• ## High-Temperature Superconductors: Garnet Chan Discusses Trying to Crack the Mystery

12/09/2016 14:50:33

# Garnet Chan Talks Quantum Chemistry and Chinese Food

Garnet Chan, Bren Professor of Chemistry, recently moved to Pasadena from New Jersey, where he was a professor at Princeton University for the past four years. Chan's specialty is quantum chemistry, a field pioneered at Caltech by the late Linus Pauling to understand the behavior of molecules. Raised in Hong Kong, Chan earned his bachelor's degree (1996) and PhD (2000) from the University of Cambridge, then was a Miller Fellow at UC Berkeley before taking a faculty position at Cornell University.

Chan sat down with us to discuss his move to Pasadena and his excitement over the Chinese culinary delights the area has to offer—and to answer a question he's heard before: What exactly does a quantum chemist do?

### How do you describe the big picture of what you do?

Broadly speaking, I'm a theorist, and I'm interested in going from the very simple equations of quantum mechanics—which are the fundamental equations of nature, the most basic equations we know about the world—to the actual behavior of molecules and materials and real matter that we can touch around us. It's a discipline that involves finding computer algorithms that allow us to simulate these equations, at least approximately.

What makes me a quantum chemist as opposed to another kind of researcher working with quantum mechanics is that the problems I'm interested in are the ones that chemists study. These can be very concrete things like what steps are involved when an enzyme catalyzes a reaction, or what makes a material absorb a specific frequency of light. Basically, we are trying to simulate complex chemistry.

### What problems are you specifically working on?

One problem we are working on is the problem of high-temperature superconductivity, which has been a mystery for 30 years. Superconductivity is the name given to the phenomenon where if you lower the temperature sufficiently in a material, you'll reach a point where the resistance to electric current all of a sudden goes to zero. We then say that the material is superconducting. In a certain class of materials called high-temperature superconductors, you do not have to lower the temperature very much. You still have to lower the temperature to minus 140 degrees Celsius. It seems cold, but that's equivalent to 130 to 140 Kelvin, and most materials are only superconducting up to about 10 Kelvin. Even though high-temperature superconductors were discovered 30 years ago, we still don't know how they work.

I have kind of an attachment to this problem because I like problems that people have banged their heads on for decades, and in many cases given up on solving. I think many people would agree that this is probably one of the single most perplexing questions about materials. The real thing that has changed in the last 30 years is the development of new computational tools for quantum mechanics. You used to solve problems by having some inspired guess. Our hope now is that we don't have to make such an inspired guess because we can get at least some of the way there by computation.

I've in some sense worked 15 years trying to build up a set of tools that can address the different challenges involved in simulating these materials. We've recently achieved success in simulating simplified models of the materials. By simplification, one can think of it as like trying to simulate the planets in the solar system, but with some of the planets taken out. We can now can simulate the models to very high precision, and you can see behavior very similar to the real high-temperature superconductors. This gives us confidence that we can soon understand what is happening in the real materials.

### Do you have any other projects?

Another one of my interests is related to the biological mechanism by which enzymes "fix" nitrogen. Most of the nitrogen on Earth is in the air as nitrogen gas [N2], but humans can't process nitrogen gas. Instead, we get much of our nitrogen indirectly from fertilizer—or from bacteria. Certain bacteria have an enzyme that naturally "fixes" nitrogen, which means that it is converted into ammonia or related compounds that fertilize plants. The plants make amino acids, and we eat the plants—or animals eat the plants and we eat the animals. In the end, the nitrogen gets into us.

What makes the biological process so fascinating is that it is able to proceed under ambient biological conditions, while industrial fertilizer production, via the Haber process, proceeds at high temperatures and pressures, and consumes enormous amounts of energy. This means that the biological enzymes are doing some very clever chemistry.  We hope to unravel the details of this process using the principles of quantum mechanics. We've recently uncovered some unexpected behavior of the electrons in these enzymes. Perhaps the answer to how they work lies there!

### What happens after you simulate the chemistry for reactions like this?

The results of computational chemistry simulations are used by many chemists, not just theorists like me. In fact, these days a very large number of experimental papers have quantum chemical calculations in them to help interpret the results—in this sense, there is a very healthy interplay between theory and experiment. However, I see the role of our simulations as having impact beyond the specific problems that we choose to study. That is because the tools that we are building to perform our simulations help push the frontier of the types of chemistry and reactions that people can study. Eventually, these tools will be usable by all chemists, and I hope they can be used to study all of chemistry.

Quantum chemistry has always evolved to make new tools to answer more and more complicated questions. In the beginning, in the 1920s and 1930s, people were mainly studying atoms. Later, they studied molecules and what holds them together—Linus Pauling, who was a professor here, started this type of work. These days we are working at a frontier where the tools are being developed to study the most complex problems of biology and materials.

### What are you most excited about in coming to Caltech?

I'm completely sold on this place. People here are focused on science, so this is exactly the right place for me. I also like the scale. It's so small that you really feel like you're in some family. Certainly the chemistry department feels like a very tight-knit community.

### What do you like about Southern California?

I think there's a reason why so many people live in Southern California. It doesn't get better than this. You have great weather. There's lots of good food. People complain about traffic, but I lived in New Jersey and traffic there is terrible. Also, this area of the country has probably the best Chinese food. There are hundreds of good Chinese restaurants in the cities of San Gabriel, Monterey Park, and Alhambra. Food is such a big part of all cultures, but certainly a big part of Chinese culture, so that's a big plus.

Written by Whitney Clavin