Understanding the totally symmetric intramolecular vibrations in $\kappa$-phase organic superconductors

We report magneto-infrared measurements of three quasi- isostructural $\kappa$-phase organic molecular solids: $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br (T$_c$=11.6 K), $\kappa$-(ET)$_2$Cu(SCN)$_2$ (T$_c$=10.4 K), and the non-superconducting $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Cl analog. Our results support the contributing role of electron-molecular vibrational coupling in the pairing mechanism of layered organic superconductors, and we identify the most important totally symmetric modes in $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br within the non-planar molecular building block picture.