Studies of the dynamics in charge-density-wave systems using inelastic X-ray scattering with meV-energy-resolution

Abstract
We report measurements of the dynamics of the charge-density-wave systems K _0.3MoO ₃ and NbSe ₃ applying the quite recent technique of meV-resolution inelastic X-ray scattering (IXS). In K _0.3MoO ₃, serving as a test-case with existing reference results from neutron scattering, the very steep slope of the phason dispersion has been reproduced by IXS together with a very good agreement for the observed "phason velocity": 26 km/sec (IXS) vs. 23 km/sec (neutrons). In NbSe ₃, where IXS allows for dispersion studies on thin whiskers, the longitudinal acoustic and one transverse acoustic (TA ₁) branch propagating along the NbSe ₃-chain direction have been determined at room temperature. Low temperature TA ₁-data have been taken around the upper-CDW satellite position (0, 0.241, 0) at temperatures down to below the upper Peierls transition (T =145 K). The low temperature results indicate the absence of a Kohn anomaly, in contrast to the CDW-related dynamics observed in K _0.3MoO ₃ and KCP, but similar as in (TaSe ₄) ₂I.