Coherent photo-induced phonon emission in the charge-density-wave state of K_0.3MoO₃

Abstract

We report on the observation of coherent terahertz (THz) emission from the quasi-one-dimensional charge-density wave (CDW) system, blue bronze (K 0.3 MoO 3 ), upon photo-excitation with ultrashort near-infrared optical pulses. The emission contains a broadband, low-frequency component due to the photo-Dember effect, which is present over the whole temperature range studied (30-300 K), as well as a narrow-band doublet centered at 1.5 THz, which is only observed in the CDW state and results from the generation of coherent transverse-optical phonons polarized perpendicular to the incommensurate CDW b-axis. As K 0.3 MoO 3 is centrosymmetric, the lowest-order generation mechanism which can account for the polarization dependence of the phonon emission involves either a static surface field or quadrupolar terms due to the optical field gradients at the surface. This phonon signature is also present in the ground-state conductivity, and decays in strength with increasing temperature to vanish aboveT 100 K, i.e. significantly below the CDW transition temperature. The temporal behavior of the phonon emission can be well described by a simple model with two coupled modes, which initially oscillate with opposite polarity.

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

“…These results strongly motivate first-principles calculations of the phonons and eph coupling, although this remains challenging for relatively complex materials such as K 0.3 MoO 3 . From the experimental side, in addition to returning to the nonequilibrium response of these modes [13,40], a rigorous determination of the ground-state phase response in the low-frequency range ( 1.5 THz) is still lacking, being complicated by the inherent issue of resolving spectral features in reflection with R ≈ 1. Here we are pursuing THz transmission studies of thin exfoliated flakes, although the small lateral dimensions of sufficiently thin samples (thickness <10 µm) are limited, requiring specialized spectroscopic methods.…”