Study demonstrates that Ta₂NiSe₅ is not an excitonic insulator
—Undefined Trio
Scientists have long been intrigued by the possibility of an excitonic insulator, a unique phase of matter that could enable superfluid energy transport without net charge. However, detecting and stabilizing this exotic order in real solids has proven challenging. In a recent study published in Proceedings of the National Academy of Sciences (PNAS), researchers from the United States, Germany, and Japan investigated the properties of Ta₂NiSe₅, a quasi-two-dimensional solid that was proposed as a candidate excitonic insulator.
Above a critical temperature of 328 K, Ta₂NiSe₅ undergoes a semimetal-to-semiconductor transition accompanied by a structural reorganization of its crystalline lattice. The scientific community has debated whether this phase transition is induced by an electronic or structural instability. To gain insights into the fundamental processes underlying this transition, the research team employed time- and angle-resolved photoemission spectroscopy, a powerful experimental technique that captures ultrafast dynamics.
By exposing Ta₂NiSe₅ to a precisely controlled laser pulse and analyzing the resulting spectroscopic fingerprints, the researchers discovered that the changes in the crystal structure hinder the development of electronic superfluidity in this material. Their findings demonstrate that Ta₂NiSe₅ is not an excitonic insulator, contrary to previous assumptions.
"This work demonstrates that Ta₂NiSe₅ is not an excitonic insulator and that dissipationless energy transport is hampered by the prominent rearrangement of the crystal structure," explains Professor Nuh Gedik from the Massachusetts Institute of Technology (MIT), who led the research.
The study also involved state-of-the-art calculations performed by multiple institutions, which provided a deeper understanding of the microscopic origin of the structural changes in Ta₂NiSe₅. The collaborative effort between experimental and theoretical researchers shed light on the mechanism driving the transition and ruled out excitonic contributions as the dominant order parameter.
While this study may have dispelled the excitonic insulator hypothesis for Ta₂NiSe₅, it offers valuable insights into the behavior of candidate excitonic insulators and provides a new approach to identifying the driving forces behind spontaneous symmetry-breaking. The research team's interdisciplinary efforts pave the way for further exploration of these unique phases of matter.
The research was carried out in collaboration between institutions such as MIT, Max Planck Institute for the Structure and Dynamics of Matter in Germany, Harvard University, Columbia University, and George Mason University. The Ta₂NiSe₅ crystals used in the study were synthesized at the Max Planck Institute for Solid State Physics in Germany and the University of Tokyo in Japan.
Reference: Edoardo Baldini et al, The spontaneous symmetry breaking in Ta₂NiSe₅ is structural in nature, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2221688120
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