Spectrally resolving the phase and amplitude of coherent phonons in the charge density wave state of 1$T$-TaSe$_2$

TL;DR

This study uses broadband reflectivity spectroscopy to spectrally resolve and analyze coherent phonons in 1T-TaSe2, revealing detailed spectral dependence, phase shifts, and mode couplings associated with charge density wave states.

Contribution

It provides the first detailed spectral and phase analysis of coherent phonons in 1T-TaSe2, linking specific modes to electronic transitions and CDW order.

Findings

Identification of dominant phonon modes at 2.19 THz and 2.95 THz.

Observation of a $$ phase shift at 2.4 eV.

Spectral dependence of phonon amplitude and phase reveals mode-specific coupling.

Abstract

The excitation and detection of coherent phonons has given unique insights into condensed matter, in particular for materials with strong electron-phonon coupling. We report a study of coherent phonons in the layered charge density wave (CDW) compound 1$T$-TaSe$_2$ performed using transient broadband reflectivity spectroscopy, in the photon energy range 1.75-2.65 eV. Several intense and long lasting (> 20 ps) oscillations, arising from the CDW superlattice reconstruction, are observed allowing for detailed analysis of the spectral dependence of their amplitude and phase. We find that for energies above 2.4 eV, where transitions involve Ta d-bands, the CDW amplitude mode at 2.19 THz dominates the coherent response. At lower energies, instead, beating arises between additional frequencies, with a particularly intense mode at 2.95 THz. Interestingly, our spectral analysis reveals a $\pi$ phase shift at 2.4 eV. Results are discussed considering the selective coupling of specific modes to energy bands involved in the optical transitions seen in steady-state reflectivity. The work demonstrates how coherent phonon spectroscopy can distinguish and resolve optical states strongly coupled to the CDW order and provide additional information normally hidden in conventional steady-state experiments.