A Q&A On Quantum Transitions
Augusto Ghiotto describes his PhD work at Columbia on 2D materials, which was just recognized by the American Physical Society.
When water molecules shift from ice to liquid to gas (and back), they undergo phase transitions related to temperature. But in certain circumstances, materials can shift from one phase to another even without their temperature changing—the result of still somewhat mysterious quantum mechanical fluctuations. At temperatures near absolute zero, phase transitions in certain materials can occur at what’s known as a quantum critical point, a rare transformation that researchers are still working to understand.
In a paper published in Nature in 2021, Augusto Ghiotto (GSAS’23), who graduated with his PhD in physics last spring, and his colleagues, found signatures of quantum criticality in a two-dimensional material called tungsten diselenide (WSe2). The material, which undergoes a tunable, continuous transition from metallic to insulating phases at ultracold temperatures, is a new platform to explore fundamental physics questions about quantum mechanics.
The work was part of Ghiotto’s dissertation, for which he recently received the 2024 Richard L. Greene Dissertation Award in Experimental Condensed Matter or Materials Physics from the American Physical Society (APS). He will accept the prize and give a talk on his work at the upcoming APS March Meeting in Minneapolis.
“Augusto’s work on the tunable metal-insulator transition in WSe2—both his experimental discoveries and his incisive analyses—is a crucial scientific step forward because it has brought a whole domain of deep theoretical questions related to metal-insulator transitions into quantitative contact with experiment,” said Columbia theoretical physicist Andrew Millis, who nominated Ghiotto for the award. “It's important to note that the work is enabled by the strengths of Columbia’s quantum community—from the ability to synthesize new materials to the technical advances that make possible the transport measurements on atomically thin materials, to the theoretical community and most importantly the on-going intensive scientific discussions and interactions that make this place so fun and stimulating to work in,” he added.
Columbia’s quantum community is one that Ghiotto has come to know well. “Everyone at Columbia was so collaborative. Regardless of their departments, other groups were always willing to help if I needed something,” he said. Born and raised in Brazil, he came to Columbia in 2012 as an undergraduate. He earned his BA in physics and math from Columbia College in 2016 and his PhD last spring. After 11 years in New York, he moved to California in September and is now a Miller Fellow at the University of California, Berkeley.
We spoke with Ghiotto about his PhD work, how he became interested in materials science, and the hobby that connects him to home.
What sparked your work at Columbia?
It was actually serendipity. I wasn’t looking for criticality but just studying electron transport and correlations. Daniel Rhodes, a former postdoc in engineer Jim Hone's lab, had spent years making high-quality samples of WSe2. With Lei Wang, a postdoc who worked with physicist Cory Dean, we first explored its electron transport in a 2020 paper published in Nature Materials.
Without correlations, the system should behave like a metal as you add or remove electrons. But there were correlations, so it acted like an insulator. A theoretical colleague, emeritus professor Chandra Varma from the University of California Riverside and Berkeley, was curious what was happening at the boundary between metal and insulator. There was a lot of data processing involved to check that, but during the pandemic, when everything was closed, I finally dug back into the data and started to see the signature of quantum criticality.
That’s what’s fun about this physics subfield of condensed matter: you never know what you will get, and you just have to measure things.
What do the results mean?
They are about building knowledge. Physicists are still trying to determine the theory that explains quantum criticality, which might contribute to superconductivity and can be found in other systems as well, like heavy fermions and cuprates.
Here, we have a highly tunable system with a very clean-cut metal-insulator transition. You can explore everything in the same sample: just by applying an electric field, you can control particle interactions very precisely and make the material more or less metallic. That should help us advance a deeper understanding of quantum phase transitions.
How did you become interested in condensed matter?
Growing up, my dad was a physicist dedicated to education. He never pushed me toward it, but I decided I wanted to study physics and earned a scholarship to come to Columbia.
Originally I was more interested in subfields like astrophysics and particle physics. As an undergraduate, I got to do some particle theory work at Fermilab outside of Chicago, and I enjoyed long chats with theoretical physicist Bill Bardeen. He described how every crystal is like its own universe, which was an inspiring idea. Later I attended a lecture by Liang Fu from MIT, who talked about how these complicated but beautiful theoretical particles called majorana fermions can be realized in a material.
"That’s what’s fun about this physics subfield of condensed matter: you never know what you will get, and you just have to measure things."
As an undergraduate, I approached Columbia Professor Abhay Pasupathy about a PhD position and even though I didn’t have any condensed matter experience, he took me on. The first time I looked at WSe2 under a microscope, it blew my mind. I’ve been looking at materials ever since.
What are you up to now?
As of this fall, I’m a postdoc at the Miller Institute at Berkeley, hosted by James Analytis. At Columbia, I was working with very well-refined materials, but here I’ll be doing more exploratory work growing and characterizing new crystals. It takes a different intuition for chemistry and what different elements can combine into.
In the future, I hope to continue in academia exploring new physics.
What did you take away from your time at Columbia?
Open-mindedness is key. In physics, it’s easy to get carried away, and you can waste a lot of time if you go in with an agenda about what you are looking for.
My thesis work took years of thought and debate. You need a community of peers and collaborators to really scrutinize your data. That creates a compass to help you tell if a result is potentially interesting or just something trivial.
Any notable hobbies outside of the lab?
For ten years, I was part of student radio at Columbia. I co-hosted a Brazilian radio show every Wednesday night for a couple of hours, and I got to interview many Brazilian artists when they came to New York.
For a second, I even considered going into music production. I helped with one concert, at Carnegie Hall in 2015, but that was enough and I decided to stick with physics. Though I will look into the music scene in the Bay Area now that I am here.