ESR STUDY OF THE ELECTRONIC-PROPERTIES OF THE NEW ORGANIC CONDUCTORS KAPPA-(BEDT-TTF)2CU[N(CN)2]X, X = BR I

The electron spin resonance data together with AC susceptibility and low field microwave absorption measurements are reported for the two organic conductors kappa-(BEDT-TTF)Cu[N(CN)2]X, with X = Br; I. The bromine containing salt is the recently discovered superconductor with T(c)onset = 11.6 K. In the compound with iodine only traces but not bulk superconductivity were found, with the onset at 9.3 K. Annealing resulted in an increase of the superconducting volume fraction but now with T(c)onset = 6.9 K. Possible interpretation of the data obtained and the discussion of the electronic structure of these compounds is given. The temperature dependences of the line width and the g-factor are discussed in connection with various mechanisms of interaction of conduction electrons with the lattice.