Physics for the 21st Century
Macroscopic Quantum Mechanics Interview with Featured Scientist Jenny Hoffman
Interviewer: Why did you become a scientist?
JENNY: I’m just curious about how things work. I think I became a scientist because my mom had been a physics teacher and she brought home cool demos—I saw a laser and I thought that was awesome. She’d bring home dry ice and I thought that it was amazing that gas could be a solid. I’ve always liked quantitative puzzles and so it’s a way to combine, sort of, curiosity about things around me with my tendency to be quantitative about things. What I study now is materials. I’m interested in why materials behave the way they do. Much of the technology in the world around us is built upon materials that have very specific properties.
So, what I do is try to study those materials and understand why they have those properties and how we can develop new materials with new properties, which will enable new technologies.
Interviewer: What is your hypothesis?
JENNY: I’m a scientist and probably the scientific method that we’ve all learned about back in grade school is that you make a hypothesis and then you dream up some kind of an experiment to test your hypothesis. Then you do your experiment and maybe you have to revise your hypothesis a little bit and come up with a new test. And you refine it until you end up with a hypothesis that fits the result of your experiment. So, that’s the scientific method and that’s hypothesis driven science.
But before you even get to a hypothesis you need to do some exploration. You need to just go out into the unknown and look at things. And once you’ve looked enough and you have enough facts to put together you can try and organize those facts and form the hypothesis. But I have been more on the exploratory end of things. I look at a new material and I try to understand what’s going on in that new material. And I just poke and I look in all different directions.
I think about the early biologists who tried to figure out how the human body works and they didn’t even necessarily have a hypothesis. They just took dead bodies apart and looked inside. They were doing exploratory research, and it’s only once you’ve done the initial exploration that you can get to the point of forming a hypothesis like: “Oh, that thing that must actually pump the blood somewhere.” But until you’ve looked in the inside and until you’ve done exploration, you can’t even form that hypothesis.
Interviewer: What’s the motivation behind your research?
JENNY: I think we all have intuition that we have built over the decades of our lives about the way the world works around us. But the world around us is big and macroscopic and so our intuition relates to the classical world where things are big and macroscopic. And we have language to describe that because we have intuition about it. Yet we don’t have intuition about the microscopic quantum world because we don’t see that in our daily lives. I think most scientists, even professors of physics at Harvard, would admit that we don’t really understand quantum mechanics intuitively. So we resort to a mathematical language where we can follow from one mathematical line to the next and understand from one equation to the next what’s going on. We can use those equations to predict things and they’re very powerful, but that doesn’t mean necessarily that we have that same kind of intuition that you have when you say, “If I take a ball and I drop it, I know it’s going to fall.”
Interviewer: Your specific research is with superconductors. What is a superconductor?
JENNY: A superconductor is a material with two main properties. One property is that it conducts electricity with zero resistance; and what that means is, if you take a typical copper cable and you send electricity through the copper cable, there’s resistance. Those electrons are traveling and they’re sort of bumping around and they’re bumping into all of the different copper atoms as they travel through that wire. And every time they bump they lose a little bit of energy. A little bit of energy is turned into heat, which is not a useful form of energy. So, for example, if you send electricity from a power plant to your home the typical loss of energy is about 10 percent. So, one property of a superconductor is that it’s a material that can conduct electricity without loss of energy. The second property of a superconductor is that it expels magnetic fields. If you apply a magnetic field to a superconductor it will just expel the field to the outside of the superconductor and inside there will be zero magnetic field.
Interviewer: How was the first superconductor discovered?
JENNY: Like many fantastic scientific discoveries, superconductors were discovered entirely by accident during another experiment. Kamerlingh Onnes, a Dutch physicist, set about to liquefy helium, and he was the first scientist to liquefy helium. As he cooled it down to 4 Kelvin, he had accessed a new temperature regime that no physicist had ever been able to explore before. And he wanted to understand, in general, what happens to the resistance of metals as you cool them down to that very low temperature regime. Does the resistance continue to drop linearly as it appears to do at a higher temperature regime?
So, one of the metals that he started working with was mercury. Why mercury? Because mercury is liquid and, therefore, it can be purified much more easily than other metals. He wanted to look at what happens to the resistivity of a pure metal as it approaches zero temperature. He assigned his graduate student to measure the resistivity of mercury and his graduate student went to the lab, and measured the resistivity. He found that it dropped suddenly to zero right around 4 Kelvin. He showed his data to his advisor and Kamerlingh Onnes said, “That’s nonsense. You must have made a mistake. Your leads must have fallen off. Go back to the lab and do a better job.” And the graduate student went back to the lab and measured again, came back to his advisor, and said, “Here it is. This is what happened.” And the advisor said, “You sloppy idiot. You’re fired.” So, the graduate student went off and Kamerlingh Onnes went down to the lab himself and did the experiment and, sure enough, it was correct. And he got the Nobel Prize for that discovery two years later.
Interviewer: He did? Not the poor graduate student?
JENNY: No, nobody remembers the name of the graduate student. I’ve tried to find out the name. I’ve asked around. Nobody seems to know the name. I think he did okay. I heard that he ended up president of Philips or something!
Interviewer: What are some of the uses for superconductors?
JENNY: Superconductors have a lot of potential uses. Their actual usage has been limited a little bit by material properties, but one of the things that superconductors are in use for today is MRI machines. When you go to the doctor and you need a scan of some interior part of your body they use magnetic resonance imaging, which requires a very large magnetic field—on the order of one or two Tesla. To give you a sense of scale, the earth’s magnetic field is about one ten thousandth of that, so, in order to do an MRI, we have to produce the field about ten thousand times the earth’s field. The way we do that is with a very large coil through which we run a very large current. If we were to run that very large current through copper cable, so much heat would be dissipated that the copper cable would just melt. Instead, what we do is run it through a superconducting cable where none of that energy is turned into heat and we can run that current forever without having to supply any additional energy.
Another application of superconductors should be in power transmission between power plants and your home. Unfortunately, this hasn’t worked out very well due to some technical difficulties, but we’re still very hopeful that this is going to be a major application in the future. Superconductors are actually in use today in cell phone relay stations. You can make a very good filter out of a superconductor and so you can put your relay stations farther apart, which is important in rural areas. You can also make better use of bandwidth by building these better filters.
Superconductors are also good for building something called a superconductor quantum interference device (SQUID), which puts two superconductors in close proximity to each other and measures the tunneling current, measures the flow of pairs of electrons between those two superconductors. This turns out to be a very sensitive detector of magnetic fields, which can be used for medical diagnostics. It can be used to read brain waves and things like that from outside your head.
There are also a few maglev trains in the world but they’re not very useful. They’re sort of short demonstration situations right now. I think there’s one at the Shanghai airport but it’s mostly just for show. The idea is that you float a train on a superconductor using the Meissner Effect, whereby the superconductor expels a magnetic field. So, if you have a very strong magnet and then you have a superconductor in the base of your train, that superconductor wants to get away from that magnet—it actually floats above the magnet. Therefore, you can propel the train without dealing with the normal friction that you would have to deal with if the train were running on tracks. It’s as if you were running your train on an air table, but it’s even better than that.
An application for physics is in building very large magnets to explore other types of materials. In high energy physics, for example at the LHC, the Large Hadron Collider, which is a 27 kilometer loop straddling France and Switzerland, high energy physicists smash particles together at very high energies. In order to do that, they steer electrons around in a giant circle using superconductor magnets.
Interviewer: What does “high temperature” mean to you?
JENNY: High temperature for one physicist is not necessarily the same as high temperature for another physicist. I think for an average person high temperature would be 100 degrees Fahrenheit outside and for a physicist maybe high temperature can be 90 Kelvin above the temperature of liquid helium. But for another physicist, even one Kelvin is hot. Physicists like Debbie Jin, who do research with the cooling of atoms, would consider a nanokelvin to be a low temperature. So, we’re off by 9 orders of magnitude in what we consider to be a low temperature!
Interviewer: Your research is specifically based around “high temperature” superconductors. What is a “high temperature” superconductor?
JENNY: Well, the first set of superconductors discovered, the so called “low temperature” superconductors, only have the extraordinary properties of zero resistance and expelling magnetic field below a temperature called the critical temperature. For these “low temperature” superconductors, that critical temperature is very low. It’s on the order of minus four hundred and sixty Fahrenheit—that’s not a temperature that you can get to without spending an extraordinary amount of money. So, a ten percent power savings that you get is not worth it for the amount of money you have to spend to get down to that temperature in the first place. But in 1986, a new category of superconductors was discovered, the so-called “high temperature” superconductors.
These “high temperature” superconductors are not at temperatures that the typical citizen would consider high. They’re still quite low temperature, but they super conduct. They have these extraordinary properties starting below temperatures of about 90 Kelvin. And the reason that that is extraordinary and potentially very lucrative is that nitrogen, which composes 80% of our air, liquefies at 77 Kelvin. Nitrogen is abundant, it’s cheap, and it’s easy to liquefy. Liquid nitrogen is actually cheaper than milk. So, if we can cool materials down to the temperature of liquid nitrogen and they have these extraordinary properties, then we have potentially a very useful technology.
Interviewer: What is the “tool of your trade”?
JENNY: The scanning tunneling microscope. The microscope part is easy. You’re trying to see small things. The tunneling part refers to the mechanism by which we’re measuring those small things. What we’re measuring is a current. So where does that current come from? It’s actually tunneling from this very sharp tip into this sample that we’re interested in. And we say tunneling because it’s not flowing like it would flow in a normal metal. It actually has to tunnel through the vacuum between the tip and the sample. And that can only happen quantum mechanically. And then we get back to the word scanning; and that just means that this tip is on what’s called a piezo. A piezo is a material that allows it to move in a controlled fashion over very small distances. So, we scan this tip in a controlled fashion over very small distances. And we measure the tunneling current as we go; and that’s what makes our microscope.
Really, all it’s about is bringing a very sharp tip very close to the sample that you’re interested in. And when I say very close, I mean angstroms. That’s ten to the minus ten, ten to the minus nine meters. And that distance is so small that it’s actually very hard to accomplish that kind of proximity. So, we have a several inch instrument that is just dedicated to approaching that tip close enough to that sample to make that measurement. And then around that little microscope we have an apparatus to cool it to low temperature. And that apparatus has to be about five feet long because we want our microscope at four Kelvin and we need some way to connect it to room temperature, which is up here at three hundred Kelvin. The closer those two things are together, the more heat leakage you’re going to have from your room temperature down to your microscope. So, you want to put those things about five feet apart so that you have a relatively slow leakage of heat from one to the other. Then you have to put this whole thing inside of a thermos bottle, which keeps it cold. That’s called a Dewar.
This whole thing is also under ultra-high vacuum. What that means is that there really are virtually no atoms flying around. The way we study these materials is we cleave open the material in this vacuum environment to expose a fresh, clean surface, which is just the material that we want to study, not the oxygen or the nitrogen or the water molecules that have landed on the surface. It’s the material itself. If we didn’t do this in ultra-high vacuum, we would instead be studying water molecules that come and gunk up our surface very quickly. If you take something that’s atomically clean and you just put it in a room, within seconds it will have a film of water on it. Water is a very poly-molecule; it likes to stick to anything.
And then you need vibration isolation, because when you have this pico-amp tunneling current between your tip and your sample, it’s very sensitively dependent on the distance between your tip and your sample; in fact, it’s exponentially sensitive to the distance between your tip and your sample. That means that if you have a little wiggle in your tip sample distance, that little wiggle is exponentially multiplied as noise in your tunneling current. So, outside of our big thermos bottle Dewar, we have an enormous apparatus just to isolate this thing from vibrations.
Interviewer: What are your current investigations?
JENNY: I’m really trying to answer a fundamental question. I’m trying to understand how do many electrons interact in a material. And I am studying it via high temperature superconductors, but there are many other materials in which electrons interact strongly. For example, there are colossal magneto resistance materials in which you apply a magnetic field and the resistivity changes by several orders of magnitude. This is how your hard drive works and this was awarded the 2007 Nobel Prize. So it’s important not just for superconductors but for many materials that are technologically relevant to understanding how we can describe the motion of many electrons at the same time. It’s not hard to describe the motion of one or two electrons but to describe the correlated motion of six times ten to the twenty-three electrons is an unsolved problem.
I’m trying to understand what the electrons are actually doing in these high temperature superconductors. And I’m trying to understand what it means that they’re correlated, and what we can learn that will enable us to come up with a mathematical framework to describe these correlations.
I use my microscope to measure the properties of the electrons and these materials and I hope that that information can be useful towards forming a broader theory to explain not only high temperature superconductivity but hopefully also other correlated electron materials.
I’m looking at Barium Cobalt Iron Arsenic, which is one of these very new high temperature superconductors discovered only in February 2008. And I’m trying to understand how the electrons interact in that material. What I’m actually looking at is a few little millimeter fragments of this material, which I’ve put into my low temperature microscope to measure the tunneling current into these materials.
Interviewer: What will the future bring to superconductor research?
JENNY: It took two decades to get from the first family of high TC superconductors to the second. And right now we’re sort of stumbling around in the dark, and there’s a few chemists who have incredible intuition, who stumble across these new compounds. But we don’t have a predictive theory that’s guiding us.
A lot of really smart people are working in this field. It’s kind of a Holy Grail to come up with a room temperature superconductor and in order to do that we think we really have to understand what’s going on in the superconductors we have. It’s definitely a hot field. I think room temperature superconductivity is going to happen, but I think it’s at least a decade out.
6.2 Macroscopic Quantum Mechanics – Video
Quantum mechanics provides a useful description of nature at the atomic and subatomic scale, but it also manifests itself in phenomena at macroscopic scales, including lasers, clouds of ultra-cold atoms, superfluids, and superconductivity. Among these phenomena, the recent discovery of new kinds of high-temperature superconducting materials holds the promise of many practical applications—but there is still no theory about how these materials work. See how researchers are approaching this problem from two different directions: a "top-down" approach, closely examining the materials themselves, and a "bottom-up" approach, looking at model systems that mimic the quantum interactions of the superconducting electrons inside the materials.
6.5 Extra: The Meissner Effect
Wolfgang Rueckner demonstrates the Meissner Effect, one of the remarkable properties of superconductors
Supplementary: Unit 6: Macroscopic Quantum Mechanics — Printable Online Text
Supplemental resource for educators and students