Discovering the Quantum States for Sensing
Topology + Excitons = Topological Excitonic Insulator
In most quantum materials, topological phases rely on preserved symmetries, while correlated electron behaviors often stem from spontaneous symmetry breaking. Rarely do these two opposing tendencies converge to create a new state of matter. Our recent discovery in Ta₂Pd₃Te₅ reveals precisely such a phenomenon: a topological excitonic insulator—a quantum phase born from interactions, not topology alone.
In this material, Coulomb interactions drive the formation and condensation of excitons (bound electron-hole pairs) below 100 K, opening a bulk energy gap and inducing a topologically nontrivial insulating state. Using scanning tunneling microscopy, we observed gapless edge modes within this insulating bulk—clear evidence of topological character. Their behavior under magnetic fields, corroborated by theory, further supports this conclusion.
Remarkably, this state emerges through spontaneous mirror symmetry breaking, without requiring strong structural distortions, as shown by our detailed structural and thermodynamic analysis. Even more intriguing is a second excitonic phase that appears around 5 K, marked by translational symmetry breaking with a tunable wavevector—an unprecedented form of magnetic field–controlled ordering.
This work establishes Ta₂Pd₃Te₅ as the first confirmed 3D topological excitonic insulator, offering a novel platform to explore the interplay between topology and strong correlations using bulk-sensitive probes. It opens the door to studying critical behavior, quantum phase transitions, and excitonic excitations in a fundamentally new regime of condensed matter physics.
Topological Charge Density Wave
Charge density waves (CDWs)—periodic modulations of the electron density—are a hallmark of many quantum materials, from high-temperature superconductors to quantum Hall systems. Yet, the boundary states arising specifically from CDW order have remained experimentally elusive.
In our recent work, we use scanning tunneling microscopy (STM) to directly image both the bulk and boundary behavior of the CDW phase in the topological material Ta₂Se₈I. Below the transition temperature (TCDW≈ 260 K), we observe an unexpectedly large insulating gap (>500 meV) in the tunneling spectra—significantly larger than predictions from standard weak-coupling theories.
Spectroscopic imaging reveals a π phase shift between valence and conduction band charge modulations, confirming the presence of a strong CDW. At a monolayer step edge, we detect a striking in-gap boundary mode with spatial oscillations matching the CDW wavevector. Remarkably, this edge state undergoes a phase shift within the gap, smoothly connecting the valence and conduction bands in energy—an effect reminiscent of topological spectral flow, though here manifested in energy-phase space rather than energy-momentum.
Temperature-dependent measurements show that both the insulating gap and edge state vanish above TCDW, establishing their origin in the charge order itself. Theoretical analysis supports a topological character for the observed boundary mode.
These findings challenge the widely discussed view of Ta₂Se₈I as an axion insulator, which would feature a surface—not edge—state. Instead, our results point to a new type of topological state driven by charge order, enriching the landscape of correlated topological phases.
Looking Ahead
At Q MIND, we aim to discover new quantum states where competing or intertwined order parameters give rise to novel emergent phenomena. By harnessing the extreme sensitivity of these exotic states to external perturbations—such as magnetic fields, strain, or light—we seek to develop next-generation quantum-enhanced sensing technologies.