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arXiv · 2511.23084

Trion gas on the surface of a failed excitonic insulator

Yuval Nitzav·Abigail Dishi·Himanshu Lohani·Ittai Sidilkover·Noam Ophir·Roni Anna Gofman·Avior Almoalem·Ilay Mangel·Nitzan Ragoler·Francois Bertran·Jaime Sánchez-Barriga·Dmitry Marchenko·Andrei Varykhalov·Nicholas Clark Plumb·Irena Feldman·Hadas Soifer·Anna Keselman·Amit Kanigel
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Abstract

Trions, three-body bound states composed of an exciton and an additional charge, are typically fragile and require external excitation to form. Here, we report the spontaneous emergence of a stable trion gas at the surface of the layered semiconductor Ta2NiS5, revealed through angle-resolved photoemission spectroscopy. We observe a sharp, highly localized in-gap feature that cannot be explained by conventional band-theory. Instead, we argue that it arises from the formation of negative trions, stabilized by surface-induced band bending and the material's quasi-one-dimensional geometry. Unlike excitons, these trions form without optical pumping and persist at equilibrium, marking a rare example of an interaction-driven surface state in a nominally conventional semiconductor. Our findings establish Ta2NiS5 as a unique platform for exploring many-body physics at surfaces and open new avenues for studying and controlling collective excitations in low-dimensional systems.

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Yuval Nitzav, Abigail Dishi, Himanshu Lohani, Ittai Sidilkover, Noam Ophir, Roni Anna Gofman, Avior Almoalem, Ilay Mangel, Nitzan Ragoler, Francois Bertran, Jaime Sánchez-Barriga, Dmitry Marchenko, Andrei Varykhalov, Nicholas Clark Plumb, Irena Feldman, Hadas Soifer, Anna Keselman, Amit Kanigel. 2026-04-19. Trion gas on the surface of a failed excitonic insulator. https://arxiv.org/abs/2511.23084

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