Longitudinal conductivity in Si/SiGe heterostructure at integer filling factors

We have investigated temperature dependence of the longitudinal conductivity $sigma_{xx}$ at integer filling factors $ u =i$ for Si/SiGe heterostructure in the quantum Hall effect regime. It is shown that for odd $i$, when the Fermi level $E_{F}$ is situated between the valley-split levels, $Delta sigma_{xx}$ is determined by quantum corrections to conductivity caused by the electron-electron interaction: $Deltasigma_{xx}(T)sim ln T$. For even $i$, when $E_{F}$ is located between cyclotron-split levels or spin-split levels, $sigma_{xx}sim exp[-Delta_{i}/T]$ for $i=6,10,12$ and $sim exp [-(T_{0i}/T)]^{1/2}$ for $i=4,8$. For further decrease of $T$, all dependences $sigma_{xx}(T)$ tend to almost temperature-independent residual conductivity $sigma_{i}(0)$. A possible mechanism for $sigma_{i}(0)$ is discussed.