Non-Coulomb strong electron-hole binding in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ revealed by time- and angle-resolved photoemission spectroscopy

Non-Coulomb strong electron-hole binding in ${Ta}_{2} {NiSe}_{5}$ revealed by time- and angle-resolved photoemission spectroscopy

Tianwei Tang, Hongyuan Wang, Shaofeng Duan, Yuanyuan Yang, Chaozhi Huang, Yanfeng Guo, Dong Qian, and Wentao Zhang

Phys. Rev. B 101, 235148 – Published 22 June 2020

Abstract

We reveal an ultrafast purely electronic phase transition in ${Ta}_{2} {NiSe}_{5}$ , which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks with enhancing the pump pulse, and above a critical pump fluence, a photoexcited semimetallic state is experimentally identified with the absence of ultrafast structural transition. In addition, the bare valence and conduction bands and also the effective masses in ${Ta}_{2} {NiSe}_{5}$ are determined. These findings and detailed analysis suggest a bare nonequilibrium semimetallic phase in ${Ta}_{2} {NiSe}_{5}$ and the strong electron-hole binding cannot be exclusively driven by Coulomb interaction.

Received 6 December 2019
Revised 1 March 2020
Accepted 1 June 2020

DOI:https://doi.org/10.1103/PhysRevB.101.235148

Physics Subject Headings (PhySH)

Electronic structure Ultrafast phenomena

Semimetals

Infrared techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Tianwei Tang ¹, Hongyuan Wang², Shaofeng Duan¹, Yuanyuan Yang¹, Chaozhi Huang¹, Yanfeng Guo², Dong Qian¹, and Wentao Zhang ^1,*

¹Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Shenyang National Laboratory for Materials Science, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
²School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China

^*wentaozhang@sjtu.edu.cn

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Vol. 101, Iss. 23 — 15 June 2020

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Figure 1
Temperature dependence of equilibrium photoemission spectra near the $Γ$ point along the Ta and Ni chains. (a) Photoemission intensity as a function of energy and momentum at temperatures from 20 to 350 K. (b) Corresponding curvature images of the spectra shown in panel (a). (c) Temperature-dependent energy distribution curve at the momenta of the band top [red dashed line cut in panel (a)]. (d) Energy gap of the bands #1 and binding energy of band #2 and the FWHM at the band top and also the FWHM at $Γ$ of the band #1 as functions of temperature from panel (c).
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Figure 2
Pump-induced energy gap change at 30 K. (a) Time-resolved photoemission spectra with the pump fluence at 0.19 mJ/ ${cm}^{2}$ . (b) Photoemission intensity as a function of energy and delay time at the momentum of band top [red dashed line cut in panel (a)]. (c) Intensity of pump-induced nonequilibrium electrons integrated in the energy window 0.1 eV above the Fermi energy as functions of delay time at different pump fluences. All the curves are normalized to the same height. (d) Energy distribution curves at the momenta of the band top and delay time 0.3 ps for the same pump fluences. (e) The pump-induced change of energy gap from the panel (d) and the decay rate of nonequilibrium electrons as functions of pump fluence.
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Figure 3
Time-resolved energy and momentum-dependent band shifts measured at 30 K. (a1)–(a4) Time-resolved band shifts at different energies and momentum with pump fluences of 0.1 mJ/ ${cm}^{2}$ (below $F_{c}$ ) and 0.32 mJ/ ${cm}^{2}$ (above $F_{c}$ ) at positions noted in the corresponding insets of each panel. The dashed lines are the incoherent part fitted to a function combined by an exponential decay function and a cubic function. (b1)–(b4) Energy shifts after removing the incoherent part shown in panels (a1)–(a4). Similar curves measured at other four different pump fluences are shown, and the curves are offset by 0.02 eV from low to high pump fluences. (c1)–(c4) The corresponding Fourier transform magnitudes of the curves shown in panels (b1)–(b4). Fourier transforms are done between 0.95 ps and 8 ps, and the results are offset by 0.1 from the bottom to the top.
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Figure 4
Pump fluence dependence of the coherent phonon vibration magnitudes from Fig. 3.
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Figure 5
The bare valence and conduction bands from time-resolved electronic qusiparticle dispersions with a pump fluence of 0.19 mJ/ ${cm}^{2}$ measured at 30 K. (a) The underlying bare valence ( $ɛ_{k}^{v}$ ) and conduction bands ( $ɛ_{k}^{c}$ ) extracted from the experimental equilibrium dispersion $E_{k}^{c}$ at $- 0.4$ ps and nonequilibrium dispersion $E_{k}^{v}$ at 0.3 ps. (b) Calculated quasiparticle energies [ $E_{k}^{c} (t)$ at $- 0.4$ , 0.3, 1.1, and 1.85 ps and $E_{k}^{v} (t)$ ] at 1.1 and 1.85 ps from the bare valence ( $ɛ_{k}^{v}$ ) and conduction bands ( $ɛ_{k}^{c}$ ) in panel (a) on top of the curvature images of nonequilibrium spectra.
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Physical Review B

covering condensed matter and materials physics

Non-Coulomb strong electron-hole binding in ${Ta}_{2} {NiSe}_{5}$ revealed by time- and angle-resolved photoemission spectroscopy

Tianwei Tang, Hongyuan Wang, Shaofeng Duan, Yuanyuan Yang, Chaozhi Huang, Yanfeng Guo, Dong Qian, and Wentao Zhang

Phys. Rev. B 101, 235148 – Published 22 June 2020

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Physical Review B

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Non-Coulomb strong electron-hole binding in Ta2NiSe5 revealed by time- and angle-resolved photoemission spectroscopy

Tianwei Tang, Hongyuan Wang, Shaofeng Duan, Yuanyuan Yang, Chaozhi Huang, Yanfeng Guo, Dong Qian, and Wentao Zhang

Phys. Rev. B 101, 235148 – Published 22 June 2020

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Non-Coulomb strong electron-hole binding in ${Ta}_{2} {NiSe}_{5}$ revealed by time- and angle-resolved photoemission spectroscopy