Eva Dale 0:01 From the heart of the Ohio State University on the Oval, this is Voices of Excellence from the College of Arts and Sciences, with your host, David Staley. Voices focuses on the innovative work being done by faculty and staff in the College of Arts and Sciences at The Ohio State University. From departments as wide ranging as art, astronomy, chemistry and biochemistry, physics, emergent materials, mathematics and languages, among many others, the college always has something great happening. Join us to find out what's new now. David Staley 0:33 Joining me today is Nandini Trivedi, Professor of Physics in the Ohio State University College of the Arts and Sciences, where she works in the area of theoretical physics, specifically on quantum Monte Carlo simulations for bosons and fermions, condensed matter theory, and cold atoms, all of which I hope we're able to discuss this morning. Welcome to Voices, Dr. Trivedi. Nandini Trivedi 0:56 Thank you. David Staley 0:57 So, you work specifically studying the phenomenon of emergence, and I wonder if you'd start there. Tell us what emergence is and what it is you study here. Nandini Trivedi 1:06 So, in quantum matter, what we are interested in is taking the elementary particles, let's say like electrons, which may have very simple local interactions, and then asking when we take many electrons and put them together, how do they organize themselves? David Staley 1:29 So, before we go in a deeper tell us what quantum matter is. Nandini Trivedi 1:33 Quantum matter is looking at materials where the essential dynamics is determined by quantum mechanics, which means that you can't precisely tell what the position and momentum of a particle are simultaneously. This does not happen in the classical world. If you take a football, you know precisely where it is, when you're trying to shoot it, and you also know what is it speed. But as you make this ball smaller and smaller, there comes a point where quantum mechanics starts playing a role. And now there's an uncertainty in the position of the ball, and that is inherent in the way quantum mechanics works. So the world I work in... David Staley 2:27 Is a very small world, it sounds like. Nandini Trivedi 2:29 It is principles that operate at a microscopic level, but you see its consequences at the macro level. David Staley 2:36 Interesting. Nandini Trivedi 2:37 So for example, magnetism superconductivity, these are things you can visualize at a macro level, your refrigerator magnet, or conductors. So these have properties which we can take advantage of, at a macro level, and yet the principles that drive them are operating at a microscopic level. David Staley 3:02 So, when you study emergence, what does that mean when you're looking at electrons, for instance? Nandini Trivedi 3:06 So if you take, let's say, electrons, in a typical conductor, these electrons will be moving along scattering of defects and producing some conduction. So that's what a single electron is doing. It's like a single person in society going about their own business, you know, through the day. But as you cool, such a conductor, like aluminum, just good old aluminum, you just start cooling it, something remarkable happens, the same defects, which were scattering of these electrons, now seem to suddenly play no role. And you start getting conduction with no resistance. So how does that happen? Because the defects are still there, you still have the same electrons. How do they now produce zero resistance? And when we say conduction, what's being conducted here? Okay, so, when you take a wire, you have electrons in the wire, and just ordinary copper wire in the copper wire or aluminum wire, you have electrons, and they are sitting sort of in a level playing field. So you have to apply a little voltage or a battery, which puts more charge on one side compared to the other. If you now connect the two sides of the wire, you can start having flow of charge from the high charge end to the lower charge end. And that flow is what we call conduction. This conduction doesn't just happen effortlessly. There is some resistance in the flow of these electrons because they are bumping around defects there are thermal vibrations in the wire which also deflect these electrons. So for the electrons to get from one sight to another, there is some resistance, and that shows up in the heating of the wire. So if you wait for some time, you will see the wire get red hot. And that's a signature, that while you are getting conduction, it's coming at a cost of producing resistance. Now the phenomenon I was talking about of emergence is the same wire as you Google it, by putting it in, say, liquid nitrogen, or even colder liquids, as you call it, so as to remove the thermal vibrations that scatter these electrons in their path of going from one end of the wire to the other end. As you reduce these fluctuations, something miraculous happens, there's no resistance, and current just flows in the loop forever. And you can calculate, how long will the current keep flowing, and you get estimates like the lifetime of the universe. Wow. So it's a very robust phenomenon. And you have to wonder and ask, it's the same material, same electrons, same defects, but a completely new property. So that's what we call emergent, because it is truly the emergence of a phenomenally new kind of property. David Staley 6:21 And where does that emergence come from? Why does that happen? Nandini Trivedi 6:26 So first discovery of this phenomenon happened in liquid mercury in 1911. So last century, it took about 50 years, to come up with a theory for this. And that's the first paradigm that established why it happens. That was done by three scientists, Bardeen, Cooper, and Schrieffer. And what they explained was that electrons, which were acting individually start to do something collective. So do electrons pair up, in sort of partnership, like a dance partner, and then not just to all the electrons pair up. And they don't just do their own pair dance, they all the pairs, sort of joined together in one coherent dance. And that's the emergence of this very collective new state. So it's like literally a marching band, walking down the street as a collective. So the small bumps and potholes that we're scattering, a single person now doesn't have that much of an effect on a large mass, marching through the street, it's a bit like that, this collective can just go through the wire without scattering of the same defects. And this coherence is literally a beautiful quantum dance that emerges from principles of physics that we understand very well, at a local level. But its consequences at a macro level, were nevertheless totally unexpected. David Staley 8:12 I was reading some background, I think, from your website, and especially when you're talking about superconductivity, you're talking about a paradigm that had governed the study of superconductivity, and then, these are your words came the Woodstock of physics in 1987. And I just have to ask, what... what is that a reference to? Nandini Trivedi 8:30 Yes. So this theory that I mentioned, which took 50 years to develop, and gave us this picture of why you get a superconductor, ruled for almost many decades till 1987. And the basic tenet of this theory was that you have two electrons that attract each other to form the pair. And, you know, this seemed like a necessary condition to get this emergent superconducting behavior. And yet, in 1987, this paradigm was strongly shattered. So there was a discovery of superconductivity in a place, nobody would have looked. So just the word tells us that to get a superconductor, you first need a conductor. Only then there's a hope that it might superconductor. And yet in 1987, the materials that ended up superconducting were in fact insulators to begin with. So they don't conduct electricity, unlike metals. And with a little bit of quantum chemistry, tinkering, these insulators were made to become superconducting and not just superconducting like the old ones. So, one other determining feature about superconductors is how low a temperature do you need to go to? David Staley 9:59 That was my one of my questions, how cold are we talking about here? Nandini Trivedi 10:01 How cold are we talking about; so for aluminum, it's on the order of few Kelvin. Now Kelvin is a different scale from what we are used to, which is the centigrade or the Fahrenheit scale. So room temperature is around like today around, say 300 Kelvin. So we are going down zero Kelvin is the absolute lowest we could ever get to. So it's just a few Kelvin above that for typical metals like aluminum. And that's really, really, really cold that is very cold. Now the beyond Antarctica cold, yes, so the lowest temperature in the universe is about three degrees Kelvin. So it's getting to those temperatures. Now, for these materials, ultimately to be useful for us, we would like the temperature to be close to room temperature. So the reason for the excitement in 1987, is a very unusual class of insulators, with a little bit of quantum chemistry, were made to go superconducting at 40 Kelvin. And a few months later, the number went up to 90 Kelvin. And today, it's around 160 Kelvin. So that was the big reason for the Woodstock of physics, which is the American Physics Society March meeting that is held every year. It has around 10,000 attendees. And practically everybody was trying to get into the same room, which is probably able to hold at most a few 100 people. So there was almost a stampede, like the Woodstock of physics, trying to hear about the latest developments of how people went after these materials, what was the reason somebody even looked there, and what was the phenomenon and so on? David Staley 11:57 What are you working on specifically in superconductivity? Nandini Trivedi 12:00 So that particular material has captured my interest for many decades, because not only is the temperature high, but when I say a paradigm is broken, and a new one needs to be established. That's the kind of situation that grabs the theoretical physicists interest. So what it says is, you need new principles to understand how an insulator could become a superconductor. Furthermore, magnetism is something that has always been sort of antithetical to superconductivity, because as I said, you need to get pairing between the two electrons. And what that involves is these electrons are actually like little tops that are spinning, one electron is spinning up, and the other one is spinning in the opposite direction. And these two spinning tops bear up, if you have a magnet, it will try to align the spins of the two electrons in the same direction and break these pairs. So typically, superconductivity and magnetism don't go exist, and yet these insulators are magnetic. So the question is okay, now you have an insulator, and you have a magnet, how is superconductivity arising in such a situation, it has to be completely new theoretical principles. So that is what has kept me interested in developing such a theory. And we have made quite some advance in that direction. David Staley 13:37 Tell us about some of these advances. Nandini Trivedi 13:39 So, what has emerged is that the insulator to begin with already has some of these bears sitting in the insulator, but they are not able to move. It's like taking, let's say, a parking lot, where you have a car in every slot. So that's an insulator nobody can move, but already there is communication between the different cars and they know that car A is paired up with car B and C with D and there is some very interesting correlated coherent state in the insulator itself. Now, as one car moves out of the parking lot, and you do this in the material with some quantum chemistry, then you can start to see these bears become mobile and bit by bit they are able to move and not just move super conduct. So the transition temperature starts out zero with some of this doping as it is called, which is removing some of these cars from the parking lot, you make space for some movement, then the transition temperature goes up and finally crashes again at even higher doping. So this phenomena and this mechanism is something that has been revealed from my research as well as other people's research. Eva Dale 15:07 Did you know that 23 programs in the Ohio State University College of Arts and Sciences are nationally ranked as top 25 programs, with more than 10 of them in the top 10? That's why we say the College of Arts and Sciences is the intellectual and academic core of the Ohio State University. Learn more about the College at artsandciences.osu.edu. David Staley 15:32 So as a theoretical physicist, obviously, you're interested in exploring these new kinds of states of matter, but I wonder, are there practical or other sorts of applications for superconductivity? Nandini Trivedi 15:44 Yes, as I said, you get conduction, which doesn't decay, the currents don't decay. So, if you take these currents and make it into a loop, take the wire and make it into a loop. So now you have currents moving in a circle, and these rotating currents produce a magnetic field. That's another fundamental property of currents. Super currents, which don't decay, produce really strong magnetic fields. So they become a way of generating magnets with stronger strength. For example, MRI machines, typically use superconductors. And if you can get these superconductors working at room temperature, you can then have a machine that can be taken to the patient anywhere, and they don't have to go into big facilities and so on. The Large Hadron Collider, which is a high energy experiment, you know, which is a very large circuit over which protons are collided in an effort to produce conditions under the Big Bang, and see what emerges from these high energy collisions, you have to move charges in a circle that is achieved again by bending magnets. And these bending magnets are made out of superconductors. And as we go toward the next generation, they will be made out of these high temperature superconductors. David Staley 17:13 And you are going to be discussing superconductivity, maybe these and other sorts of matters, for science Sunday on December 1, 2019. Give us a little bit of a sneak preview of what you'll be presenting that day. Nandini Trivedi 17:26 Yes. So I want to talk a lot more about real experiments and explain what are conductors what affect their properties and give a real visual picture of a superconductor. And I'm going to show a train, which has a magnetic track. And on that I will place a superconductor which has been cooled to below this transition temperature and show that there is a repulsive force between the magnet and the superconductor. And that repulsive force levitates the superconductor on top of the tracks. So this is really amazing that you can have something levitating. Now you can quickly see how this could be really applicable. For real life. If you have a train moving on a track, there's a lot of friction. But if this train could be levitated, and you think, holy moly, that's like bizarre how could you ever have a train which is up in the air, but you could do it with such a superconducting train on a magnetic track. And then there won't be any friction and it could move at speeds that have been estimated to be as high as those of airplanes. David Staley 18:40 So, magnetic levitation, maglev, is that sort of thing we're talking about? Nandini Trivedi 18:44 Yes. So maglev, currently is done not with superconducting trains, but with just the electromagnetic induction, and that can be taken to the next level of putting a superconducting train. David Staley 18:56 I have to go back to analogy you gave you talked about these electrons sort of moving in order, and you describe it as a beautiful quantum dance. What's beautiful about it? Nandini Trivedi 19:06 So I think what is beautiful about it is that no one tells the electrons, you have to dance, there's no instructor giving these instructions. The instructions are literally in the fundamental physical forces between electrons. So these are coulomb interactions, interactions with the lattice or with magnetic interactions, but these are all well understood interactions at the local level. And yet, the collective F effect that we have been talking about this emergence is something that because of quantum mechanics, and what statistical mechanics which is the effect of having many degrees of freedom, this collective dance appears. So to me, that is really beautiful. David Staley 20:06 So what got you interested in theoretical physics in the first place? Nandini Trivedi 20:10 Before theoretical physics, I got interested in just physics in high school. I liked tinkering with, you know, different gadgets that would go bad in my home. And my mother was nice enough to let me fix the toaster, which I was able to do, and then completely ruin the speaker and the audio system. But I learned from these experiences. So that's how I first got interested in physics, or science, I should say. And then later, I think the turning point for me were the Fineman lectures, and specifically, Richard Fineman, lectures, and even there, I remember the precise D, that I knew I would be a physicist. And it was this experiment of two slits, which an electron has to go through. And you know, this comes back to our football, if our football had to go through two slits, it would have to pick either slit one or slit two. But an electron produces an interference pattern at the other side on the detector, which clearly indicates that it has not chosen one of the slits, it has gone through both slits simultaneously. And that can only happen if a single electron is a wave. And that was an eye opener for me, you know, to imagine a water wave. It's not hard, you know, there are many atoms, some are going up in the crest, some going down. But here, a single electron is acting like a wave going through both slits. And yet, when you measure it at the other end, it produces a ding, definite identification at the detector, which is like a particle. When I read that, I was stunned. And I thought about that for a long time. And I decided this was a mystery I had to understand deeper. And mathematics is really the language for physics. So that's what got me interested and showed me the path of getting into deeper physics through deeper mathematics. David Staley 22:31 I'm also interested in your outreach work, which is rather extensive. So, first, I know you are the founder and director of Scientific Thinkers, which is an outreach program at Innis Elementary School, tell us more about this work. Nandini Trivedi 22:44 So I want to take this excitement that I've felt about physics from an early age to young people. So that's why I decided to go to the elementary schools. And we were told that some of these schools are under dire conditions, Academic Alert. And I said, you know, if we can just make them excited about learning, maybe other things will fall in place. So I didn't want to go after improving grades and performance. But I just wanted to open their eyes to the excitement of science. So in this in this elementary school, what we do, the motto is, meet a scientist, be a scientist, think like a scientist. So what we have are undergrads and grad students from Ohio State. And we take kids with particular experiments to the class, distribute these kids, for example, how to make an electromagnet to all the students, and then they work on it. They put this electromagnet together, and we spend about three hours. So it's not just a quick demo, and then you're out of it. But we discussed the demo in great detail. How does the magnet work? As you add more windings to the nail and more wire windings? Do you pick up more pins? Why do you pick up more pins and so on. So we blend in some mathematical skills of plotting. We also bring in some language and like taking electromagnet and breaking the word up into smaller words. So we try to integrate everything and make it fun, and it's been a real eye opener for the students at Ohio State, the students in the school and also the teachers who learn a lot from the volunteers from Ohio State. David Staley 24:32 You do public outreach events at COSI, including "My Quantum Mom" and "Emergence" and I've gotta ask, what is "My Quantum Mom"? Nandini Trivedi 24:40 Well, it comes back to that two slit experiment which has never left me, but the precise title came about because of a Harry Potter story. So when that was the rage, my daughter was always reading Harry Potter even at dinner time and one day I said to her, you know you have to stop that at dinner and she He looked at me very unhappy with a mother who couldn't share in her excitement of Harry Potter. And he said, But why are you not interested in it? And I said, you know, I absolutely love the book, with its nine, three quarter platform and all these stories. But at the same time, the world I live in the world I work in is so much more exciting than, you know, Harry Potter, which is just fictional. So she said, Tell me one thing about your world that is so exciting. And I said, Well, you know, an electron can be in two places at the same time. So she stops for a while, and she says, Oh, you know, Hermione can do that. She can be in two classrooms at the same time. And I was like, Yeah, that's a good story. But it's just a story. In my world, it's real. And you can do this experiment to test it. So I knew I had got her she didn't have much to say at that time. But couple of days later, when I was about to travel, I go to tuck her into her bed. And she says, Mom, couldn't you be a quantum mom, as I thought about that, such a delicious story. And I said, you know, we have to take it out to go sigh and make an event out of that. So that's what became my quantum mom. We had a one day event where a lot of the graduate students did small skits for the public, explaining how something can be a particle in a wave, some of the elementary things about bits and qubits. Because ultimately, cubed is a quantum bit. Yes. qubit is a quantum bit and that forms the foundation of a quantum computer. And one of the architectures for making that is a superconducting qubit. So that's how my work then ties in all the way from the theoretical underpinnings to experiments, to you know, applications, like levitated trains, and now to the absolutely spellbinding applications, like quantum computers, that's still in the future. But it's already happening with Google and other companies even putting money into it. David Staley 27:12 And you've also given talks at conferences for undergraduate women in physics called "Balancing Career and Family", and maybe you can get from the title, what you're talking about, but what are you been talking about here? Nandini Trivedi 27:23 Well, I want to address the issue of women in physics and why there are so few women in physics, what are the challenges women face, everybody faces challenges in a difficult field. But women face particular challenges, because there are so few of us. And on top of that, we also have to balance our families. So that's the second big challenge. And how do you make it all happen? What kind of strategies can you develop, you can't do it alone. And when you are very isolated, that's what you tend to do is to become more alone. But you have to reach out from where you are, and network. So in this talk, I talk about strategies for surviving and not just surviving, but blossoming in the field, and leaving a stamp of your own and being a culture changer. Because we've come in with a very different way of looking at physics and a different way of inspiring other people. And instead of falling on on ourselves, we have to sort of reach out. So that is kind of my message in this talk. David Staley 28:42 Nandini Trivedi. Thank you. Nandini Trivedi 28:44 Thank you. Eva Dale 28:46 Voices from the Arts and Sciences is produced and recorded at The Ohio State University College of Arts and Sciences Technology Services Studio. Sound engineering by Paul Kotheimer, produced by Doug Dangler. I'm Eva Dale. Transcribed by https://otter.ai