Transport properties of MgB2 single crystals doped with electrons and holes

The thermoelectric power of C, Mn. C:Li, and Al:Li substituted MgB2 single crystals has been investigated in the temperature range 10-300K. Both the in-plane (S-ab) and the out-of-plane (S') thermopowers are positive for the non-substituted crystal and both S-ab and S, change a sign for crystals doped with electrons where C is substituted for B in the amount larger than 5 at%. When Li is substituted for Mg, the pi band rather than the sigma band is doped with holes and the doping effects are much more subtle. The anisotropy ratio of the non-substituted crystal Sab/Sc approximate to 3 and this ratio is strongly reduced by the substitution of C. Isovalent magnetic Mn ions, which Substitute for Mg with a drastic reduction of T-c, do not influence the values and temperature dependences of both S-ab and S, (C) 2008 Elsevier Ltd. All rights reserved.