Phonon anomalies in quasi-one-dimensional blue bronze, K_(0.3)MoO_3

摘要

Collective electron transport due to the sliding of the charge density wave (CDW) has been a topic of experimental and theoretical interests. K_(0.3)MoO_3 exhibits a second-order metal-insulator transition (Peierls transition) into the CDW state around 180 K (T_P), besides highly anisotropic quasi-one-dimentional properties [1]. K_(0.3)MoO_3 also undergoes an incommensurate to commensurate (IC-C) transition near 100 K as revealed by x-ray and neutron scattering studies [2]. The Young's modulus of K_(0.3)MoO_3 along [102] direction shows a large dip at T_P whereas along the chain direction [010] direction (b-axis), the anomaly is much smaller [3]. The elastic constant C_(22) and the sound velocity of the longitudinal accoustic (LA) mode along [20-1] direction measured by ultrasonic absorption shows softening with increasing temperature without any anomaly [4]. However, the sound velocity of the LA mode along [102] direction shows a dip at T_P. Similarly, thermal expansion coefficient along the [102] direction shows an anomaly at T_P [5]. These measurements, however, were limited to 80 K and hence did not cover the interesting consequences of the IC-C transition. We have carried out Brillouin scattering experiments on K_(0.3)MoO_3, for the first time, in order to investigate the nature of changes around T_P and to explore whether the IC-C transition manifests itself in some way.