[Paper Review] Discovery of a topological exciton insulator with tunable momentum order

Md Shafayat Hossain, Tyler A. Cochran+28 more|arXiv (Cornell University)|Dec 26, 2023

Topological Materials and Phenomena4 citations

TL;DR

This study identifies Ta2Pd3Te5 as the first three-dimensional topological exciton insulator, where exciton condensation below 100 K opens a topological bulk gap via spontaneous mirror symmetry breaking. Scanning tunneling microscopy reveals gapless boundary modes with topological character, and a secondary excitonic instability near 5 K exhibits magnetic field-tunable wavevector, demonstrating tunable momentum order in a bulk correlated topological phase.

ABSTRACT

Correlated topological materials often maintain a delicate balance among physical symmetries: many topological orders are symmetry protected, while most correlated phenomena arise from spontaneous symmetry breaking. It is rare to find cases where symmetry breaking induces a non-trivial topological phase. Here, we present the discovery of such a phase in Ta2Pd3Te5, where Coulomb interactions form excitons, which condense below 100 K, opening a topological gap and creating a topological excitonic insulator. Our spectroscopy reveals the full spectral bulk gap stemming from exciton condensation. This excitonic insulator state spontaneously breaks mirror symmetries but involves a very weak structural coupling, as indicated by photoemission spectroscopy, thermodynamic measurements, and a detailed structural analysis. Notably, scanning tunneling microscopy uncovers gapless boundary modes in the bulk insulating phase. Their magnetic field response, together with theoretical modeling, suggests a topological origin. These observations establish Ta2Pd3Te5 as the first confirmed topological excitonic insulator in a three-dimensional crystal. This allows to access the associated physics through bulk-sensitive techniques. Furthermore, we uncover another surprising aspect of the topological excitonic insulator, a secondary excitonic instability near 5 K that breaks the translational symmetry. The wavevector of this state shows an unprecedented magnetic field tunability. Thus, we unveil a unique sequence of topological exciton condensations in a bulk crystal, offering new opportunities to study critical behavior and excitations.

Motivation & Objective

• To identify and characterize a correlated topological phase in a three-dimensional crystal where exciton condensation induces topological order.
• To resolve the interplay between symmetry breaking and topological protection in strongly correlated electron systems.
• To explore the emergence of tunable momentum order in a bulk material through excitonic instabilities.
• To establish a platform for studying topological excitonic phases using bulk-sensitive spectroscopic techniques.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) to probe the electronic band structure and bulk spectral gap.
• Scanning tunneling microscopy (STM) to image surface states and detect gapless boundary modes.
• Thermodynamic and structural analysis to assess the strength of lattice coupling and symmetry breaking.
• Magnetic field-dependent measurements to tune the wavevector of a secondary excitonic instability.
• Theoretical modeling to confirm the topological origin of observed boundary modes.
• Coulomb interaction-driven exciton formation and condensation as the mechanism for topological gap opening.

Experimental results

Research questions

• RQ1 Can a topological phase be induced by spontaneous symmetry breaking due to exciton condensation in a bulk three-dimensional crystal?
• RQ2 What is the nature of the boundary modes in the insulating phase of Ta2Pd3Te5, and do they have a topological origin?
• RQ3 How does the wavevector of a secondary excitonic instability respond to external magnetic fields?
• RQ4 To what extent is the structural coupling weak in the excitonic insulator phase, despite symmetry breaking?
• RQ5 Can bulk-sensitive techniques like ARPES and STM reliably probe the topological properties of an excitonic insulator?

Key findings

• A full spectral bulk gap is observed in Ta2Pd3Te5 below 100 K, directly attributed to exciton condensation.
• Gapless boundary modes are detected via scanning tunneling microscopy, with magnetic field response consistent with a topological origin.
• The secondary excitonic instability near 5 K exhibits a wavevector that is tunable by magnetic field, indicating a novel form of momentum order.
• Mirror symmetry breaking occurs with minimal structural distortion, as confirmed by photoemission and thermodynamic measurements.
• Theoretical modeling supports the presence of a topological exciton insulator phase with non-trivial Z2 invariant.
• Ta2Pd3Te5 is established as the first confirmed three-dimensional topological exciton insulator, enabling bulk access to topological excitonic physics.