Upper critical field and the effect of Li doping on the activation energy in MgB2

Abstract MgB 2 and Mg 0.93 Li 0.07 B 2 superconductors have been prepared by solid-state reaction. The temperature dependence of electrical resistivity in MgB 2 and Mg 0.93 Li 0.07 B 2 have been measured in magnetic field up to 14 T. They have almost the same superconducting transition temperature T c . The upper critical fields H c2 (0) were determined. The activation energy of MgB 2 and Mg 0.93 Li 0.07 B 2 bulk sample have been calculated from the Arrhenius activation law ρ = ρ 0 exp(− U / k B T ). Compared to MgB 2 , the activation energy of Mg 0.93 Li 0.07 B 2 rises by 25.0%, 20.4% and 16.5% at the magnetic field 14, 10 and 7 T, respectively. A small amount of Li doping in MgB 2 that almost not changes the T c of MgB 2 has raised the activation energy U , especially at high magnetic field.