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.
@article{arxiv.2511.23084,
title = {Trion gas on the surface of a failed excitonic insulator},
author = {Yuval Nitzav and Abigail Dishi and Himanshu Lohani and Ittai Sidilkover and Noam Ophir and Roni Anna Gofman and Avior Almoalem and Ilay Mangel and Nitzan Ragoler and Francois Bertran and Jaime Sánchez-Barriga and Dmitry Marchenko and Andrei Varykhalov and Nicholas Clark Plumb and Irena Feldman and Hadas Soifer and Anna Keselman and Amit Kanigel},
journal= {arXiv preprint arXiv:2511.23084},
year = {2026}
}