Scientists used few-femtosecond XUV spectroscopy to track a quantum material’s atoms and electrons in the femtoseconds (10-15 seconds) just before it transforms. The research addresses a 50-year-old debate surrounding the material 1T-TiSe2 and its unusual behavior at approximately 200 Kelvin. Sheng-Chih Lin, a co-lead author of the study, said the question surrounding 1T-TiSe2 has persisted for over five decades.
Few-Femtosecond XUV Spectroscopy Tracks Real-Time Quantum Material Shifts
Few-femtosecond XUV spectroscopy enabled direct observation of electron-hole pair disruption in 1T-TiSe2, revealing their persistence even at temperatures above the material’s 200 Kelvin phase transition. This capability to monitor changes on timescales of seconds, essentially capturing the initial moments of a material’s transformation, overcomes the limitations of previous methods that only registered transitions after they occurred. The technique gently perturbs the material with XUV light, allowing scientists to track the behavior of electron-hole pairs, or excitons, as the material shifts between states.
Michael Zuerch, an associate professor of Chemistry and the study’s senior author, described the methodological significance of this approach. He stated that this work demonstrates a broader strategy for investigating complex quantum materials, where electronic, structural, magnetic and other degrees of freedom often evolve simultaneously. This strategy is particularly valuable given the simultaneous electronic and structural changes that complicate the study of materials like 1T-TiSe2, where isolating the role of excitons has been a long-standing challenge. The team’s ability to measure excitonic disruption directly addresses this difficulty, providing new insight into the material’s behavior.
Alfred Zong, co-lead author and now an assistant professor of Physics and Applied Physics, explained that the researchers were initially concerned when they first saw the high-temperature results because they showed the same trend observed at low temperature. However, observing enhanced susceptibility of these pairs near 200 K clarified the results, suggesting the material was approaching an instability that drives the phase transition. This characteristic also suggests the potential for applying the few-femtosecond XUV spectroscopy method to other low-dimensional excitonic insulator candidates.
Zong said the same approach could be used to probe other transitions in the future. The researchers plan to continue investigating 1T-TiSe2, focusing on the interplay between excitonic correlations and the unique orders that emerge at lower temperatures, further solidifying the material’s position as a subject of ongoing scientific inquiry. Despite all that work, there is still no consensus on the role excitonic interactions play in its phase transition.
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.
Michael Zuerch, an associate professor of Chemistry and the new study’s senior author
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