Scientists reveal the role of structural symmetry breaking in a candidate correlated quantum material for dissipationless energy transport

June 01, 2023

An international research team has demonstrated unambiguously that the bulk crystal Ta₂NiSe₅ is not an excitonic insulator, settling the debate around the microscopic origin of symmetry breaking in the material. Their findings, published in PNAS, pose a cornerstone in the correct identification of the driving force behind spontaneous symmetry breaking in a wide class of quantum materials that are candidate platforms for dissipationless energy transport.

The excitonic insulator is an electronically driven phase of matter that can occur in solids. Scientists are searching for ways to detect and stabilize this exotic order in candidate quantum materials because it could pave the way towards superfluid energy transport with no net charge (which is distinct from superconductivity). If realized, this phenomenon could lead to a new generation of devices where energy is transported at the nanoscale with high coherence and minimal dissipation.

Characteristic electronic structure observed in candidate excitonic insulators. The shape of the electronic bands originates from the combined action of structural and electronic symmetry breaking. In Ta2NiSe5, the contribution of structural symmetry breaking is dominant and hinders any prospect of dissipationless energy transport. — Characteristic electronic structure observed in candidate excitonic insulators. The shape of the electronic bands originates from the combined action of structural and electronic symmetry breaking. In Ta₂NiSe₅, the contribution of structural symmetry breaking is dominant and hinders any prospect of dissipationless energy transport.

© Jörg Harms, MPSD

Characteristic electronic structure observed in candidate excitonic insulators. The shape of the electronic bands originates from the combined action of structural and electronic symmetry breaking. In Ta₂NiSe₅, the contribution of structural symmetry breaking is dominant and hinders any prospect of dissipationless energy transport.

© Jörg Harms, MPSD

However, spotting this phase in real solids has proven difficult so far. For the past two decades, it had been proposed that the quasi-two-dimensional solid Ta₂NiSe₅ may support an excitonic insulator phase above room temperature. Above a critical temperature T_C = 328 K, this material crystallizes in a layered structure that consists of parallel Ta and Ni chains. At T_C, the system undergoes a semimetal-to-semiconductor transition, accompanied by a structural reorganization of the crystalline lattice. The scientific community has been engaged in an intense debate regarding whether this phase transition was induced by a purely electronic or a structural instability.

In a recently published study on PNAS, researchers in the U.S., Germany, and Japan probed the fundamental processes underpinning that transition via a joint experimental-theoretical approach. Using an advanced experimental tool called time- and angle-resolved photoemission spectroscopy under highly controlled conditions, they exposed Ta₂NiSe₅to a tailored laser pulse and recorded a real-time movie of the fundamental components of the excitons (i.e., electrons and holes) as well as the structural degrees of freedom. To resolve these microscopic phenomena, the movie had to achieve an ultrafast time resolution of less than a millionth of a billionth of a second.

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