Moore Foundation grant to enable direct experimental investigation of correlated pairs of electrons in quantum materials

“Strongly correlated materials” are a special kind of quantum matter—matter that can’t be described in terms of individual particles, only in terms of relationships among multiple particles—and they’ve been a tough nut for scientists to crack. Why has it been so hard to plumb their mysteries? A big reason is that instruments don’t exist that can perform the needed study of more than one electron at the same time.

Thanks to a new $1.6 million grant from the Gordon and Betty Moore Foundation, that’s about to change. Fahad Mahmood, an assistant professor of physics and researcher in the Illinois Quantum Information Science and Technology Center (IQUIST), will lead an effort to develop an instrument, called “Double-ARPES,” that can reveal the elusive workings of entangled pairs of electrons.

Why is that important? In strongly correlated materials, “electrons don’t really behave like electrons anymore,” says Mahmood. “There are very strong interactions with them. And then because of those strong interactions, we get emergent behavior, which can be described as ‘macroscopic quantum behavior,’ that’s happening on much larger scales.”

One example of such macroscopic behavior is superconductivity. Scientists have some understanding of how superconductivity works for metals that superconduct at very low temperatures, just a few degrees above absolute zero. However, for higher-temperature superconductors, it’s still a mystery why electrons bind together—and that has been a major impediment to the use of superconductors in locations where they can’t be kept extremely cold. Mahmood asks, “How do you make something that’s room temperature superconducting? Well, first, you want to understand what is causing electrons to bind together.”

“Strange metals” are another example of strongly correlated materials that the new instrument will be able to study. They behave very differently from normal metals, and no one knows why. “There’s some belief that... something very fundamental about quantum mechanics is causing strange metal behavior. But... we don’t know what that fundamental thing is, or how electrons are interacting to give us that behavior,” says Mahmood.

He explains that existing ARPES (Angle-Resolved Photoemission Spectroscopy) instruments can measure the spectral function of a single electron that’s been kicked out of a material by the impact of a photon. By looking at the electron’s energy and momentum, researchers can figure out what the electron was doing inside the material.

Under the new award, Mahmood will develop the “D-ARPES” (Double-ARPES) instrument, which will be able to detect two entangled electrons that are kicked out of a material as a pair, again by the impact of a single photon.

“Rather than getting one electron out, we’re going to get two electrons out,” he says. “And we’re going to measure both of them simultaneously.” By doing so, “we can directly tell what the electron pair was doing inside of the material.”

The Gordon and Betty Moore Foundation has a mission of fostering path-breaking scientific discovery, environmental conservation, patient care improvements, and preservation of the special character of the Bay Area. It awarded the grant through its Emergent Phenomena in Quantum Systems Initiative (EPiQS) Flexible Funding program. EPiQS prioritizes high-risk, high-reward fundamental research in quantum materials, and its Flexible Funding program aims to drive innovation by elevating experimental capabilities and supporting timely projects.

The new award is the latest in a series of generous awards to UIUC research from the Moore Foundation. Physics professors Peter Abbamonte and Vidya Madhavan, for example, recently received EPiQS Experimental Investigator awards. As early-career faculty, Mahmood is particularly grateful for the double-blind review process used by the EPiQS Flexible Funding program. “It gives assistant professors like me a chance to just put our ideas on fundamental science out there!”

He notes that the Moore Foundation is special because other funding agencies tend to be more reluctant to support fundamental science that doesn’t already have predictable direct applications.

“It’s hard to convince people that this is worthy,” he says. “The kind of science that the Moore Foundation promotes and supports is very important and can be difficult to get funded otherwise.”