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Nanotechnology Now – Press Release: Study shows that Ta2NiSe5 is not a excitatory insulator international research team settles a decade-long debate around the microscopic origin of symmetry breaking in bulk crystals

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Home > press > Study shows that Ta2NiSe5 is not a excitatory insulator international research team settles a decade-long debate around the microscopic origin of symmetry breaking in bulk crystals

The characteristic electronic structure observed in excitonic insulator candidates.  The electronic band form comes from the combined action of structural and electronic symmetry breaking.  In Ta2NiSe5, the contribution of structural symmetry breaking is dominant and precludes any prospect of energy transport without dissipation.  CREDITS Jörg Harms, MPSD
The characteristic electronic structure observed in excitonic insulator candidates. The electronic band form comes from the combined action of structural and electronic symmetry breaking. In Ta2NiSe5, the contribution of structural symmetry breaking is dominant and precludes any prospect of energy transport without dissipation. CREDITS Jörg Harms, MPSD

Abstract:
Exitonic insulators are electronically actuated phases of matter that can occur in solids. Scientists are looking for ways to detect and stabilize this exotic order in potential quantum materials as it could pave the way to superfluid energy transport without a net charge (which is different from superconductivity). If realized, this phenomenon could lead to a new generation of devices in which energy is transported on the nanoscale with high coherence and minimal dissipation.

Study showing that Ta2NiSe5 is not a excitatory insulator international research team resolves a decade-long debate around the microscopic origin of broken symmetry in bulk crystals

Hamburg, Germany | Posted on May 12, 2023

However, finding this phase in real solids has so far proved difficult. Over the past two decades, it has been proposed that a quasi-two-dimensional dense Ta2NiSe5 can support excitatory phase insulators above room temperature. Above the critical temperature TC = 328 K, this material crystallizes in a layered structure consisting of parallel chains of Ta and Ni. In TC, the system undergoes a semimetal-to-semiconductor transition, accompanied by a structural reorganization of the crystal lattice. The scientific community has been engaged in a heated debate as to whether these phase transitions are caused by purely electronic or structural instability.

In the recently published study on PNAS, researchers in the US, Germany, and Japan investigate the fundamental processes underlying that transition through a shared theory-experimental approach. Using a state-of-the-art experimental tool called time- and angle-resolved photoemission spectroscopy under highly controlled conditions, they exposed Ta2NiSe5 to customized laser pulses and recorded real-time movies of the elementary components of the excitons (i.e., electrons and holes) as well. as structural degrees of freedom. To overcome this microscopic phenomenon, the film must achieve an extremely fast time resolution of less than a millionth of a second.

Tracking of the dynamics of the electronic and crystalline structures of materials after light excitation reveals spectroscopic fingerprints compatible only with the dominant order parameters of the structural properties. This implies that changes in crystal structure actually hinder the development of electronic superfluidity in these quantum materials.

“This work demonstrates that Ta2NiSe5 is not an excitonic insulator and energy transport without dissipation is impeded by rearrangements of prominent crystal structures,” said Nuh Gedik, Professor of Physics at the Massachusetts Institute of Technology (MIT), who coordinated the research. “Our experiments provide a novel approach to identifying the driving forces behind spontaneous symmetry breaking in a variety of excitatory insulator candidates,” added lead author Edoardo Baldini, a former postdoctoral fellow at MIT and now an Assistant Professor of Physics at the University of Texas at Austin.

This finding is supported by state-of-the-art calculations at several institutions that combine different theoretical techniques to understand the microscopic origin of these Ta2NiSe5 changes with unprecedented accuracy. “Confirming the microscopic mechanisms that drive this transition to be structural requires highly demanding and interwoven electronic and structural modeling that also provides relevant information on the impact of possible excitatory contributions,” said Director of Theory Angel Rubio of the Max Planck Institute for the Structure and Dynamics of Matter. (MPSD) in Hamburg, Germany.

The groups of Eugene Demler at Harvard University, Andrew Millis at Columbia University, and Igor Mazin at George Mason University were partners in the theoretical collaboration. Experimental investigations were carried out at MIT, and the Ta2NiSe5 crystals used for this study were synthesized at the Max Planck Institute for Solid State Physics in Stuttgart, Germany, and at the University of Tokyo in Japan.

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Jenny Witt
Max Planck Institute for the Structure and Dynamics of Matter

Office: 0049-408-998-88044
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Edward Baldini
University of Texas at Austin

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