nature of chemistry. And what's more, some of the things that we work on also turn out to be useful. 391 00:38:59,800 --> 00:39:05,140 But if I'm honest, that's not what really draws us into it. It's not just trying to find a room temperature superconductor. 392 00:39:05,140 --> 00:39:09,730 It's the fascination of understanding these many universes. 393 00:39:09,730 --> 00:39:11,590 The one very final example. 394 00:39:11,590 --> 00:39:18,880 I should say that not only do these different crystals, each one gives us a new universe, but when we have a crystal, we can also tune it. 395 00:39:18,880 --> 00:39:23,440 So this is a particular material that is tuned by both magnetic field and pressure 396 00:39:23,440 --> 00:39:27,250 and temperature to make all these different phases that I'm not going to talk about. 397 00:39:27,250 --> 00:39:36,070 The superconductivity spin density waves, metallic behaviour, field induced and spin density waves and enormous amounts of complexity. 398 00:39:36,070 --> 00:39:43,780 And we have lots of control parameters, temperature, magnetic field. We can press the sample to change the interactions between the atoms. 399 00:39:43,780 --> 00:39:55,600 We can use chemical doping. We can use strain by and isotopically tuning the materials in one of the talks we heard today in our symposium. 400 00:39:55,600 --> 00:39:59,670 We heard how you can use coherent light to tune materials. 401 00:39:59,670 --> 00:40:08,450 So all of these things are available to us. So, yes, we have the many universes of quantum materials. 402 00:40:08,450 --> 00:40:14,780 But I should finish with a conclusion. And so my conclusion is from William Blake. 403 00:40:14,780 --> 00:40:19,490 Yes, he wrote Jerusalem and some of the some of the words in that a lot of nonsense. 404 00:40:19,490 --> 00:40:21,410 But I think this is rather profound. 405 00:40:21,410 --> 00:40:30,590 So to see a world in a grain of sand and a heaven in a wild flower, hold infinity in the palm of your hand and in eternity in an hour. 406 00:40:30,590 --> 00:40:39,302 Thank you very much.