Metal-insulator transition in the low-dimensional organic conductor (TMTSF)2FSO3 probed by infrared microspectroscopy

摘要

We present measurements of the infrared response of the quasi-one-dimensionalorganic conductor (TMTSF)2$SO3 along (E||a) and perpendicular (E||b') to thestacking axis as a function of temperature. Above the metal-insulatortransition related to the anion ordering the optical conductivity spectra showa Drude-like response. Below the transition an energy gap of about 1500 cm-1(185 meV) opens, leading to the corresponding charge transfer band in theoptical conductivity spectra. The analysis of the infrared-active vibrationsgives evidence for the long-range crystal structure modulation below thetransition temperature and for the short-range order fluctuations of thelattice modulation above the transition temperature. Also we report about a newinfrared mode at around 710 cm-1 with a peculiar temperature behavior, whichhas so far not been observed in any other (TMTSF)2X salt showing ametal-insulator transition. A qualitative model based on the coupling betweenthe TMTSF molecule vibration and the reorientation of electrical dipole momentof the FSO3 anion is proposed, in order to explain the anomalous behavior ofthe new mode.