Critical magnetic field H c2 and electron scattering in MgB2

The correlation between the upper critical field H _c2 and the residual resistivity ρ of the MgB₂ compound is studied in a wide range of ρ values. The slope ?dH _c2/dT of the temperature dependence of H _c2 near T _c is found to increase steadily with increasing residual resistivity to 100 μΩ cm. Over the range from 0 to 50 μΩ cm, the dependence of ?dH _c2/dT on ρ is fitted well by a linear function, which is typical of a single-band superconductor. The fit is performed using the electronic parameters of the two bands (π and σ) that form the Fermi surface. The field H _c2 is assumed to depend on the electronic parameters of the σ band only. Using this approximation, the following quantities are estimated: the relative contribution of σ electrons to the total conductivity along the boron planes (which turns out to be about 1/2), the ratio of the mean free paths of electrons in the σ and π bands l _σ/l _π ≈ 1.5, and the ratios between some other parameters describing electron scattering.