Mallika Randeria captures images of the quantum world

Focus on Graduate Research

By Tom Garlinghouse

Mallika Randeria first fell in love with quantum physics during her undergraduate years at MIT. When she took a course that required students to replicate some of the fundamental physics experiments of the past — including the 20th-century experiments that laid the groundwork for quantum mechanics — she was hooked.

“That really lit the spark,” said Randeria, who recently earned her Ph.D. in physics from Princeton. “That’s when I decided I definitely wanted to be an experimentalist, and not just do theory.”

This focus on experimentation has allowed Randeria to peer deep into the counterintuitive world of quantum physics.

Person standing by equipment
Mallika Randeria studies quantum materials in Princeton’s Department of Physics

Working under the direction of Ali Yazdani, the Class of 1909 Professor of Physics, Randeria and her colleagues have, for the first time, imaged the collective behavior of electrons in a high magnetic field. In this unique environment, all the electrons begin to swirl in a distinctive elliptical orbit, a behavior never previously observed. The research is detailed in an article in the Feb. 6, 2019, Nature.

To observe this behavior, the researchers used a device called a scanning tunneling microscope, which is capable of detecting objects at the atomic scale. The experiment consisted of placing a bismuth crystal in the microscope and subjecting it to an incredibly low temperature — a few shades above absolute zero (-459 degrees Fahrenheit) — and a high magnetic field, about a thousand times the strength of a common refrigerator magnet.

What Randeria and colleagues saw was astonishing. The electrons’ wave functions, which describe the probability of an electron being around an atom at any one point in time, all began to align themselves in the same direction. This is an example of collective behavior, Randeria said, because all the electrons are sensing what the other electrons are doing.

This behavior might be an example of the electrons attempting to minimize the energy costs of many electrons overlapping in different directional orbits, according to Randeria. “It becomes energetically favorable for all the electrons to point in the same direction,” she said. “This experiment opened up a completely different way of studying these systems.”

Randeria believes the images and behavior she and her colleagues observed could help researchers explore other behaviors in the quantum world. “The real purpose of these experiments is to push the frontiers of our understanding of physics,” she said. Support for the research was provided by the Gordon and Betty Moore Foundation and the U. S. Department of Energy.