October 8, 2026 -- Phase transitions, in which the electrons and atoms that make up a material collectively reorganize into a new state, give rise to many of the interesting properties in quantum materials. But they can be incredibly challenging to study. Most of the time, physicists and chemists only know a phase transition happened after it's over—the way you'd notice a firework after it explodes.

Now, Berkeley scientists have spearheaded a new way to catch a phase transition starting to happen, using ultrafast pulses of extreme-ultraviolet (XUV) light to watch a quantum material's atoms and electrons in the femtoseconds (10^-15 seconds) just before it transforms.

The team used the method, called few-femtosecond XUV spectroscopy, to gain new evidence in a 50-year-old mystery about how the material 1T-TiSe₂ forms an ordered, exotic state below about 200 Kelvin. The team also said the same approach could be used to probe other transitions in the future.

"From a methodological perspective, this work demonstrates a broader strategy for investigating complex quantum materials, where electronic, structural, magnetic, and other degrees of freedom often evolve simultaneously," said Michael Zuerch, an associate professor of Chemistry and the new study's senior author.

1T-TiSe₂ has been a target of debate since the 1970s, when researchers first proposed that its transition at 200 K might be driven by "excitons" — bound pairs consisting of an excited electron and the positively charged "hole" it leaves behind. Under the right conditions, a material such as 1T-TiSe₂ can become an "excitonic insulator," in which excitons spontaneously form and condense, revealing unique quantum properties. But in 1T-TiSe₂ , this electronic shift happens at the same time as the atoms rearrange into a repeating pattern, forming what is known as a charge-density wave. Electronic and structural changes therefore emerge together, making it difficult to isolate the contribution of excitonic interactions.

To get around this, the team used pulses of XUV light lasting just a few femtoseconds to gently perturb 1T-TiSe₂ and track, in real time, how its electron-hole pairs were broken up at a wide range of temperatures above and below the known transition.

"It was exciting to hunt for this exotic state because researchers have been asking this question in 1T-TiSe₂ for over five decades," said Sheng-Chih Lin, a recent PhD graduate from the Zuerch group and co-lead author of the study. "Despite all that work, there is still no consensus on the role excitonic interactions play in its phase transition."

By directly measuring how excitonic pairs were disrupted by the laser pulses, this work revealed that excitonic pairs exist even above the transition temperature, in the material's ordinary, disordered phase. That suggested that the pairing isn't just a byproduct of the ordered state forming. And near the 200 K transition temperature, the excitonic correlations became highly susceptible to even small perturbations, showing that the material was approaching an instability.

"To be honest, we were quite concerned when we first saw the high-temperature results because they showed the same trend we had observed at low temperature," said Alfred Zong(link is external), co-lead author of the study, who conducted the research as a Miller Research Fellow at UC Berkeley and is now an assistant professor of Physics and Applied Physics at Stanford University. "But when we measured the material near the transition temperature and observed the enhanced susceptibility, things started to make sense."

The findings strengthen the case that electron-hole interactions play an active part of the mechanism that drives the transition in 1T-TiSe₂ .

While the work is fundamental, the authors said that understanding the excitonic mechanism in 1T-TiSe₂ may also shed light on related materials.

"The reason for the existence of excitonic fluctuations above the transition temperature in 1T- TiSe₂ is that the material is quasi-two-dimensional instead of three dimensional," said Prof. Zuerch. "Many excitonic insulator candidate materials are quasi-low-dimensional, so the same XUV approach could also help identify similar excitonic behavior in other candidates before they fully order."

The material of 1T-TiSe₂ itself also remains a highly interesting subject, the researchers add. Future work could probe the interplay between excitonic correlations and nontrivial orders that develop in the material's low-temperature state.