Study reveals Ta2NiSe5 is not an excitonic insulator due to dominant structural order parameter

Scientists are actively searching for ways to detect and stabilize the excitonic insulator, a unique phase of matter that could revolutionize energy transport. This phase has proven elusive, but Ta2NiSe5 has been proposed as a potential candidate for hosting it. However, there has been much debate as to whether the transition in this material is caused by electronic or structural instability.

To gain insight into this, researchers from the U.S., Germany, and Japan conducted a joint experimental-theoretical study using time- and angle-resolved photoemission spectroscopy. By using a tailored laser pulse and achieving ultrafast time resolution, they were able to track the dynamics of the excitons and structural degrees of freedom in Ta2NiSe5.

Their findings suggest that the transition in this material is caused by structural instability rather than electronic instability, which impedes the development of electronic superfluidity. This new understanding could help researchers find other materials that could potentially host the excitonic insulator phase and pave the way for new energy transport technologies.

According to Nuh Gedik, a Physics Professor at MIT who led the research, the study demonstrates that Ta2NiSe5 cannot be classified as an excitonic insulator due to the significant rearrangement of its crystal structure, which impedes dissipationless energy transport.

Lead author Edoardo Baldini, a former postdoctoral fellow at MIT who now works as an Assistant Professor of Physics at the University of Texas at Austin, added that the research provides a new method for identifying the underlying cause of spontaneous symmetry-breaking in candidate excitonic insulators.

The study’s findings were corroborated by state-of-the-art calculations carried out at multiple institutions, which utilized various theoretical techniques to gain unprecedented insight into the microscopic origin of the changes in Ta2NiSe5.

Angel Rubio, the Theory Director at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, noted that confirming the structural nature of the transition required intricate and demanding electronic and structural modeling that also provided crucial information on the potential excitonic contributions.

The research was a collaborative effort between several groups, including Eugene Demler at Harvard University, Andrew Millis at Columbia University, and Igor Mazin at George Mason University, who were involved in the theoretical calculations. The experimental investigations took place at MIT, and the Ta2NiSe5 crystals used in the study were synthesized at the Max Planck Institute for Solid State Physics in Stuttgart, Germany, and the University of Tokyo in Japan.

Source: Max Planck Institute for the Structure and Dynamics of Matter

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