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Physicist Chuck Agosta and the students in his research lab study how a material's ability to conduct electricity is affected by changes in temperature, pressure and magnetic field. |
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Meet the researchers: Relying on ingenuity
Interview with Professor Chuck Agosta and Mike Viotti
Sophomore Mike Viotti began working in physicist Chuck Agosta's lab during the first semester of his sophomore year. In a recent conversation, summarized below, they described their interest in devising new instrumentation to study the properties of novel conductors.
Chuck, could you describe what you study in your lab?
We study novel conductors (conductors are materials that conduct electricity). Most of the conductors we study are anisotropic, that is, they conduct electricity better in some directions than in others. Most of the ones we've been investigating recently conduct electricity better in two dimensions (within a plane) than in three dimensions.
In three dimensions, it's much easier for electrons to avoid each other. When you confine electrons to just one or two dimensions--for example if you can imagine electrons moving along a line--they have to interact with each other much more often. It's very hard for an electron to get somewhere when it's trying to move along a line and there are other electrons in its way. So electrons interact more strongly when they're confined to lower dimensions. We want to understand how electrons interact, and part of the way we do that is to confine them to lower dimensions to intensity those interactions.
We investigate the behavior of electrons by subjecting them to extreme conditions--very low temperatures, very high magnetic fields, and very high pressures.
Are these conditions varied to confine electrons to low dimensions?
All of these extreme conditions do different things. Pressure allows us to tune the anisotropy of the material; that is, the direction(s) in which the electrons flow. We have material where the electrons mostly travel in layers. When we increase the pressure, we push the layers closer together and it allows the electrons to more easily pass between layers. Ideally, if we pushed them together enough, we could make the material 3-dimensional. So we can use changes in pressure to continuously vary the dimensionality of the material. Variations in pressure change the density of electrons.
Lowering the temperature confines the electrons more because there's less energy at low temperatures and it's less likely that an electron will "jump out of line," so to speak. In most cases, lowering the temperature is just to dampen down the background noise so that the interactions between electrons are more apparent.
We create high magnetic fields to perturb the electrons. Normally when you set up a magnetic field, electrons want to move somewhere (what we call electric current), and when you put them in an electro-magnetic field they want to move in a spiral. By turning on the magnetic field, you do two things. You make the electrons change their paths, and you also cause the electrons to align. Electrons act like little magnets and they want to align with the direction of the magnetic field. That's important to know, because one of the things we study is superconductivity. In a superconducting material--one that lets electricity flow with very little or no resistance--electrons have to be paired and oppositely aligned. By turning on a magnetic field, you can destroy superconductivity.
Do you vary these parameters one at a time, or in combination?
Usually one at a time. In general we keep pressure and temperature constant while we vary the magnetic field. It's rare that we hold the magnetic field constant and vary the pressure or temperature.
I assume you use special equipment to create these different conditions?
Yes. Every piece of apparatus that we use is in some sense unique because we're trying to work with all three parameters at once. In particular, our magnetic fields are unique because they're high-pulse magnetic fields. They're only on a short period of time. There are only a few pulsed-magnetic field laboratories in the United States. In fact, on a world scale there are only about 20-30. At Clark we've achieved 50 tesla, the strongest magnetic field produced at any university lab in the U.S.
Low temperatures are not unique to Clark; however, creating low temperatures in a pulsed magnetic field poses certain challenges. Specifically, we have to minimize the use of metal in our apparatus.
Why is that?
When you have a quickly changing magnetic field, you produce an electric current in any material such as metal that conducts electricity. Also, the current makes the material heat up, which is not what you want when you're trying to create low temperatures. The current also creates forces on the object that can actually push it out of the center of the magnetic field. Essentially you're creating a little projectile launcher. So you want to avoid put conducting objects in the magnetic field as much as possible. We have to use some very thin copper wires leading into our sample chamber, but we try to keep them as small as possible to minimize any effects. Sometimes the samples themselves conduct electricity, which can cause problems.
In what kinds of materials are you studying the properties of electrons?
We use a few different classes of materials, but most commonly we look at materials that are organic (carbon-based) conductors. In addition to carbon, they contain hydrogen and usually selenium or sulphur, and then small amounts of many other elements. Selenium and sulphur atoms have large electron orbits that overlap and form a conducting path through the material.
How do undergrads fit into your research laboratory?
When undergrads ask if they can work in my lab, I'm happy to include them. They can assume many different roles, from building equipment to running experiments, depending on how long they stay, their previous experience, and their facility with tools and computers. They get involved at all different levels. Several times undergrads have traveled with me to other labs to do experiments. It's extremely varied.
I assume there are graduate students working in the lab as well?
Yes. As an example, right now there are four grad students and three undergrads working in my lab. One of the graduate students is getting his master's degree through Clark's 5th year free program.
Mike, how did you come to work in Chuck's lab?
Chuck was my professor last year for Introduction to Physics and at the end of the year he took us on a tour of his lab. I found it interesting and asked if I could get involved.
What is your current role?
I'm working closely with another undergrad, senior Dave Barbee, and we're in charge of setting up the pressure system. We took an old system that Chuck got from another university. Dave pretty much redid the system with new valves and tubing, while I was in charge of setting up the actual pressure cell that would contain the sample of material we wanted to study. The sample goes inside a pressure cell made of ceramic and high durability plastic, and the cell is put inside a magnetic field. Then we set a constant pressure, and vary the magnetic field. As the strength of the field varies, we measure the properties of the sample using RF (radio frequency) penetration.
Chuck: We have to put a little coil of wire in the pressure cell and lead wires out so we can make our measurements. It's because we learned how to do that that we decided to try this new pressure cell. The idea behind the pressure cell is that we can measure samples that are extremely small. We can fit our entire measurement apparatus-the sample in a tiny coil-into a hole that is only .5 millimeter in diameter. We can achieve very high pressures because we're only pressurizing a very tiny space.
How did you learn how to take this piece of equipment apart and put it back together?
Chuck: The company that made some of the original parts is no longer in business. But there's a company in Massachusetts that made similar equipment that is still in business. I went to visit them, which was part of our learning process. But part of it was Dave just diving in.
Most of the components are old, but the arrangement is new. We used the old system to understand how you would put one of these together. That was the hardest part. Once we knew what everything was, it was just a matter of configuring it for our particular use.
Mike: Most of the way the system is pathed-that is, the way the gas moves through the system--is completely different from in the original equipment.
It sounds like you build a lot of your own instrumentation.
Chuck: That's not true of every physics lab, but it's particularly true of ours. Designing and building instrumentation is what I do. I'm not interested in making measurements using apparatus that's currently available. I'm interested in finding things that haven't been measured before, because no one has figured out how to measure them. So our goals include developing new apparatus to advance measurement science, and measuring things that other people haven't measured before.
Mike, can you comment on advantages and disadvantages of participating in research?
We meet as a group--grads, undergrads and Chuck--once a week for an hour or so and talk about each other's projects. There are three or four different projects going on at one time. I learn a lot about everyone else's work. Even though some of it is outside of my realm of comprehension, I still learn something. What I really like is that a lot of the problems that we have to solve require ingenuity more than really advanced physics. When it comes to equipment, the first question always is, 'can we make it?' and the answer is generally yes. I enjoy that.
I have a lab once every two weeks for a physics class I'm taking now, and it's really interesting. But working in Chuck's lab is just completely different.
I think many students assume that they can't start participating in research as early as sophomore year.
Mike: You can't expect to discover something incredible your first year, but there's always something you can do. I know from working with Chuck, and from friends who work in biology and chemistry labs, that the professors are very good at explaining what you need to know on a level you can understand.
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Additional Resources
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Chuck Agosta and Mike Viotti
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 Mike Viotti and David Barbee '04 present their research in a poster session at Academic Spree Day. Their presentation, available here, was titled "Development of a High Pressure System and Cell for Use at Low Temperature and High Magnetic Field."
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| The pressure system front (top) and back (bottom). See the poster link for more information. |
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Things that haven't been measured before