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Exciton Photoemission from a Ground State of a Solid: Ta2Pd3Te5

Siwon Lee1,2, Kyung-Hwan Jin2,3, SeongJin Kwon1,2, Hyunjin Jung1,2, Choongjae Won4,5, Sang-Wook Cheong5,6, Gil Young Cho1,7, Jaeyoung Kim2, and Han Woong Yeom1,2,*

  • *Contact author: yeom@postech.ac.kr
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Phys. Rev. Lett. 135, 116401 – Published 9 September, 2025

DOI: https://doi.org/10.1103/5kd3-trkg

Abstract

Excitons are bosonic quasiparticles with a variety of applications in optoelectronics, photosynthesis, 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.

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Phys. Rev. Lett. 135, 116401– Published 9 September, 2025
  • Received 6 March 2025
  • Revised 30 May 2025
  • Accepted 7 August 2025
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