Microwave Surface Impedance Measurements of the Electronic State and   Dissipation of Magnetic Vortices in Superconducting Iron-Based LiFeAs Single   Crystals
  LiFeAs is one of the iron-based superconductors having multiple gaps with the possible sign reversal. To clarify how those novel natures affect the energy dissipation of magnetic vortices, we investigated the microwave surface impedance of LiFeAs single crystals under finite magnetic fields. The flux-flow resistivity enhanced rapidly at low magnetic fields, which is similar to the case of MgB$_{2}$. This is probably the consequence of the multiple-gap nature and the gap anisotropy. This suggest that the sign-reversal is not important for the flux-flow even for multiple-gap superconductors. As for the electronic state, the vortex core of LiFeAs turned out to be "moderately clean". Furthermore, the mean free path inside the vortex core was much shorter than that outside, and was close to the core radius. These results strongly suggest a process specific to the core boundary is important for a scattering mechanism inside the vortex core. 
T. Okada, H. Takahashi, Y. Imai, K. Kitagawa, K. Matsubayashi, Y.   Uwatoko, A. Maeda
cond-mat.supr-con