The Physics of Magnetic Monopoles - with Felix Flicker

[Music] thanks very much so tonight I'd like to tell you about some very recent and ongoing work in which teams from around the world are pulling together to explore the ISIS in search of the North Pole now I know what you're thinking didn't we find the North Pole already back in the 19th century and indeed we did I'm not going to be telling you about finding the North Pole of the earth but about finding the North Pole of a magnet without the South Pole what we'd call a magnetic monopole and rather than search the ices of the Arctic we're going to search certain materials called spin ices so even though the work is cutting-edge in terms of both theory and experiment it is deeply rooted in ideas from the 19th century and in particular a lot of the ideas that were developed right here in the Royal Institution by Michael Faraday so I'll tell you a bit about that as it comes up so if you'd care to join me I'd like to continue the pretense of being an arctic explorer returned from his team's adventures partly because I always wanted to be an arctic explorer and partly because I already bought the suit thanks very much that'll be the quality of the jokes you'll be receiving this evening ok so let's begin I'll present you with the the map of our adventures where are they going to take us we're in search of the North Pole so we're going to begin down here at the Royal Institution just around the corner from Powell mal and I'm going to present to you the challenge what our magnetic monopoles and why should we be interested in them next we'll set sail across the sea and I'll present to you with the route so we're going to take to try and find these mana poles and the route will take us across these spin ice materials and I'll tell you a little bit about those next I'll present the expedition itself so this is a particular experiment that's taken place over the past couple of years which I've been fortunate enough to be involved in in my capacity as a theorist and I'll present the the work of my experimental collaborators where we've tried to identify these magnetic monopoles and finally rather than return to rainy Old England we'll shoot off the edge of the map in search of new frontiers so if we find these magnetic monopoles what use is it to us and how can it help us in our day to day life and to give you a bit of a preview I'll try to argue that they may be able to lead to more efficient computation with benefits to both the economy and potentially lessening our impact on the environment and also potentially some aspect some beneficial aspects to things such as health so for example less intrusive and more efficient MRI scanning so I'll make those augments at the end so let's begin with me presenting the challenge to you what are magnetic monopoles and why should we be interested in them so that you won't need much background for this talk but I want to check one thing right at the start so if I were to take a magnet like this and I would to pull it in half one of two things could happen either I could end up with the North Pole in my right hand and the South Pole in the left or I could end up with both the north and a South Pole in both hands so who thinks the first thing would happen all right good not too many hands that's good who thinks the second thing would happen I'd get yeah great okay very good that's all the background we need it for the talk all right so we traditionally paint the end red and blue of course that's totally arbitrary and if I take it my try and pull it in half look pop there we go we get or magnet in each hand there will be magic tricks tonight and that will be the quality of those magic tricks so a slightly fancier name for a magnet is a magnetic dipole meaning we have two poles north and south so the question is why do we always have the two poles why can't we isolate the North Pole of the magnet for example by pulling it off the end so the modern theory of magnetism I think it's safe to say dates back to the work of this man Michael Faraday here he is stood in totally unfamiliar surroundings ah see if I can do the hand here is demonstrating something here in the Royal Institution and Faraday kept meticulous logbooks which we have available to us here they're stored in the Royal Institution and we can show a couple of them to you tonight so if we could switch to visualizer one please Ross well here we go perfect so over here we have what I originally thought was a drawing of iron filings around to magnet in fact I've just been told that it's not a drawing those are literally iron filings so Faraday sprinkled those iron filings onto the page there's a sticky page and you see how they've landed there so where there's a magnet underneath the page on this this little book right here so you see that as they've landed they've shown us that there are two poles of the magnet there which we know to be north and south so the iron filings a little magnets of their own and they'll line up with the magnetic field locally so the question is why is this so here's a picture of a magnet with its North and South I'll sometimes depict it with a little arrow just for ease of drawing in later slides so why if we cut this in half and separate the two halves we get to north and a South on both halves well we can zoom in to a scale of say a millimeter down to about a thousandth of a millimeter and we'll find that the magnets made up of little magnetic domains each of which has its own magnetic field and both the north and a South Pole okay so the domains may not line up perfectly but on average they'll give the the overall magnetic field of the magnet so the first answer to the question is if I cut the magnet in half and separate the two halves well both halves are still made of domains and each domain has a north and a south so I end up with a north and a South and both halves okay so that's not going to help us but we could say let's take one of those individual domains and try to cut that in half and perhaps we can separate the North Pole of that so if we zoom in again down to the atomic scale we'll see something like this so within a domain we have a regular periodic array of atoms so this defines one of those domains to be a crystal I've shown the atoms here as little balls and each of those atoms I've depicted with a little arrow on it to indicate that each of the atoms itself has its own little magnetic field so the magnetic field of a single atom is called a spin and within one of the domains the spins will all line up and each of those spins again has a North into South Pole so if you cut the domain in half and separate the halves you again have both North and South in both halves so that doesn't help us either so next you could say let's try and cut one of those atoms in half and separate the two halves but of course you can't do that so is that it at the end of the talk you the answer is you can't do it we can all go home well not quite let's think about how it's possible that these atoms can have these little magnetic filled these spins so I can motivate that a little bit with the work of Faraday again so if we switch over to the visualizer again quickly to visualizer one please so here is a notebook of Faraday's from 1831 and again it's his actual handwriting in here he actually bound the book himself as well so we can see here there's a this one right here is his experiment where he took a coil of wire and he moved a magnet through the coil and he found that when he moved a magnet through a coil of wire it induced an electric current into the wire okay so this led to the understanding that we now have that electricity and magnetism are deeply interconnected they're really two sides of the same coin so I have a little demonstration of this experiment this Faraday coil which was built for us by Jeff lead guard in the Oxford experimental undergraduate teaching labs so what we have here is a set of coils of wire and through each of them we've connected them through an LED so if a current passes through the coil we should get the LED to light up okay and I have here a nice powerful magnet so if I drop it through here hopefully it's going to light up how was that did it work good all right let's this right over here okay we colored the rainbow colors so hopefully you can see one lighting up after the other here we go over there did it work okay maybe one one set so that was made for us by Jeff lead guard if we just switch to the slide serious I've made him look like he's from the 19th century to fit in with the theme but he's very much alive and mobilis and beat and created this for us so perhaps we could give a round of applause to Jeff in Casey's watching at home okay so it works we've confirmed Faraday's observation that if you move a magnet through a coil of wire you induce an electric current into the wire now the converse is also true if you pass an electric current through the coil of wire you'll generate a magnetic field so this gives us an inkling as to how it is these atoms can have their own little magnetic fields these spins because if we were to take an electron represented here by this ball of rubber bands an electron of course has an electric charge and so it has an electric field emanating from it okay and so if I take my electron and I move it in a little loop like this let's get rid of Jeff for the time being we move our electron loop then of course this defines an a little electric current moving in a loop because an electric current after all is just the passage of electric charge so you take the electron and we move it in a loop and that must then look like a magnetic field and in particular if I'm moving it in a clockwise direction it should look like the North Pole of a magnet okay so if I were to move it in an anti-clockwise direction it would look like the South Pole of a magnet so a deep run so we can give to the question as to why we can't separate the poles of the magnet thinking down on the atomic scale is that if I take my electron and I move it in a clockwise direction to make it look like a North Pole and I continue doing exactly the same thing with my hand but I turn around then hopefully now it should look like it's going in an anti-clockwise direction and therefore looks like a South Pole okay so now you see that the North Pole viewed from one side is really just the South Pole viewed from the other and so you really can't separate the poles because it's just a paradox okay but we're still going to try okay so it seems like we've hit a bit of an impasse here on our on our Arctic exploration but there's a bit of sleight of hand that went on there slightly more impressive than the sleight of hand with the magnets earlier and that's the following I said that Faraday's work had shown us that electricity and magnetism are two sides of the same coin and in fact if you have to write a list of phenomena that appear in the two theories you'd find a perfect correspondence between them but with one exception and that was the following to get an electric field I can take a unda mental particle like an electron in this case represented by a ball of elastic bands and the electric field just emanates from the electron but if I wanted to make a magnetic field I had to take an electrically charged particle and start moving it around in a little loop so why is it that I can have electric fields just coming out of fundamental particles those are particles that can exist by themselves off in the vacuum of space but if I want to make a magnetic field I have to take an electric particle and move it around it's not true that if I want an electric field I have to go and get a magnet and start moving it so there is this one asymmetry between the two theories in fact the electron is what's called an electric monopole it just has one pole of the electric field the negative pole that it's negatively charged and you don't require a positive charge to come along for the ride but whenever we have magnets we always seem to have magnetic dipoles we always get two of them so at the deepest level in turn are trying to answer this question why can't we pull the North Pole off a magnet well the deeper stance we can give is we don't know in principle it seems possible that we could have some other kind of fundamental particle that can exist by itself in the vacuum of space which would just have the North Pole of a magnet without the south and that would be a magnetic monopole it would have magnetic charge to select the electron house an electric charge so it seems compatible with all the other laws of physics that these things could exist as far as we know there's no fundamental reason to think that they shouldn't exist but there are various ongoing experiments looking for magnetic monopoles and so far we haven't seen any so there's two basic approaches to looking for them one is to just build a detector and hope one flies through it and it's not as silly as it sounds you know we've we've not seen one so far and we've been looking for about 50 years but they're clearly quite rare but you know they could occasion fly-through that he may just exist in the universe just like electrons exist the other route is to try and take other stuff and smash it together so of course this is what happens at CERN and there are various ongoing searches looking for magnetic monopoles at CERN where we hit other stuff together we see what comes out and maybe some of its a magnetic monopole or two probably we get both north and south so in both of those cases if we just switch back to visualizer one again so recall we had this Faraday coil setup and Faraday observed that when he moved a magnetic dipole through his coil of wire it induced an electric current into that wire so it turns out that if you were to move a magnetic monopole through the coil of wire you would again induce an electric current but it would have a very distinctive form in particular it leads to a persistent current in the wire so it's a very obvious signature so these searches that are going on including at CERN so you know really cutting-edge stuff they actually just use this Faraday coil set up again they tried to create magnetic monopoles and then move them through a Faraday coil and look for this distinctive signature so it's cutting-edge physics but really rooted in Faraday's work in the 19th century okay so there are good reasons to think these magnetic monopoles may exist one is this symmetry between electricity and magnetism if you were to write a big list of comparisons between two things and they agree on every term except for one you tend to think that you've made a mistake in that one entry and that's what physicists think we see everything else degrees and electricity and magnetism maybe they should agree on the existence of fundamental charges another reason to think they might exist is that some of our fanciest modern theories of physics rely on their existence so string theories if you've heard of these or grand unified theories both imply the existence of magnetic monopoles as fundamental particles but it could be that they're very heavy if they have a lot of mass it's very hard to make them in CERN for example we have to put a lot of energy in and it certainly seems that they're quite rare another reason for thinking they might exist is a little bit a bit harder to explain and frankly I'm not going to explain it in any sort of detail so don't worry if it doesn't make too much sense but it dates back to a hundred years after this work was done so Faraday's work in 1831 from this notebook precisely a hundred years later the quantum theory of the magnetic monopole was worked out by this man Paul Dirac so Derek said okay maybe they exist maybe they don't but if they do and their fundamental particles then they should be described by the laws of quantum mechanics what are the consequences of that so he deduced a number of consequences but the one I'm going to tell you about tonight is the following so if you take a balloon and you rub it on your jumper or your tweed suit you can charge it up with electric charge and then you can detect that electric charge for example by putting it near your hair and you'll feel it sticking now it may seem that you can charge it up basically arbitrarily you can put as much charge on there as you want and you might imagine that it's taking a kind of continuous range of values but if you were to measure it very precisely you'd find that the charge on the balloon is always an exact integer multiple of the electrons charge okay so this phenomenon is what we call the quantization of electric charge quantum in quantum mechanics or quantization these things mean something coming in discrete units so a priori you may be able to imagine that you could have a charge that say 1.2 times the electrons charged or pi times electrons charged but in experiments and whenever we've measured it we find that it always comes charged always comes in integer multiples of some discrete unit so it turns out that it doesn't seem to be inconsistent with all the other laws of physics for us to have a continuous range of charges so we're not really clear why it is that we only see integer multiples of some basic amount so Derek's realization with regard to magnetic monopoles was the following he realized that the existence of even a single magnetic monopole as a fundamental particle in the entire observable universe would be enough to explain the quantization of all the electric charges okay it's pretty out there and I'm not going to explain it in enough detail but this was the observation of course there could be more than one that one would do so in the 90 years since Derek's theory we haven't actually come up with any better explanation as to why electric charge is quantized there are different explanations but they all seem to involve magnetic magnetic monopoles in one way or another so it doesn't prove their existence but since we can't come up with any other explanation for the quantization of electric charge it's so of you you it's certainly compatible that say that okay so there's some good reason to think they might exist but so far we haven't seen them so let's recap slightly so the challenge is to find magnetic monopoles and to understand what they are and schematically there's something like this we take our magnet to me just snap it break it in half we somehow get the North Pole off separate from the South Pole and the trick would be to find fundamental particles those are things that can exist by themselves off in the vacuum of space which would have a magnetic charge instead of an electric charge okay let's proceed along adventurous route to look for where we might find these magnetic monopoles so what I've told you about so far is what you'd call theoretical particle physics so I'm a theoretical physicist but not a particle physicist I'm what's called a condensed matter physicist so where as particle physics studies fundamental particles again we can think of that as things that can exist by themselves in condensed matter physics we also study particle like things but they're not fundamental they're emergent properties of complex systems okay so I have an example of this to try and to try and explain it so if we take the theory of light and we write it as a quantum theory then the quantum description of light has it described in terms of particles that we call photons some of you may have heard of these and photons are fundamental particles they can certainly shoot off through space by themselves as you can tell because you can see the Sun now if we write down the theory of sound we can actually write a quantum theory of sound as well and when we write the quantum theory of sound we describe it in terms of particles which are called phonons rather than photons now phonons can't be fundamental because sound can't propagate through space it can only go through a median but if you're existing in a medium like right now and I'm talking to you with sound or if we lived inside a crystal remember crystal is something which on the atomic scale has a regular periodic array of atoms then sound can propagate through those crystals and we can write that theory in terms of a quantum theory of the sound and then it can have a particle description in terms of these phonons but they can't be fundamental and one way to look at what's going on is that it's really a vibration passing Christel okay but it admits this perfectly good description in terms of particles that's an example of something we might study in condensed matter physics now you might think that these emergent particles are sort of less good than the fundamental ones I only want to know about the fundamental ones and I'd make the following argument I'm not a zealot over this stuff but most if not all of the things you've ever experienced are actually emergent phenomena right you don't really experience fundamental particles and a lot of emergent phenomena you think exist so I don't have to take this glass of water right here this is an emergent phenomenon there's no fundamental particle in the standard model which is the glass of water particle this is may this is this emerges out of other fundamental particles but we still think it's real right so the idea of these emergent properties or emergent particles is going to be that when we carry out experiments on materials then we're going to have a better description in terms of what we're seeing in terms of the emergent properties rather than the fundamental ones in many cases okay so how we're going to use this to try and make magnetic monopoles when the only things that racing to exist a magnetic dipoles and the trick is going to be very much along the lines of a famous 19th century magic trick soaring a person in half actually think it might be early 20th century but you know that doesn't fit with the theme and I can give you a rather crude demonstration of this using my thumb so if we take my thumb and we say this is like a magnetic dipole here's the North Pole of my thumb and here's the South Pole of my thumb what I'd like to do is try and pull the North Pole off the therm without causing myself incredible pain and making a mess so what I can do is I can take hold of my thumb and I can sort of do this and off it comes like I got the North Pole off the thumb and I can put it back on and isn't that magic I did warn you about the quality of the magic tricks tonight okay so I hope it doesn't ruin that trick if I reveal how it was done I think probably only how it was done so you thought I had this one thumb over here this magnetic dipole I had secreted about my other hand a second thumb and by having two thumbs I was able to make it appear that I pulled the North Pole on the tip off one of the thumbs like that okay the trick was having multiple thumbs so we can do the same basic trick with material to try and make these magnetic monopoles so here is a line of magnets and they'd like to line up north-south north-south as we all know and if I take one of those magnets and I flip it then I get a concentrated region of North Pole next to a concentrated region of South Pole to North together and to south together so if you imagine blurring your eyes just the right amount that you see whenever North's and South are next to each other it kinds of averages out to nothing then you'd still be able to see the two North's in the two south now if I flip the next magnet along I can move the concentrated region of south away from the regions north and they can move separately like this this is going to be the basic trick by having multiple magnets we can make it appear that we've broken one magnet in half we've separated the north from the South Pole okay and of course these magnetic monopoles that I've made here can only live inside the line of magnets they can't come out they're emergent properties so on visualize r2 we have a chess board and it's being covered up by dominoes so there are 64 squares on the chess board and I have 32 dominoes here and of course I can cover all the squares up with dominoes let me ask you a question quickly if I were to take two drafts and I block off opposite corners like this well there's still an even number of squares left on the chess board is 62 now is it going to be possible for me to cover all of the squares I don't need this particular arrangement I can rearrange things how I like let's see a show of hands who thinks it's possible no one who thinks it's impossible everyone okay Wow okay we're really really on it tonight now people who said it's impossible you're correct would any of you be willing to suggest why it's impossible I've got a suggestion right there yes that's exactly right I couldn't have put it better myself what's your name so doc could we Siddhartha's it we have a round of applause per cup [Music] thanks very much that's exactly right so placing a domino takes out two squares one black and one white and the opposite squares on the chess board or of course the same color and if I block those off I'm unable to kind of cover the whole chess board so that's what's called the mutilated chessboard problem which is a bit graphic that's what they call it so it son mutilate it and we'll cover up the squares with dominoes there we go okay so what the mutilated chessboard problem showed us is that placing a domino covers two squares one black one white and we can kind of think of that as positive and negative charges and then by blocking off the opposite corners which is the same charge you have a net charge left on your board so if I block off two black squares I'm left with a total amount of white charge left over okay so if I cover every square of the chessboard like this then I can take one Domino out out it comes okay and that reveals two squares one black one white okay and of course they're next to each other so in this analogy we're going to think of the fundamental things that are really there are dominoes and or equivalently a black and white square neighboring each other okay so you might think that whenever I remove dominoes I always must end up with black squares next to white squares but in fact because I had lots of dominoes and lots of squares I can move subsequent dominoes around like this and I can separate the black from the white like that okay and we can move that one over there so it's a little bit like the following if we cover the whole chessboard in dominoes it's a bit like the vacuum of space with nothing there when I take a Domino out it's like creating a particle antiparticle pair where the antiparticle has the opposite charge of the particle and then by subsequent rearrangements of dominoes we can move the particle away from the antiparticle moving around in space okay so I can create a second pair like this and if I can move any black next to any white let's do it this way let's move that white over there then I can move that white over there here we go so I've got a black next to white up here any blackness 20 white I can cover them with a domino again and annihilate them back to the vacuum okay so this is like a particle meeting its antiparticle in the vacuum and annihilating and disappearing so the thing to take away from this is that even though you might have thought the fundamental thing was the black white squares next to each other by having lots of dominoes and lots of squares we can make them separate out into their two parts okay so when this happens in condensed matter physics we call the phenomenon fractionalization the fundamental things that live in the system get fractionalized broken up into fractions of themselves and separated out and the trick is to have lots of copies okay so let's look at that again with magnets so here are a load of magnets lined up onto a grid and there are four of the meeting at each of those intersections and so the lowest energy state of the system is going to have to north pointing in and to South pointing in because of course North's like to line up with south so in terms of North's it's two in and two out at each of those meeting points okay and that would be the lowest energy State if I take one of those magnets like this one and I flip it then I create a three in one out in terms of North's which is a concentrated region of North Pole next to a three out one in a concentrated region of South Pole and then by subsequent flips of magnets I can separate the region of north from the region of south okay so it's a bit like two in two out in terms of north is like the vacuum of space nothing there flipping a magnet creates a concentrated region of north this is like our North magnetic monopole and a three out one in which is like the south magnetic monopole or the anti monopole if you want to think of it in terms of particles and antiparticles and if we again flip another magnet to create another north/south pair if we can get any north next to any south like this we can flip the magnet connecting them and send them back to the vacuum they disappear again okay so that's going to be the basic story that we're going to try and tell in a real material so let's try and do it with a real material and this is going to involve a bit of moving things around on my part let's give this a go and I'll stick this here and auto focus so what we'd really like to do is have this come about in a real material and as you may have guessed I have one of these real materials right here so this at the moment is dysprosium titanate ab so can switch to the lamp is much nicer isn't it there we go this is dysprosium titanate a crystal and remember crystal means it has a regular periodic array of atoms down on the atomic scale now if we call this dysprosium tighten it down to extremely cold temperatures actually to kelvins that's two degrees centigrade above absolute zero the theoretical lowest temperature that could exist then it becomes what's called a spin eyes okay so just to put that temperature into some context you might think of liquid nitrogen is quite cold right but liquid nitrogen boils at 77 Kelvin so 77 degrees centigrade above absolute zero whereas these need to be cooled down to below 2 Kelvin or you might think that if you went off into the vacuum of space miles away from any stars or anything so there's no light and that sort of thing you might think that was about as cold as you could get but in fact that would be more like 2.7 Kelvin that's the temperature of the Cosmic Microwave Background so you actually need to cool these crystals down to colder than the universe itself to get them to turn into a spin ice but we can do that or rather I can't do that because I'm a theoretical physicist and I can't do practical things but my experimental colleagues can do that and when you do that you you turn it into a spin ice so this crystal of spin ice let's switch to the lamp here just we can see it good it doesn't occur naturally you can't find this in the ground you have to grow it and there's only a handful of people in the world who can grow this but fortunately there's one in Oxford and he grew this for us tonight so if we switch back to the slides again quickly so this is the person who grew that Forrest Prabhakar an Oxford crystal grower again made to look like he's from the 19th century not from the 19th century very much alive and well and he grows that crystal tonight so if we look down on the atomic scale of our dysprosium titanate we find something like this remember it's made up of a regular periodic array of atoms I haven't shown all the atoms here in fact what I've shown arts and magnetic ions so if an atom has an electric charge is called an ion and I've shown the magnetic ions inside this dysprosium titanate now I've shown a bit of the structure here it's this regular periodic array and the way these ions are arranged is they sit on a lattice which is made up of corner sharing tetrahedra so tetrahedron is a regular 3-dimensional solid with four corners and each of these magnetic ions sits on the corners of two tetrahedra and each of these ions has a little spin a little magnetic field of its own and the environments of these ions there's other atoms and things that I've not shown here the environments constrain the spins so that they point directly towards or directly away from the Centers of the tetrahedra on which they lie okay so just as before the lowest energy state of the system will have two spins point in and two spins point out because you'd like to line up north to the south and that's the best you can do so if I put a bit of energy into the system for example by heating it up a bit I take one of those spins and I flip it so I've colored yellow on the outside as you can see the flipped one and when I do that then I'll have three in one out next to a three out one in just like in the 2-dimensional case a concentrated region of North next to a concentrated region of South and these are our magnetic monopoles or monopole auntie monopole pair north-south paths subsequent flips of spins can separate the north from the south like this and if I can get any north next to any South I can flip the magnet connecting them the spin on the ion and annihilate them send them back to the vacuum so as before to in to out is like the vacuum of space nothing there three in one out is like north magnetic monopole three out one is like a South magnetic monopole and we believe this should come about naturally in these materials at low temperatures so this theory was developed by these authors in 2008 and the work was published in Nature and I think this created the first opportunity to experimentally observe and potentially control these magnetic monopoles in real materials so I want to say a few words about how this relates to the fundamental magnetic monopoles at the start of the talk so these room bar are emergent properties you can see that the things that are really existing in these crystals on the down on the smaller scales are magnetic dipoles the things we've always measured before but the collective behavior of all those magnetic dipoles can make it behave as if there are magnetic monopoles in the system and what we believe is that when we do experiments on these crystals to see what's really down there at those small scales will measure the emergent properties rather than the fundamental ones so we should measure these magnetic monopoles however at the start of the talk I was talking about for example string theories and grand unified theories these emergent magnetic monopoles don't have anything to say about that they're really a different thing and I talked about the quantization of electric charge and how that came about through the existence of magnetic monopoles again these don't say anything about that that's only fundamental magnetic monopoles ever do that so we don't get any of those benefits but these things do in other ways behave like you'd want magnets in one holes to behave for example the different monopoles have a Coulomb's law interaction between one another which is just like how the electrons in two electrons they repel one another and it's in exactly the same way so they really do behave as if they're these entities that have a magnetic charge okay so these materials are called spin ices you may wonder why that is we know that spins now are there names for the magnetic fields of single ions or atoms the reason it's called icy you might have thought was because it's very cold but it's much colder than you'd normally make ice in fact the reason they're called spin ices is that if we look at the structure of ice down on the atomic scale it looks something like this so the the rods there just show how the bonds would connect these things up the oxygens are tetrahedrally coordinated so you see that the oxygen has lies at the center of retro hydron and then you have oxygen sat on their corners of the tetrahedron and of course each oxygen has two hydrogen's close to it because it's really h2o molecules making up the ice but that means that each oxygen has two hydrogen's close but as you can see there's also too far away along with the other two bonds and their tetrahedron so the reason these things are called spin ices is because of this two into out rule in the spin ice it spins the point two into out in normalize its hydrogen's being two into out relative to the oxygens what are called the banal fouler ice rules so that's where the name spin ice comes from okay so let's recap again I've told you about the route we're going to take to try and find these magnetic monopoles and the route has taken us across these materials called spin ices and and the basic idea is going to be that we hope these magnetic monopoles will come about as emergent properties when you call these materials these crystals of dysprosium titanate down to cold enough temperatures so the theory is well believed by the community I think it's safe to say but what we need is an experiment to confirm that theory so let's move on to part three of the talk the expedition itself and what we're going to see is that there's a sense in which you don't look for these magnetic monopoles you listen for them okay so as I've tried to emphasize I'm a theoretical physicist and and so therefore don't do practical things I was very fortunate to be involved in some work by some very excellent experimental colleagues of mine to look for these magnetic monopoles and spin ices so the work was primarily carried out by these two people so on the on the left here is rity ker desaad who was the lead experimentalist on this work so she built the experimental apparatus and took the measurements and so on and on the right is Fran Kirchner who I hope is sat here tonight and so Fran was the lead numerous astonished she wrote computer simulations to work out what we would expect to see when we carried out those experiments again very much not from the 19th century in fact much younger than me so these are the two lead authors of the paper so here's the experiment that Radhika built so she took this spin ice sample and wrapped it in a coil of wire and then connected up to this measurement device which we'll talk about in the second so hopefully this setup looks rather familiar to you from the first part of the talk hopefully it looks a little bit like this Faraday coil that we showed before and that's because really this is a Faraday coil so even though this is really cutting-edge stuff actually the best way to look for these magnetic monopoles is to use a coil of wire and wait for a magnetic monopole to wander through it and when it does you'll induce this very clear signal in the current in the wire so it's really rooted in this 19th century physics idea developed by Faraday right here in the Royal Institution the one thing in this picture which is very much not 19th century is this loop thing after the top the detector so this is what's called a superconducting quantum interference device or squid for short let's make it look a bit more 90 century there we go it's a squid I know that's actually an octopus but it'll look nicer so the squid is an extremely sensitive detection device for measuring magnetic flux so the basic idea is you we can predict what we'd expect to see from this very distinctive signature of a magnetic monopole wandering through our Faraday coil so what happened in this situation and let me give you my kind of subjective experiences to how this work came about I was working with Fran Kirchner and her PhD supervisor professor Stephen Blundell in Oxford and we were working with Professor and Mayor Coby in Harvard and professor nor Miao in Berkeley so an excellent team plus myself and we'd predicted some signatures from a fairly similar experiment for trying to measure these magnetic monopoles what we found is that and the conditions in which you can operate these these types of setups even though we're at extremely low temperatures you're actually at a relatively high temperature for what the spent lights would like so rather than having a single magnetic monopole you actually have lots and lots of these mana poles in there so the problem is that you actually have loads of mana poles moving through your Faraday coil so it's a little bit like you've predicted the existence of raindrops and you've worked out an experiment to work to go and detect these raindrops and that's to listen for the sound of a single raindrop you go outside to listen to the rain and it's not just a single drop of course it's loads and loads of drops at the same time so rather than getting a nice clean signal of one you get noise from loads of them so your signal is now pure noise so again this seems like it might be a bit of an obstacle on our adventures something that we can't get around but Frank came it was a very good way to deal with this and that's the following even though the signal is noise there are actually different types of noise and there can be structure in noise even though it's somewhat random okay so let me tell you a bit about types of noise one type of noise we've probably all heard of his white noise does that familiar with this phrase white noise okay so white noise would be an equal mix of all different frequencies of sound if it were noise and sound okay and it's named an analogy to white light where if we took an equal mix of all different frequencies of visible light then that would look white okay the different frequencies being the different colors so I have a program for you here which will play us a bit of white noise it's not the nicest thing to listen to but will listen to a second of it so here's a second of white noise let's listen to it again and in fact I can take another random mix of frequencies within any given frequency interval of the same size we expect to have the same amount of of power and let's hear another white noise sound about the same now a type of noise which you may or may not have heard of is pink noise as anyone can give me a bit of a nod for pink noise okay so pink noise is again a random mix of different frequencies but there's a statistical bias - in it in that we expect to have more of the longer wavelength sound okay so these are the lower tones so I can make you some pink noise now by dragging the slider over and I'd like to say that this program which I'm using was written for us tonight by my former summer student Leon supports key you'll get a slide in a second and you're free to use this online so here's a bit of pink noise now it sounds pretty similar I'll give you that if we move all the way down to red noise we have even more bias towards the lower frequencies the lower tones let's try that again okay so let's go up to white again and down to red okay the jitteriness is not part of the effect so if the tone drops on average and if we go to the other end we can have violet noise which has more of the high frequencies that is how violet white red okay so that was made for us by this person Leon Jaworski wrote it just for you tonight so perhaps we could a round of applause to Leon who saw her in the audience okay so there are different types of noise now it turns out that you can work out on pen and paper what would happen if you took this setup with your peridot coil and you inserted not to spin ice but a normal magnet into it in particular type of magnet called a para magnet that's something that's not magnetic by itself but if you put a magnet next to it it becomes magnetic or you put it in a magnetic field it becomes magnetic and if you put a para magnet into this setup it turns out that you would expect to hear perfect red noise now what Fran predicted in her computer simulations is that if you put spin ice into the setup you won't detect perfect red noise you'll detect instead pinky red noise that becomes Pinker as the temperature increases so let me just be clear again about why I'm talking about noise all of a sudden the squid the thing that's doing the detection is measuring magnetic flux as a function of time and it's a noisy signal because we're getting lots of these monopoles moving through the coil or that's the way we were thinking about it so rather than getting the signal from one you're getting lots and lots at the same time so this is what you would call a noisy signal now we can look at that signal as a function of time and we can say okay how much of each different frequency of magnetic flux is a function of time in this case do we have and so in that sense we can think of it as noise because noise is just some distribution of frequencies of sounds it's a bit of an analogy again but in this case we can make the analogy a little bit clearer in the sense that the squid has a set of frequencies of magnetic flux that it can detect in fact it detects a few Hertz up to about 2.5 kilohertz and your ear has a set of frequencies it's sensitive to in terms of sound and that's about 20 Hertz up to about 20 kilohertz so the set of frequencies you're sensitive to overlaps that of what the squid is sensitive to yours is in terms of actual sounds the squid is in terms of magnetic flux so function of time but what we can do is take the signals that the squid is detecting and we can turn it into a literal audio sound by saying okay here's the mix of frequencies is detected let's just take that same mix of frequencies of sounds and then we should be able to listen to the output of the squid as it's doing its measurement so Richie here did this out this formatting into an audio file as the experiment was carried out so I'm going to play you an audio file in a second in the first three seconds of it it's before the sample is inserted and you're hearing what the squid would hear if there's nothing in there which is basically white noise the second three seconds the sample is inserted and you'll hear that the average tone of the the noise will drop as we detect this pinky red noise that Fran had predicted so the experiment very nicely confirmed Fran's numerix and our theories so let me play that audio file now so three seconds of white noise no sample three seconds of pink noise sample although my hand when it happens there you go there's my exciting yeah you know this is the first recording of these magnetic monopoles in their real material you're just hearing lots of those monopoles shooting through your Faraday coil as a function of time also we think okay so let's recap again I've presented you with the expedition we went on in search of the North Pole and we found that along the way we ran into a giant squid or brother a very small squid and we had to enlist the help of that squid in trying to detect these magnetic monopoles so it's time now to shoot off the edge of the map in search of new frontiers so where do we go from here well the first question is have we proven the existence of these magnetic monopoles and spin ices and I'd say the answer is no that would be too strong a statement we found very good evidence for them we predicted this signal and we detected the same signal we predicted but that doesn't rule out the possibility that something else could be giving the same signal so I should say that there have been a number of very nice expeditions in search of these magnetic monopoles in the ten years since they were theorized and there's been lots of evidence for their existence but there's no definitive experimental proof I'd say right now so something that would constitute that proof I think would be the detection of a single magnetic monopole and there are experimentalist getting ready to do that measurement is going to take a few years if we do find them and we can harness their power what use is that to us so famously probably stood on this spot Michael Faraday gave an answer to this type of question it's somewhat apocryphal that he said it's about electromagnetism but that's the way the story is told so he's hurt he presented in 1831 his results in electromagnetism and someone said well what's the use of that what use can I make of electricity and magnetism and supposedly he answered what is the use of a newborn child so it's only apocryphal because he said it out a different subject maybe he did say something like this so it's always a tempting answer to give but I think these magnetic monopoles in spin ices do have applications that we could see coming about in the next few years so one of them is that is the following electricity and magnetism are both very useful and use them all the time perhaps we use electricity a little bit more I think it's probably featured slightly more of a role in in presenting the work tonight and part of the reason for that is that if I want electricity I can go to a plug socket and turn it on and I can just have electricity on tap and sort of pour it into my devices and make some work now I can't do that with magnetism because I can't have magnetic currents so the reason I can have an electric current is that I have an electric monopole the electron and I can use it to create electric current by removing my electric charges around the reason I can't have magnetic currents is that if I have a North Pole of a magnet I always have a South Pole with it if I try to pass a north pole along a wire of course I always get the South Pole along for the ride and it kind of cancels it out it's one way to think of it so you can't have magnetic currents but you could in spin ices and the name has been coined for this rather than electricity we call it magnet tricity it's quite a nice name so we're hardly going to be building big power lines out of spin ices and calling them down to two degrees centigrade above absolute zero I don't think that's very practical but what we could do is perhaps make like integrated circuit boards for example so for doing things like computation using magnetic monopoles rather than electrons sir as the moving electric charge around moving magnetic charge around so why would you want to do that well it's part of a broader theme in physics at the moment which is to switch from using the electrons charge to do computations to using its spin its little magnetic field and rather than electronics we call this spintronics so I think it fits into that broader theme of spintronics and part of the interest in doing these spintronic applications has something to do with Moore's law which something people may have heard of so Moore's law in in one of its forms that it states that the density of transistors on integrated circuit boards should double approximately every two years so this was originally an empirical observation back in the early 1970s and since then it's became become a kind of self-fulfilling prophecy so it's been abated accurately for the past 50 years and large parts of the computer industry and therefore the wider economy are predicated on Moore's law being obeyed and it's not clear what would happen if Moore's law stopped being obeyed but of course it must at some point you can't keep making things denser and denser and in fact we're hitting up against those fundamental theoretical limits right now we already have single atom transistors so it's hard to imagine going much denser than we currently can and and what happens to the economy I don't know so what we're looking for now is ways around Moore's law or always to continue making things denser and denser and one way is to switch fundamentally from using charges to using spins so there are potential ways to do that and potential increases in efficiency we can get with spintronics a similar application which has already been achieved is the following so we can make artificial spin ices so these are little arrays of magnets in this case the magnets are about the length of a micrometer so about a thousandth of a millimeter and that's a picture of some magnets there in black and white and again they're made so they lie on these grids but they can flip along those directions so just like in the picture I showed before by flipping them we can create these magnetic monopoles and we can move them around so again why would you want to do that well it's been shown that you can encode logic gates in the movement of these monopoles by flipping these little magnets so if you have a logic gate you have the basis of computation and it's been shown that the computations you can carry with these artificial spin Isis act very close to what's called the land our limit the fundamental theoretical maximum efficiency you can do computations at so it's not something we tend to think about day to day but the act of computation will create heat and use energy so if I want to do a search on the internet using my computer that will necessarily use energy now partly my computers wired up to the mains and so it's using power here but the actual act of carrying out that computation is carrying it out somewhere else and that's using energy both in terms of trying to get the heat out of the data centers where it's where it's taking place and just in general using energy so it's actually a huge amount of power that goes into those types of computations I looked this up I think I have this figure right so I believe last year the the data servers of the world these are things that are doing things like internet searches for us when you type it in they used 40 percent more energy than the entire United Kingdom so it's a lot of power that goes into carrying out those internet searches and related things so if you can do it more efficiently it helps the economy but it also helps negate some of our effect on the environment because if we're using less power then we may not cause so much of a detriment to the environment so more efficient computation is important for that reason another reason you can give for these theoretical types of study is that the experimental advances that will go into realizing them will have knock-on effects in wider society and I think that's true as well as these spin Isis so one way in which this could potentially help is in terms of MRI scanning so if you've ever been for an MRI scan you know you go into a big tunnel and there's a giant machine there that makes these sort of clunky noises and it's quite an intrusive process it's not very nice and these machines cost a lot of money at least a million pounds and they use a lot of power to run them so what's going on there is that that big machine is generating a huge magnetic field and that magnetic field can flip the spins inside the nuclei inside the atoms inside your brain or whatever's being scanned and we can detect those spin flips and use that to measure certain properties of the body that's then used in those medical diagnoses so the reason the field needs to be so large and and for all this machinery is that the coupling between that magnetic field and those spins is very weak so to cause an effect you can detect you need a huge field but an alternative would be to use a smaller magnetic field but a much more sensitive detector of magnetic fields to detect the effect now the experiment that was carried out by rity Kerr and the team when it carried out last year was the most sensitive detection of magnetic flux ever performed so a lot of work went into that it may still be the the most sensitive performed so it's really pushing the boundaries of what we can achieve in that direction and there's already been work showing the proof of principle that MRI scanning can be done at much lower field so there was a paper in the Proceedings of the National Academy of Sciences in 2016 showing this MRI scanning was much smaller field so it'd be less intrusive cost less power and so on okay so hopefully I've convinced you that magnetic monopoles are interesting in their own right hopefully some of you wondered when you were children why can't I just pull the North Pole off a magnet well now you can sort of and hopefully I've convinced you that there's a wider benefit to doing this work as well so that I'd like to thank a few people without whom this talk not have been possible so Brendan Holmes is the new college carpenter and he made us all these dominoes just for tonight so thanks Brendan Holmes for doing that Prabhakar engrossed the spin ice crystal or really he grew at you know international teams of scientists but he let us use it tonight which is very nice of him my friend Dominique Scarpas just at the 19th century explorer theme i hope it worked for you Jeff lead guard built and designed us this Faraday coil which worked very nicely and my student Leon Topolsky who's sat here tonight wrote us that colors of noise program which you can play around with on my website in terms of the experiment that was carried out I've just listed all the author's here you got my subjective experience of how that worked him about but of course they'll all have their own experiences as to how they came to do the work it was published in nature last year and you can take a look at that online as well but most of all I'd like to thank all of you for your time so thank you very much [Applause] [Music] you you

Channel: The Royal Institution

Views: 761,215

Rating: 4.8662696 out of 5

Keywords: Ri, Royal Institution, magnetic monopole, spin ice, magnetism, magents, physics, theoretical physics, wave function, lecture, science lecture, how do magnets work, north pole

Id: S3xH97Su-KY

Length: 53min 47sec (3227 seconds)

Published: Wed May 20 2020