Combined investigation of collective amplitude and phase modes in a quasi-one-dimensional charge-density-wave system over a wide spectral range

Abstract

We investigate experimentally both the amplitude and phase channels of the collective modes in the quasi-1D charge-density-wave (CDW) system, K0.3MoO3, by combining (i) optical impulsive-Raman pump-probe and (ii) terahertz time-domain spectroscopy (THz-TDS), with high resolution and a detailed analysis of the full complex-valued spectra in both cases. This allows an unequivocal assignment of the observed bands to CDW modes across the THz range up to 9 THz. We revise and extend a time-dependent Ginzburg-Landau model to account for the observed temperature dependence of the modes, where the combination of both amplitude and phase modes allows one to robustly determine the bare-phonon and electron-phonon coupling parameters. While the coupling is indeed strongest for the lowest-energy phonon, dropping sharply for the immediately subsequent phonons, it grows back significantly for the higher-energy phonons, demonstrating their important role in driving the CDW formation. We also include a reassessment of our previous analysis of the lowest-lying phase modes, whereby assuming weaker electronic damping for the phase channel results in a qualitative picture more consistent with quantum-mechanical treatments of the collective modes, with a strongly coupled amplitudon and phason as the lowest modes.

Suppression of Stacking Order with Doping in 1T-TaS2−xSex

“…In the typical Peierls model for CDW order [7], the instability of the coupled electron-lattice system results in a structural phase transition that is driven by strong electron-phonon coupling [8][9][10][11]. CDW formation can result in electron localization, where displacements along phonon modes lower the total electronic energy by opening up a gap at the Fermi level, E F [12,13]. This scenario, although applicable to simple one-dimensional systems, does not fully describe the case of 1T-TaS 2−x Se x , in which the CDW behavior is intertwined with the opening of a Mott gap [14].…”