OPTICAL STUDY OF THE ELECTRONIC-STRUCTURE AND ELECTRON MOLECULAR VIBRATIONAL COUPLING IN A NOVEL ORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)2CU[N(CN)2]CL0.5BR0.5

Reflectance spectra of a new ambient-pressure superconductor, kappa-(BEDT-TTF)2Cu[N(CN)2]Cl0.5Br0.5 (T(c) = 11.3 K), where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, are measured in the range from 750 to 5500 cm-1 and from 9000 to 40 000 cm-1 for different polarizations. The detailed analysis of the infrared conductivity, obtained after a Kramers-Kronig transformation, is performed in the framework of the approach, based on the 'phase phonons' theory of M.J. Rice, with the help of special transformation of the experimental data. Parameters of the electronic subsystem and electron-molecular vibration coupling are determined. The determined 'total' dimensionless coupling constant (lambda) is compared with those of the organic super conductors kappa-(BEDT-TTF)2Cu(NCS)2 (T(c) = 10.4 K) and (BEDT-TTF)4Hg2.89Br8 (T(c) = 4.3 K). It is found that a larger lambda produces a larger T(c) in accordance with the known Bardeen-Cooper-Schriffer formula for T(c).