Theory of time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5

Abstract

We theoretically investigate photonic time-crystalline behaviour initiated by optical excitation above the electronic gap of the excitonic insulator candidate $\mathrm{T}{\mathrm{a}}_2\mathrm{NiS}{\mathrm{e}}_5$. We show that after electron photoexcitation, electron-phonon coupling leads to an unconventional squeezed phonon state, characterised by periodic oscillations of phonon fluctuations. Squeezing oscillations lead to photonic time crystalline behaviour. The key signature of the photonic time crystalline behaviour is THz amplification of reflectivity in a narrow frequency band. The theory is supported by experimental results on $\mathrm{T}{\mathrm{a}}_2\mathrm{NiS}{\mathrm{e}}_5$ where photoexcitation with short pulses leads to enhanced terahertz reflectivity with the predicted features. We explain the key mechanism leading to THz amplification in terms of a simplified Hamiltonian whose validity is supported by ab-initio DFT calculations. Our theory suggests that the pumped $\mathrm{T}{\mathrm{a}}_2\mathrm{NiS}{\mathrm{e}}_5$ is a gain medium, demonstrating that squeezed phonon noise may be used to create THz amplifiers in THz communication applications.

arXiv commentMain: 24 pages, 2 figures. Supplementary: 8 pages, 3 figures, 2 tables

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Cited by in corpus (4)

• Photonic time crystals: Theory and applications
• Squeezed Josephson plasmons in driven YBa${}_2$Cu${}_3$O${}_{6+x}$
• Collective modes and Raman response in Ta${}_2$NiSe${}_5$
• Strong long-wavelength electron-phonon coupling in Ta${}_2$Ni(Se,S)${}_5$