Anomalous flux flow resistivity in the two-gap superconductor ${\mathrm{MgB}}_2$

The flux flow resistivity ${\rho }_f$ associated with purely viscous motion of vortices in high-quality ${\mathrm{MgB}}_2$ was measured by microwave surface impedance. Flux flow resistivity exhibits unusual field dependence with strong enhancement at low field, which is markedly different to conventional s-wave superconductors. A crossover field which separates two distinct flux flow regimes having different ${\rho }_f$ slopes was clearly observed in $\mathit{H}\Vert \mathrm{ab}$ plane. The unusual H dependence indicates that two very differently sized superconducting gaps in ${\mathrm{MgB}}_2$ manifest in the vortex dynamics and almost equally contribute to energy dissipation. The carrier scattering rate in two different bands is also discussed with the present results, compared to heat-capacity and thermal-conductivity results.