Highlighting the hidden properties of quantum materials

Some materials have desirable properties that are hidden, and just as you would use a flashlight to see in the dark, scientists can use light to discover these properties.

Researchers at the University of California, San Diego, used an advanced optical technique to learn more about a quantum material called Ta.₂NiSe₅ (TNS). Their work appears in Natural materials.

Materials can be disturbed by different external stimuli, often with changes in temperature or pressure; However, because light is the fastest element in the universe, materials respond very quickly to optical stimuli, revealing properties that would otherwise remain hidden.

“Essentially, we shine a laser at a material and it’s like stop-action photography where we can gradually track a certain property of that material,” said physics professor Richard Averitt, who led the research and is the one of the authors of the article. “By looking at how the constituent particles move in this system, we can discover these properties that are really hard to find otherwise.”

The experiment was conducted by lead author Sheikh Rubaiat Ul Haque, a 2023 graduate of UC San Diego and now a postdoctoral researcher at Stanford University. He and Yuan Zhang, another graduate student in Averitt’s lab, improved a technique called terahertz time-domain spectroscopy. This technique allows scientists to measure the properties of a material over a range of frequencies, and Haque’s improvements have allowed them to access a wider range of frequencies.

The work was based on a theory created by another author of the paper, Eugene Demler, a professor at ETH Zürich. Demler and his graduate student Marios Michael developed the idea that when certain quantum materials are excited by light, they can transform into a medium that amplifies light at the terahertz frequency. This led Haque and his colleagues to take a close look at the optical properties of TNS.

When an electron is excited to a higher level by a photon, it leaves behind a hole. If the electron and hole are bonded, an exciton is created. Excitons can also form a condensate, a state that occurs when particles come together and behave as a single entity.

Using Haque’s technique, supported by Demler’s theory, and using density functional calculations carried out by Angel Rubio’s group at the Max Planck Institute for the Structure and Dynamics of Matter, the team was able to observed an anomalous amplification of terahertz light, which revealed some of the hidden properties of matter. Exciton condensate TNS.

Condensates are a well-defined quantum state and the use of this spectroscopic technique could make it possible to imprint some of their quantum properties on light. This could have implications in the emerging field of entangled light sources (where multiple light sources have interconnected properties) using quantum materials.

“I think it’s a very open area,” Haque said. “Demler’s theory can be applied to a suite of other materials with nonlinear optical properties. With this technique, we can discover new light-induced phenomena that have never been explored before.”