Unraveling the ultrafast phase transition of vanadium dioxide

Understanding which interactions at the atomic and subatomic level underlie the features of quantum materials can be incredibly challenging. Vanadium dioxide (VO₂) and its ultrafast phase transition from insulator to metal exemplifies this complexity. For over 50 years it has been debated whether this transition is driven by electronic changes (that is, modifications to the material’s energy ‘landscape’, or band structure) or structural changes (alterations in how the atomic lattice is arranged).

Now, for the first time, a team of researchers led by Heriot-Watt University and IMDEA Nanoscience, with the collaboration of ICFO researchers Dr. Lin Zhang, Dr. Utso Bhattacharya, Maria Recasens, Dr. Johann Osmond, and ICREA Prof. Maciej Lewenstein, has directly observed both structural and electronic transitions in VO₂. They have discovered that the electronic transformation triggers the structural one, in an intricate process that unfolds in under 100 femtoseconds. In the study, which has been published in Nature Communications, several other institutions have participated, including the University of Memphis, ETH Zurich, Donostia International Physics Center, Adam Mickiewicz University, and Vanderbilt University.

Until now, previous experiments had only been able to capture either the electronic or the structural transition, but not both. As a result, researchers were forced to infer what the unseen component was doing, leading to the apparent conclusion that VO₂ experiences a step-like switch from insulator to metal. “We can now see both electronic and structural changes directly, which occur faster than previously thought. Moreover, our new approach has revealed an entire transition pathway. This means that attempts to control phase transitions might be more complicated to implement, but also have many more potential outcomes. Instead of A or B, maybe we can drive materials into a whole alphabet of states”, shares Dr. Allan Johnson, IMDEA Nanoscience researcher and corresponding author of the study.

The complete picture of VO₂ phase transition

Using their novel method, the team captured the VO₂ phase transition on its natural timescale. According to their observations, the material begins as an insulator, then passes through a ‘bad metal’ phase (just 10 femtoseconds after being excited with light), oscillates between insulating and semi-metallic states, and finally settles into a conventional metallic phase around 100 femtoseconds later. The study not only mapped these rapid electronic changes, but also demonstrated that they are closely connected to shifts in the material’s atomic structure, where vanadium ions go from a twisted to a neutral position in an intricate process.

These insights were made possible by the use of ultrashort laser pulses—just 1 to 5 femtoseconds long—with an exceptionally broad spectrum. The broad spectral range was crucial to resolve all energy bands simultaneously, allowing researchers to build a complete picture of the transition.

One of the biggest challenges, the team noted, was interpreting the novel data. “There was such a dramatic break from previous experiments that we had to develop entirely new theoretical models to describe the transition at such short timescales”, explains Dr. Lin Zhang, ICFO researcher and co-author of the study. For this, the researchers from ICFO Quantum Optics Theory group developed an efficient theoretical approach to the light-induced phase transitions of VO₂, which they had published a few months earlier in npj quantum materials. The theoretical method, which incorporates all the essential physical ingredients of VO₂, proved to be a powerful tool for explaining the experiment’s complex dynamics.

Importantly, the fact that the proposed technique revealed such a rich transition pathway in a well-studied material like VO₂ suggests that something similar could occur in other quantum materials too, and that more hidden complexities might be uncovered in the near future.

Reference:

Brahms, C., Zhang, L., Shen, X. et al. Decoupled few-femtosecond phase transitions in vanadium dioxide. Nat Commun 16, 3714 (2025).

DOI: https://doi.org/10.1038/s41467-025-58895-z

Acknowledgements:

This work was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme: Starting Grant agreement HISOL no. 679649 and ERC Consolidator Grant XSOL no. 101001534. C.B. and J.C.T. acknowledge support from the United Kingdom’s Engineering and Physical Sciences Research Council: Grant agreement EP/T020903/1. C.B. acknowledges support from the Royal Academy of Engineering through Research Fellowship No. RF/202122/21/133. This work was funded by the Spanish AIE (projects PID2022−137817NA-I00 and EUR2022−134052). A.S.J. acknowledges the support of the Ramón y Cajal Program (Grant RYC2021-032392-I). IMDEA Nanociencia acknowledges support from the “Severo Ochoa” Programme for Centers of Excellence in R&D (MICIN, CEX2020-001039-S). Computational resources were provided by the High-Performance Computing Center at the University of Memphis (X.S.). U.B. is also grateful for the financial support of the IBM Quantum Researcher Program. R.W.C. acknowledges support from the Polish National Science Centre (NCN) under the Maestro Grant No. DEC −2019/34/A/ST2/00081. T.G. acknowledges funding by Gipuzkoa Provincial Council (QUAN-000021-01), by the Department of Education of the Basque Government through the IKUR strategy and through the project PIBA_2023_1_0021 (TENINT), by the Agencia Estatal de Investigación (AEI) through Proyectos de Generación de Conocimiento PID2022-142308NA-I00 (EXQUSMI), by the BBVA Foundation (Beca Leonardo a Investigadores en Física 2023). S.T.P. acknowledges funding from the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Directorate grant No. DEFG02- 09ER46554 and by theMcMinn Endowment at Vanderbilt University. K.A.H. and R.F.H. acknowledge support from the U. S. National Science Foundation (EECS-1509740) and the Stevenson Endowment at Vanderbilt University. The ICFO group acknowledges support from: ERC AdG NOQIA; MCIN/AEI (PGC2018-0910.13039/501100011033, CEX2019-000910-S/10.13039/501100011033, Plan National FIDEUA PID2019-106901GB-I00, Plan National STAMEENA PID2022-139099NBI00 project funded by MCIN/AEI/10.13039/501100011033 and by the “European Union NextGenerationEU/PRTR” (PRTR-C17.I1), FPI); QUANTERA MAQS PCI2019-111828−2); QUANTERA DYNAMITE PCI2022-132919 (QuantERA II Programme co-funded by European Union’s Horizon 2020 program under Grant Agreement No 101017733), Ministry of Economic Affairs and Digital Transformation of the Spanish Government through the QUANTUM ENIA project call – Quantum Spain project, and by the European Union through the Recovery, Transformation, and Resilience Plan – NextGenerationEU within the framework of the Digital Spain 2026 Agenda; Fundació Cellex; Fundació Mir-Puig; Generalitat de Catalunya (European Social Fund FEDER and CERCA program, AGAUR Grant No. 2021 SGR 01452, QuantumCAT \ U16-011424, co-funded by ERDF Operational Program of Catalonia 2014-2020); Barcelona Supercomputing Center MareNostrum (FI-2023-1-0013); EU Quantum Flagship (PASQuanS2.1, 101113690); EU Horizon 2020 FET-OPEN OPTOlogic (Grant No 899794); EU Horizon Europe Program (Grant Agreement 101080086 — NeQST), ICFO Internal “QuantumGaudi” project; European Union’s Horizon 2020 program under the Marie Sklodowska-Curie grant agreement No 847648; “La Caixa” Junior Leaders fellowships, “La Caixa” Foundation (ID 100010434): CF/BQ/PR23/11980043.