Optical Properties of β″-(ET)2SF5CH2CF2SO3: A Novel Superconductor with Large Discrete Counterions

The optical spectra of the organic superconductor β″-(ET)2[SF5CH2CF2SO2 are measured over a wide spectral range (30–35000 cm−1) as a function of temperature and polarization. The optical anisotropy is quite large compared with other ET-based organic superconductors, and the spectra are far from Drude-like over the full temperature range. A broad electronic band centered near 1000 cm−1 is observed at low temperature along the a axis, prior to the superconducting transition. The changes of vibrational features near 120 K are attributed to a weak reorientation of the counterion, which may affect hydrogen bonding in the material.