Researchers uncover a topological excitonic insulator with a tunable momentum order

Topological materials are a class of materials that exhibit unique electronic properties at their boundary (surface in 3D materials; edge in 2D materials) that are robust against imperfections or disturbances and are markedly different from their bulk properties. In other words, these materials could be insulators (i.e., resisting the flow of electrons or heat), and yet be conducting at their boundary (i.e., allowing electrons or heat to easily flow through them).

Topological phases in materials arise from their overall quantum properties, which are in turn influenced by symmetries, the structure of electronic energy bands, and interactions. So far, very few topological phases have been found to emerge from spontaneous symmetry breaking, a process that causes a material’s ground state (i.e., lowest energy state) to exhibit a lower symmetry compared to that at high temperatures.

Researchers at Princeton University, Beijing Institute of Technology, University of Zurich, National Magnet lab, and other institutes recently uncovered one of these symmetry breaking–induced phases, namely a topological excitonic insulator phase, in the compound Ta₂Pd₃Te₅.

Their observations, outlined in a paper published in Nature Physics, could open new possibilities for the study and engineering of quantum phases in solid-state systems, which could in turn also inform the development of quantum technologies and spintronic and excitonic devices.

“The competition or cooperation between different orders often gives rise to novel quantum phases,” Md Shafayat Hossain, first author of the paper, told Phys.org. “For example, in high-T_csuperconductors and twisted bilayer graphene, a rich landscape of charge and spin orders—arising from spontaneously broken symmetries—has provided new insights into correlated electron behavior. We sought platforms where electronic topology could interplay with such symmetry-breaking orders.”

The main goal of the recent study by Hossain and his colleagues was to better understand how the topological properties of materials respond to the emergence of additional broken symmetries. To do this, they used a technique called scanning tunneling microscopy (STM) to explore the symmetry breaking-induced topological phases in the compound Ta₂Pd₃Te₅.

“STM measurements revealed the development of an insulating energy gap as the temperature dropped below 100 K,” explained Hossain. “Complementary angle-resolved photoemission spectroscopy (ARPES) identified this gap as resulting from a zero momentum excitonic condensation, which breaks the material’s mirror symmetries.

“STM further enabled us to detect topological edge states, and below 5 K, we observed the development of an additional finite-momentum excitonic condensate. These findings were corroborated by thermodynamic signatures in heat capacity measurements, confirming the phase transitions.”

The excitonic insulator phase is a quantum phase of matter characterized by a collective insulating state prompted by the spontaneous formation of excitons (i.e., hole-electron pairs). This phase has been widely theorized about, yet it has so far proved to be highly elusive and difficult to observe experimentally.

The recent work by Hossain and his colleagues gathered evidence of an excitonic insulator phase in Ta₂Pd₃Te₅, with an added twist. Specifically, this phase was found to coexist with the material’s nontrivial electronic topology (i.e., its already observed topological phase).

“Until now, no material has been shown to naturally host both strong excitonic correlations and topological band structure in a single quantum phase,” said Yuxiao Jiang, co-first author of the study. “Most proposed candidates suffer from complications such as structural distortions that mask the signature of an excitonic insulator. This is the first time we’re seeing both topology and excitonic correlation dance together in a bulk 3D material.”

Earlier studies had successfully observed excitonic behavior in carefully engineered 2D heterostructures, such as monolayer WTe₂. However, the behaviors they observed relied on the artificial confinement of electrons and holes in a very thin layer, typically a few atoms thick.

In contrast, Hossain and his colleagues found that Ta₂Pd₃Te₅ spontaneously undergoes excitonic condensation in its bulk form. Notably, this condensation is entirely driven by the material’s internal electronic interactions, without the need for any intervention and engineering on the part of the researchers.

“Remarkably, we also discovered a second excitonic instability—one that breaks the underlying crystal’s translation symmetry and produces a superlattice modulation in real space,” added Zijia Cheng, co-first author of the study. “This finite-momentum condensate emerges alongside the zero-momentum excitonic phase. Having two excitonic condensates- one with zero momentum-and the other with a finite momentum- has not been observed in any other known system.

“Moreover, we demonstrate that the wavevector of the finite-momentum condensate can be continuously tuned with an applied magnetic field. This tunability is a smoking gun for a finite-momentum exciton condensate, referring to a quantum fluid of bound electron-hole pairs that carry momentum—akin to a crystal forming not at rest, but while moving.”

This recent study uncovered a new class of quantum materials in which the spontaneous condensation of excitons coexists with a nontrivial topological phase. In the future, the compound studied by the researchers and other materials found to exhibit similar characteristics could prove valuable for the development of various advanced technologies, including dissipation-less electronics, quantum computing components and electrically tunable optical devices.

“Our discovery may open a promising new direction in quantum materials research,” added Hossain. “It’s akin to finding an exoplanet with water—if one exists, there are likely others waiting to be discovered.

“We are now exploring related compounds to identify new platforms where electronic correlations, symmetry breaking, and topology coexist or compete, potentially giving rise to even more exotic quantum phases. In parallel, we are fabricating devices based on Ta₂Pd₃Te₅ to investigate the transport properties of its quantum states.”

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