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Detecting photoelectrons from spontaneously formed excitons

Nature Physics volume 17, pages 1024–1030 (2021)Cite this article

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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. They are also predicted to form spontaneously in a small-gap semiconductor or a semimetal, leading to a Bose–Einstein condensate at low temperature, but there has not been any direct evidence of this effect so far. Here we detect the 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 the anisotropic Bohr radius. The present result provides evidence for so-called preformed excitons and guarantees the excitonic insulator nature of Ta2NiSe5 at low temperature.

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Fig. 1: The symmetry-selective ARPES experimental set-up and the band structure of TNS.
Fig. 2: Results of the symmetry-selective ARPES.
Fig. 3: Polarization-dependent ARPES results at T = 380 K (above Tc).
Fig. 4: Analysis of excitonic photoemission at T = 380 K.
Fig. 5: Analysis of excitonic photoemission along the \({\bar{{\Gamma}}}\)–\(\bar {\rm{Y}}\) direction and overview of preformed excitons in real and momentum space.

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Data availability

Data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

Code availability

The numerical computation codes used to obtain the ARPES simulations in Fig. 4c,d are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank B.-G. Park for experimental help and C. Park, A. Kemper and A. Rutsagi for fruitful discussions. This work was supported by the Institute for Basic Science (IBS), Korea, under project code IBS-R014-D01 (H.W.Y.), and also by the National Research Foundation of Korea (NRF) through the SRC (no. 2018R1A5A6075964; J.S.K.) and the Max Planck-POSTECH Center (no. 2016K1A4A4A01922028; J.S.K.).

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Author notes

  1. Keisuke Fukutani

    Present address: Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki, Japan

Authors and Affiliations

  1. Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang, Republic of Korea

    Keisuke Fukutani, Roland Stania, Chang Il Kwon, Jun Sung Kim, Ki Jeong Kong, Jaeyoung Kim & Han Woong Yeom

  2. Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea

    Keisuke Fukutani, Roland Stania & Jaeyoung Kim

  3. Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea

    Chang Il Kwon, Jun Sung Kim & Han Woong Yeom

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Contributions

K.F. and R.S. measured the experimental data. K.F. analysed the data and performed the computations on ARPES spectra simulations. K.J.K. performed the electronic structure calculations. C.I.K. and J.S.K. prepared the samples. H.W.Y. and J.K. were responsible for experimental infrastructures. K.F., R.S. and J.K. maintained the ARPES endstation. K.F. and H.W.Y. wrote the manuscript with input and discussions from the co-authors. H.W.Y. was responsible for overall project planning and direction.

Corresponding author

Correspondence to Han Woong Yeom.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–6 and Tables 1–4.

Source data

Source Data Fig. 2

Numerical source data for Fig. 2e–h.

Source Data Fig. 3

Numerical source data for Fig. 3e,f.

Source Data Fig. 4

Numerical source data for Fig. 4b,e,f.

Source Data Fig. 5

Numerical source data for Fig. 5b.

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Fukutani, K., Stania, R., Il Kwon, C. et al. Detecting photoelectrons from spontaneously formed excitons. Nat. Phys. 17, 1024–1030 (2021). https://doi.org/10.1038/s41567-021-01289-x

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