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

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.

20 pages, 4 figures

References in corpus (5)

• Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1T-TaSe${}_2$ mediated by mode-selective electron-phonon coupling
• Coherent phonons and the interplay between charge density wave and Mott phases in 1$T$-TaSe${}_2$
• Correlation between crystal purity and the charge density wave in 1$T$-VSe${}_2$
• Interplay of crystal thickness and in-plane anisotropy and evolution of quasi-one dimensional electronic character in ReSe${}_2$
• Observation of the charge density wave collective mode in the infrared optical response of VSe${}_2$

Cited by in corpus (6)

• Strong Coupling of Coherent Phonons to Excitons in Semiconducting Monolayer MoTe${}_2$
• Exploring the Charge Density Wave phase of 1$T$-TaSe${}_2$: Mott or Charge-transfer Gap?
• Identification of soft modes across the commensurate-to-incommensurate charge density wave transition in 1$T$-TaSe${}_2$
• Unconventional photo-induced charge-density-wave dynamics in 2H-NbSe${}_2$
• Anomalous amplitude mode dynamics below the expected charge-density-wave transition in 1$T$-VSe${}_2$
• A versatile setup for symmetry-resolved ultrafast dynamics of quantum materials