Observation of two different -C-C-H bending modes in the Ambient-Pressure organic superconductor κ-(ET) 2Cu(NCS) 2 by polarized infrared reflectance measurements and implications on its structural and physical properties

Abstract

We have carried out micro-FT-IR polarized reflectance measurements on the (100) faces of the ambient pressure organic superconductors κ-(ET) 2Cu(NCS) 2 and d 8-κ-(ET) 2Cu(NCS) 2, T c ⋍ 10.4 K and 10.8 K, respectively. The spectrum of κ-(ET) 2Cu(NCS) 2 exhibits absorptions due to -C-C-H bending at 1280 and 1302 cm -1. The frequency difference of these -C-C-H bending modes correlates with the fact that κ-(ET) 2Cu(NCS) 2 contains two types of ET molecules widely different in the “softness” of their hydrogen…anion interactions which in turn affects T c. Large shifts in the vibronic envelope are observed for d 8-κ-(ET) 2Cu(NCS) 2, and these may be due to the softer -C-D…anion interactions in the non-equivalent d 8 -ET molecules.