MgB2 single crystals substituted with Li and with Li-C:: Structural and superconducting properties

The effect of Li substitution and Li-C co-substitution on the superconducting properties and crystal structure of MgB2 single crystals has been investigated. Hole doping with Li decreases the superconducting transition temperature T-c at a slower rate than that of the electron doping with C or Al. For crystals co-substituted with Li for Mg and C for B, T-c decreases more rapidly than for crystals substituted with C alone. This means that holes introduced with Li cannot counterbalance the influence of electrons doped with C and are not able to prevent T-c to decrease. The possible explanation is that holes coming from Li occupy mainly the pi band while electrons coming from C fill the sigma band. The resistivity in both Li substituted and Li-C co-substituted crystals is significantly increased, reflecting scattering due to substitutional defects, including distortion of the lattice. Li substitution decreases the upper critical field H-c2(T)(parallel to ab) while H-c2(T)(parallel to c) does not change. The temperature dependences of H-c2 for the Li and Al single-substituted crystals with the same T-c show a similar H-c2(T) behavior despite the different substitution level.