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

TL;DR

This paper presents a theoretical and experimental study of photonic time-crystalline behavior in Ta2NiSe5, showing that electron-phonon interactions induce squeezed phonon states leading to THz amplification of reflectivity.

Contribution

It introduces a novel mechanism where phonon squeezing causes time-crystalline behavior and demonstrates potential for THz amplification in Ta2NiSe5.

Findings

Photoexcitation induces squeezed phonon states.

Squeezing oscillations cause THz reflectivity amplification.

Experimental results confirm theoretical predictions.

Abstract

We theoretically investigate photonic time-crystalline behaviour initiated by optical excitation above the electronic gap of the excitonic insulator candidate $\rm{Ta_2 Ni Se_5}$. 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 $\rm{Ta_2 Ni Se_5}$ 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 $\rm{Ta_2 Ni Se_5}$ is a gain medium, demonstrating that squeezed phonon noise may be used to create THz amplifiers in THz communication applications.