Exciton photoemission from a ground state of a solid Ta2Pd3Te5

TL;DR

This paper reports the direct observation of excitons in the ground state of the candidate excitonic insulator Ta2Pd3Te5 using angle-resolved photoemission spectroscopy, revealing their properties and potential for quantum phase exploration.

Contribution

It provides the first direct photoemission evidence of excitons in a ground state of a solid, confirming their role in the phase transition of Ta2Pd3Te5.

Findings

Excitons have lower energy than the valence band maximum.

Excitons exhibit an unusual odd parity wave function.

Observation of excitons supports the excitonic insulator phase.

Abstract

Excitons are bosonic quasiparticles with a variety of applications in optoelectronics, photosyn thesis, and dissipationless informatics, and their lifetime can become sufficiently long to form a quantum condensate. While exciton condensation has been predicted to occur as a ground state of a solid, so called an excitonic insulator, whose material realization has been elusive. Here we report the observation of direct photoemission signals from excitons in a ground state of a very recent excitonic insulator candidate Ta2Pd3Te5 below its metal-insulator transition temperature using orbital-selective angle-resolved photoemission spectroscopy. It is confirmed that the excitons have a lower energy than the valence band maximum to possibly drive the phase transition. This measurement further discloses the size and the unusual odd parity of the exciton wave function. The present finding opens an avenue toward applications of coherent excitons in solid systems and searching for exotic quantum phases of exciton condensates.