Research ArticleCONDENSED MATTER PHYSICS

Coherent order parameter oscillations in the ground state of the excitonic insulator Ta2NiSe5

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Science Advances  23 Mar 2018:
Vol. 4, no. 3, eaap8652
DOI: 10.1126/sciadv.aap8652
  • Fig. 1 Structure of Ta2NiSe5.

    (A) The Ni (red) and Ta (blue) 1D chains are aligned along the a axis and forming sheets in the ac plane. The electronic transport forms along the chains in a direction (Se atoms are marked green). (B) Structure and exciton formation along the chains: The Ni chains supply the valence band, and the Ta chains supply the conduction band. In the semiconducting phase, all Ni sites are doubly occupied. An exciton is formed between an electron on the Ta chains and a hole on the Ni chain. The Higgs amplitude mode (frequency ωΔ) corresponds to a collective hopping of electrons and holes between the chains.

  • Fig. 2 Pump-robe response.

    (A) Time trace of photoinduced reflectivity changes at different temperatures and an excitation density of 0.35 mJ/cm2. The signal is made up of the electronic response and the coherent oscillations. The dotted black lines represent fits to the measured data. (B) The inset shows only the coherent oscillations, which were extracted by substracting the fits in (A) from the measured data. The main panel of (B) presents the corresponding FFTs at 80 and 350 K. (C) Phonon-coupled amplitude mode at 1 THz, which was extracted using an FFT band-pass filter. The amplitude (AEI) of the coupled mode was determined by fitting a damped harmonic oscillator to the data (dotted lines).

  • Fig. 3 Temperature dependence.

    (A) Amplitude of the coupled mode (AEI) at 1 THz over temperatures at different excitation densities. The fits (dotted and dashed lines) denote a mean field-like order parameter that was fitted to the low-temperature data points. For the measurement at 0.18 mJ/cm2, only points above 225 K were used for the fit. The arrows illustrate the position of TC at the respective excitation density. (B) Amplitude of the uncoupled phonon mode (Aph) at 3 THz over temperatures at different excitation densities.

  • Fig. 4 Fluence dependence.

    (A) Amplitude of the coupled mode (AEI) at 1 THz over excitation density at 120 and 250 K. As discussed in the main text, the fit (dotted and dashed lines) reveals the threshold (ρs) that characterizes the onset of the coupling to the excitonic condensate. The shaded area indicates the regime in which the coupling of the condensate to the phonon is not effective. (B) Separate fit components (A1 and A2) for the measurement at 120 K. The shaded area describes the width of the step function, which characterizes the threshold. (C) Amplitude of the uncoupled phonon mode (Aph) at 3 THz over excitation density at 120 and 250 K.

  • Fig. 5 Coupled EI and phonon potentials.

    The double-well potential represents the EI, and the single-well potential represents the phonon. Because of the strong electron-phonon coupling, a new amplitude mode emerges, which combines phonon and order parameter dynamics. The excitation mechanism can be understood as follows: (A) At negative time delays, no effective coupling between the potentials can be observed. (B) When the pump pulse arrives, it changes the potential energy landscape of EI adiabatically, that is, without exciting a Higgs amplitude mode directly. The potential shrinks, the order parameter reduces, and a coupling in the nonlinear excitation regime (represented as spring) between the EI and the phonon becomes effective. (C) This leads to an impulsive excitation of the 1-THz phonon because the change occurs faster than its intrinsic 1-ps response time. Coupling to the EI results in an oscillation of the coupled condensate-phonon system.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaap8652/DC1

    section S1. Phonon spectrum of Ta2NiSe5 and characterization of the 1-THz phonon

    section S2.1. Amplitude behavior of the 1-THz mode at low excitation densities

    section S2.2. Frequency shift of the 1-THz mode

    section S3. Coherent phonon: The 3-THz mode

    section S4. Polarization parallel to the chains

    section S5. Coherent phonon oscillation of the 1-THz mode in Ta2NiS5

    section S6. Excitation density dependence of the electronic amplitude

    fig. S1.1. Linear Raman measurements at different analyzer settings.

    fig. S1.2. Real-space representation of the 1-THz A1g phonon.

    fig. S2.1. Amplitude of the 1-THz mode at low excitation densities.

    fig. S2.2. Frequency of the phonon-coupled amplitude mode for all measurements discussed in the main text.

    fig. S3. Properties of the 3-THz coherent phonon.

    fig. S4. Properties parallel to the chains.

    fig. S5. The 1-THz coherent phonon in Ta2NiS5.

    fig. S6. Maximum of the electronic signal as a function of the excitation density at different temperatures.

    table S1. Phonon spectrum of Ta2NiSe5 in the low-temperature phase at q = (0,0,0).

  • Supplementary Materials

    This PDF file includes:

    • section S1. Phonon spectrum of Ta2NiSe5 and characterization of the 1-THz phonon
    • section S2.1. Amplitude behavior of the 1-THz mode at low excitation densities
    • section S2.2. Frequency shift of the 1-THz mode
    • section S3. Coherent phonon: The 3-THz mode
    • section S4. Polarization parallel to the chains
    • section S5. Coherent phonon oscillation of the 1-THz mode in Ta2NiS5
    • section S6. Excitation density dependence of the electronic amplitude
    • fig. S1.1. Linear Raman measurements at different analyzer settings.
    • fig. S1.2. Real-space representation of the 1-THz A1g phonon.
    • fig. S2.1. Amplitude of the 1-THz mode at low excitation densities.
    • fig. S2.2. Frequency of the phonon-coupled amplitude mode for all measurements discussed in the main text.
    • fig. S3. Properties of the 3-THz coherent phonon.
    • fig. S4. Properties parallel to the chains.
    • fig. S5. The 1-THz coherent phonon in Ta2NiS5.
    • fig. S6. Maximum of the electronic signal as a function of the excitation density at different temperatures.
    • table S1. Phonon spectrum of Ta2NiSe5 in the low-temperature phase at q = (0,0,0).

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