Anomalous profile of amplitude mode in K_0.30MoO₃ at low temperatures studied by Raman spectroscopy

We precisely reinvestigate the Charge-Density Wave (CDW) amplitude mode in molybdenum blue bronze (K_0.30MoO₃) by using Raman spectroscopy down to 4 K. The Raman profile of the amplitude mode is gradually changing from one harmonic oscillator form to an anomalous asymmetric one like a `fin' with decreasing temperature. This is probably due to a splitting of the amplitude mode and contrasted with the previous study, where the CDW amplitude mode was analyzed with one harmonic oscillator down to 10 K. This anomaly probably reflects a spatial variation of the CDW amplitude in the bulk of the crystal at low temperatures.