The pressure dependence of electron-phonon coupling in the organic superconductor $\kappa$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$: A comparison of high pressure infrared reflectivity and Raman scattering experiments

摘要

We determine the pressure dependence of the electron-phonon coupling constantin $\kappa$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$ by comparison of high pressure Ramanscattering and high pressure infrared reflectivity (IR) measurements. We usecomparison of the IR reflection spectrum from deuterated and protonated samplesto aid the deconvolution of several overlapping phonon modes. These coupledmodes are modelled with a Green's function to extract the linear pressuredependence of the individual modes. The Raman active molecular vibrations ofthe BEDT-TTF dimers stiffen by 0.1-1 %GPa$^{-1}$. In contrast, thecorresponding modes in the IR spectrum are observed at lower frequency, with apressure dependence of 0.5-5.5 %GPa$^{-1}$, due to the influence of theelectron-phonon interaction. The stronger pressure dependence of the centralC=C mode of the BEDT-TTF molecule in the IR, is discussed. Our analysissuggests that reduction of electron-phonon coupling under pressure does notaccount for observed suppression of superconductivity under pressure.