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Topology and correlations are fundamental concepts in modern physics, buttheir simultaneous occurrence within a single quantum phase is exceptionallyrare. In this study, we present the discovery of such a phase of matter inTa2Pd3Te5, a semimetal where the Coulomb interaction between electrons andholes leads to the spontaneous formation of excitonic bound states below T=100K. Our spectroscopy unveils the development of an insulating gap stemming fromthe condensation of these excitons, thus giving rise to a highly sought-aftercorrelated quantum phase known as the excitonic insulator. Remarkably, ourscanning tunneling microscopy measurements reveal the presence of gaplessboundary modes in the excitonic insulator state. Their magnetic field responseand our theoretical calculations suggest a topological origin of these modes,rendering Ta2Pd3Te5 as the first experimentally identified topologicalexcitonic insulator in a three-dimensional material not masked by anystructural phase transition. Furthermore, our study uncovers a secondaryexcitonic instability below T=5 K, which differs from the primary one in havingfinite momentum. We observe unprecedented tunability of its wavevector by anexternal magnetic field. These findings unlock a frontier in the study of novelcorrelated topological phases of matter and their tunability.

Discovery of a topological exciton insulator with tunable momentum order