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The effect of Li substitution for Mg and of Li-C co-substitution on thesuperconducting properties and crystal structure of MgB2 single crystals hasbeen investigated. It has been found that hole doping with Li decreases thesuperconducting transition temperature Tc, but at a slower rate than electrondoping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li forMg and C for B decreases more than in the case where C is substituted alone.This means that holes introduced by Li cannot counterbalance the effect ofdecrease of Tc caused by introduction of electrons coming from C. The possiblereason of it can be that holes coming from Li occupy the pi band whileelectrons coming from C fill the sigma band. The temperature dependences of theupper critical field Hc2 for Al and Li substituted crystals with the same Tcshow a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite ofmuch different substitution level. This indicates that the mechanismcontrolling Hc2 and Tc is similar in both hole and electron doped crystals.Electrical transport measurements show an increase of resistivity both in Lisubstituted crystals and in Li and C co-substituted crystals. This indicatesenhanced scattering due to defects introduced by substitutions includingdistortion of the lattice. The observed behavior can be explained as a resultof two effects, influencing both Tc and Hc2. The first one is doping related tothe changes in the carrier concentration, which may lead to the decrease or tothe increase of Tc. The second one is related to the introduction of newscattering centers leading to the modification of the interband and/orintraband scattering and therefore, to changes in the superconducting gaps andto the reduction of Tc.

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