Time-Resolved Microscopy Reveals Charge-Density-Wave Fluctuations at 295 K in 1T-TiSe₂

The behaviour of charge-density waves (CDWs) in materials holds the key to understanding the interplay between electrons and atomic structure, and titanium diselenide (TiSe₂) presents a particularly promising system for investigation. Sotirios Fragkos from Université de Bordeaux, Hibiki Orio from Universität Würzburg, and Nina Girotto Erhardt from the Centre for Advanced Laser Techniques, alongside their colleagues, have now directly observed spectral features indicative of these CDW fluctuations occurring at surprisingly high temperatures. Their research demonstrates that these fluctuations, even well above the temperature at which the CDW normally forms, are primarily driven by the interaction between electrons and the material’s atomic vibrations, or phonons. This discovery provides new insights into the fundamental mechanisms governing the behaviour of CDWs and offers a deeper understanding of the complex relationship between electronic and lattice properties in materials.

Excitonic Insulator Formation in 1T-TiSe2

Understanding the interplay between electronic and lattice properties is crucial for deciphering complex quantum phases and phase transitions in materials. The charge density wave (CDW) phase in 1T-TiSe2 provides an excellent platform for investigating this relationship, potentially unlocking unconventional properties like tunable optical responses and even superconductivity. Below 200 K, 1T-TiSe2 undergoes a transformation into a CDW state, characterized by a reconstruction of its atomic lattice. Recent studies employing ultrafast laser techniques suggest that both electron-hole and electron-lattice interactions are intertwined in driving the CDW transition.

Researchers have now directly observed spectral features associated with these CDW fluctuations at temperatures as high as room temperature (295 K) using time-resolved extreme ultraviolet momentum microscopy. This method allows scientists to observe the transient melting and recovery of CDW fluctuations upon excitation with light. Surprisingly, the results reveal that a coherent vibrational mode, modulating the recovery of the CDW, persists even at these elevated temperatures. This observation, supported by theoretical calculations, indicates that electron-phonon interactions continue to play a significant role in the behavior of the material well above the temperature at which the CDW phase typically appears. These findings offer a deeper understanding of the nature of this quantum phase transition and potentially pave the way for harnessing the unique properties of fluctuating CDW states.

High-Temperature Charge Density Wave Fluctuations Observed

Researchers have directly observed charge density wave (CDW) fluctuations in the material 1T-TiSe₂ at temperatures significantly above those previously thought possible, reaching 295 Kelvin. This discovery challenges existing understanding of how these electronic and atomic orderings behave. The team employed time-resolved extreme ultraviolet momentum microscopy to monitor these fluctuations, revealing their dynamic behavior in both energy and momentum space. The research demonstrates that these CDW fluctuations are not simply a weakening of the ordered phase, but a distinct phenomenon driven primarily by interactions between electrons and the material’s lattice structure.

Surprisingly, the team found evidence of a coherent amplitude mode persisting at these elevated temperatures, indicating a strong coupling between the electronic order and lattice vibrations. This supports the idea that electron-phonon interactions play a dominant role in maintaining these fluctuations, even when the material is well above the temperature where a traditional CDW phase would form. Previous studies hinted at the existence of CDW fluctuations at higher temperatures, but this work provides the first direct observation of their dynamic behavior. The results show a rapid melting and recovery of these fluctuations upon excitation with a laser pulse, and the team’s analysis confirms that this process is governed by electron-phonon interactions. This provides new insights into the complex interplay between electronic and atomic degrees of freedom, potentially unlocking the material’s full potential for applications such as novel electronic devices and unconventional superconductivity.

Room Temperature Charge Density Wave Fluctuations Observed

This study provides direct evidence of charge density wave (CDW) fluctuations in 1T-TiSe2 at room temperature, using time-resolved extreme ultraviolet momentum microscopy. Researchers observed the persistence of spectral features related to these fluctuations well above the temperature at which the CDW typically forms, demonstrating their detectability even at elevated temperatures. The rapid melting and recovery of these fluctuations reveal the importance of interactions between electrons and lattice vibrations, or phonons, in maintaining the CDW phase. Analysis combining experimental results with theoretical calculations indicates that electron-phonon interactions are the dominant factor driving the CDW mechanism at room temperature.

This finding refines the understanding of CDW physics in 1T-TiSe2 and has broader implications for studying similar fluctuating phases in other materials. The authors acknowledge that while electron-hole coupling may contribute, it appears to be a secondary effect. Future research employing advanced ultrafast techniques could further investigate the microscopic mechanisms governing these fluctuations and potentially reveal new ways to control correlated electron systems in quantum materials, particularly given the connection between CDW fluctuations and unconventional superconductivity.

Ultrafast Momentum Mapping of CDW Fluctuations

Researchers employed a sophisticated time-resolved extreme ultraviolet (XUV) momentum microscopy technique to investigate charge-density-wave (CDW) fluctuations in the material 1T-TiSe2, even at room temperature. This method allowed them to directly observe the behavior of these fluctuations in both energy and momentum space, providing a detailed picture of their dynamics. The technique combines a precisely timed infrared laser pulse to initiate changes within the material with an XUV probe to capture snapshots of the electronic structure as it evolves, enabling observation of ultrafast processes. A key innovation was the use of momentum microscopy, which maps the distribution of electrons within the material, revealing how the CDW fluctuations affect the electronic band structure.

By analyzing changes in a specific electronic band, researchers could track the melting and recovery of these fluctuations after excitation with the laser. This approach allowed them to distinguish between different mechanisms driving the fluctuations, specifically disentangling the roles of electron-hole and electron-phonon interactions. To validate their experimental findings, the team performed theoretical calculations using density functional theory, incorporating the dynamic interaction between electrons and phonons. These calculations confirmed that the observed fluctuations are strongly linked to the material’s lattice vibrations, specifically a soft phonon mode, even at elevated temperatures. This combined experimental and theoretical approach provides new insights into the complex interplay between electronic and lattice degrees of freedom, and helps to resolve the ongoing debate about the origin of these fluctuations.