Infrared-reflectivity-measurements on the organic superconductors h(8) andd(8) kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br

A comparison of the temperature dependant reflectivity spectra of the organ ic superconductor kappa-(h(8)-BEDT-TTF)(2)Cu[N(CN)(2)]Br and kappa-(d(8)-BE DT-TTF)(2) Cu[N(CN)(2)]Br on rapid cooling is presented. kappa-(d(8)-BEDT-T TF)(2)Cu[N(CN)(2)]Br is on the border of a superconductor-magnetic-insulato r transition. On rapid cooling, the low temperature ground state is a purel y magnetic phase. The reflectivity spectra then are very similar as for kap pa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl which has an antiferromagnetic ground state. It is proposed that localized charge carriers contribute to the mid-infrar ed reflectivity of the low temperature magnetic phase of kappa-(d(8)-BEDT-T TF)(2)Cu[N(CN)(2)]Br. (C) 1999 Published by Elsevier Science B.V. All right s reserved.