Interfacial Charge-transfer Excitonic Insulator in a Two-dimensional Organic-inorganic Superlattice

TL;DR

This paper reports the discovery of a novel interfacial charge-transfer excitonic insulator in a 2D organic-inorganic superlattice, providing clear evidence of exciton condensation without lattice distortion, advancing the understanding of excitonic insulators.

Contribution

It introduces a new platform for excitonic insulators that isolates excitonic effects from lattice effects, enabling clearer study of exciton condensation in equilibrium.

Findings

Observation of narrow gap opening without lattice distortion

Visualized evidence of exciton condensation in equilibrium

Identification of spontaneous interfacial charge transfer as a new strategy

Abstract

Excitonic insulators are long-sought-after quantum materials predicted to spontaneously open a gap by the Bose condensation of bound electron-hole pairs, namely, excitons, in their ground state. Since the theoretical conjecture, extensive efforts have been devoted to pursuing excitonic insulator platforms for exploring macroscopic quantum phenomena in real materials. Reliable evidences of excitonic character have been obtained in layered chalcogenides as promising candidates. However, owing to the interference of intrinsic lattice instabilities, it is still debatable whether those features, such as charge density wave and gap opening, are primarily driven by the excitonic effect or by the lattice transition. Herein, we develop a novel charge-transfer excitonic insulator in organic-inorganic superlattice interfaces, which serves as an ideal platform to decouple the excitonic effect from the lattice effect. In this system, we observe the narrow gap opening and the formation of a charge density wave without periodic lattice distortion, providing visualized evidence of exciton condensation occurring in thermal equilibrium. Our findings identify spontaneous interfacial charge transfer as a new strategy for developing novel excitonic insulators and investigating their correlated many-body physics.