Isotope effect study of {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2}: Labeling in the anion

Abstract

Since the initial discovery of organic superconductivity in 1979, a large number of organic superconductors have now been synthesized. However, the mechanism of electron-pairing in these novel superconductors has remained largely unresolved. Isotope effect studies constitute an important experimental tool for the investigation of whether or not the electron-pairing mechanism in organic superconductors is phonon-mediated, as in conventional superconductors. Recent isotope effect studies in the authors` laboratory, involving seven different isotopically labeled BEDT-TTF (or ET) derivatives, have demonstrated the following: (1) intramolecular phonon modes involving C{double_bond}C and C{single_bond}S stretching vibrations in the ET donor molecule are not the dominant mediators of electron-pairing, and (2) in {kappa}-(ET){sub 2}Cu(NCS){sub 2}, there exist two competing isotope effects--a normal mass effect, i.e., lowering of {Tc} upon isotopic labeling, when the ET molecular mass is increased by concurrent {sup 13}C and {sup 34}S labeling, in addition to an inverse isotope effect upon deuterium labeling in ET. It is of great interest to investigate if there is an isotope effect when the charge-compensating anions, which are also located within the non-conducting layer in the superconducting cation-radical salts, are isotopically labeled. The existence of an isotope effect when the anions are labeled would be indicative of electron-pairingmore » with the mediation of vibrational frequencies associated with the anions. In this paper, the authors present the results of the first isotope effect study in which isotopic labeling in the anion portion of {kappa}-(ET){sub 2}Cu(NCS){sub 2} is carried out. The authors find no isotope effect when the carbon and nitrogen atoms of the thiocyanate groups in the anion are replaced with {sup 13}C and {sup 15}N isotopes.« less