Trion gas on the surface of a failed excitonic insulator

TL;DR

This study reports the spontaneous formation of a stable trion gas on the surface of Ta2NiS5, a layered semiconductor, observed via photoemission spectroscopy, revealing a novel interaction-driven surface state.

Contribution

It demonstrates the existence of equilibrium trion gases on a semiconductor surface without optical excitation, a rare example of many-body surface phenomena.

Findings

Stable trion gas observed at the surface of Ta2NiS5.

In-gap feature attributed to negative trions stabilized by surface effects.

Trions form without optical pumping and are persistent at equilibrium.

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.