Detecting photoelectrons from spontaneously formed excitons

TL;DR

This paper reports the direct detection of spontaneously formed excitons in Ta2NiSe5 using angle-resolved photoemission spectroscopy, providing evidence for excitonic insulator behavior and detailed exciton properties.

Contribution

It presents the first direct photoemission evidence of spontaneous excitons in a candidate material, confirming its excitonic insulator nature.

Findings

Detection of excitonic features above transition temperature

Identification of anisotropic Bohr radius of excitons

Evidence of preformed excitons in Ta2NiSe5

Abstract

Excitons, quasiparticles of electrons and holes bound by Coulombic attraction, are created transiently by light and play an important role in optoelectronics, photovoltaics and photosynthesis. While they are also predicted to form spontaneously in a small gap semiconductor or a semimetal, leading to a Bose-Einstein condensate at low temperature, their material realization has been elusive without any direct evidence. Here we detect the direct photoemission signal from spontaneously formed excitons in a debated excitonic insulator candidate Ta2NiSe5. Our symmetry-selective angle-resolved photoemission spectroscopy reveals a characteristic excitonic feature above the transition temperature, which provides detailed properties of excitons such as anisotropic Bohr radius. The present result evidences so called preformed excitons and guarantees the excitonic insulator nature of Ta2NiSe5 at low temperature. Direct photoemission can be an important tool to characterize steady-state excitons.