Final Activity Report Summary - EYA (Optical properties of MgB2-based superconductors) Our research interest has been focused on the study of the vibrational and the superconducting properties of various MgB2-based superconductors. This effort includes the synthesis of novel MgB2-based compounds derived from the chemical modification of the parent material and their experimental study with various complementary techniques including Raman spectroscopy both at ambient and high pressure conditions, XRD, muSR, NMR and inelastic neutron scattering. NMR data indicates a considerable increase of Hc2c with carbon doping and a drastic reduction of coherence with respect to pure MgB2. The spin-lattice relaxation rate 1/(T1T) extracted by NMR analysis demonstrates clearly the presence of a coherence peak right below Tc in pure MgB2, followed by a typical BCS decrease on cooling. Also, in the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T1T) weaken, an effect attributed to the gradual shrinking of the hole cylinders of the Fermi surface with electron doping. Raman spectroscopic studies show that significant changes in the Raman spectra of the MgB2xCx compounds occur for carbon concentrations x > 0.04 and these changes can be associated with carbon substitution-induced disorder in the investigated samples. The most prominent feature of our results concerning the Mg1-xAlxB2-yCy compounds is that the co-addition of Al (up to 0.02) and C (up to 0.08) retains the two-band superconductivity in the presence of significant scattering. Complementary Raman measurements show significant evidence of electron-phonon coupling attenuation in these compounds. Furthermore, inelastic neutron scattering measurements on Mg11B2-xCx samples shows that the high energy part of the generalised phonon density-of-states is hardly affected upon boron doping and that the temperature dependence of the phonon spectrum for the MgB1.96C0.04 compound clarifies the importance of certain modes in the superconducting process. Finally, we have extended our studies toward the synthesis and characterisation of carbon nanotube based composites in order to obtain low-weight composites of extraordinary mechanical properties.
