Topological Excitonic Insulator with Tunable Momentum Order

 

Topological materials are a unique class of materials that display extraordinary electronic properties at their boundaries. These edge or surface states remain robust against disturbances or imperfections and are often in sharp contrast with the insulating behavior in the bulk of the material. In essence, such materials can resist the flow of electrons within but allow smooth conduction along their edges.

The topological phases observed in such materials emerge due to quantum mechanical effects governed by the structure of electronic bands, symmetries, and inter-particle interactions. Spontaneous symmetry breaking The known topological phases that arise due to spontaneous breaking of symmetry are few.

Recently a new quantum phase was discovered, a topological excitonic insulator phase, in a material called Ta 2 Pd 3 Te 5. The finding may open possibilities on exploring quantum materials further as well as being a breakthrough in future discoveries of quantum technologies, spintronics, and excitonic devices.
This investigation aimed to explore how topological characteristics of a material evolve when spontaneous symmetry-breaking orders are introduced. Using a powerful imaging technique called scanning tunneling microscopy (STM), researchers observed the emergence of an insulating energy gap below 100 K in the material.

Complementary measurements using angle-resolved photoemission spectroscopy (ARPES) linked this gap to a zero momentum excitonic condensation. This condensation breaks the mirror symmetry of the material. STM also detected topological edge states, and at temperatures below 5 K, a finite-momentum excitonic condensate emerged, further supported by heat capacity data confirming phase transitions.

An excitonic insulator phase forms when bound pairs of electrons and holes (excitons) spontaneously condense, creating a collective insulating state. While this phase has been theoretically predicted, its experimental verification has proven to be extremely challenging.

This particular compound not only demonstrated the existence of such an elusive excitonic insulator phase but also showed that it coexists with a nontrivial topological electronic structure. It is a pioneer in material science.

The excitonic behavior previously had been noted only in carefully designed 2D materials, with the electrons and holes confined in layers so thin, less than a nanometer, that it was comparable to 2D behavior. By contrast, the present work demonstrated that Ta 2 Pd 3 Te 5 exhibits spontaneous excitonic condensation in the 3D bulk, under entirely internal interactions with no external engineering at all.