Anisotropy of upper critical fields and thermally-activated flux flow of   quenched KxFe2-ySe2 single crystals
  We report the anisotropy of the upper critical fields mu0Hc2(T) and thermally-activated flux flow (TAFF) behavior of quenched KxFe2-ySe2. Even though the post-annealing and quenching process enhances the superconducting volume fraction, it has a minor effect on the upper critical fields for H//c and H//ab. Analysis of the angular-dependence of resistivity rho_ab(theta,H) indicates that it follows the scaling law based on the anisotropic Ginzburg-Landau (GL) theory and the anisotropy Gamma(T) increases with decreasing temperature with Gamma(T) ~ 3.6 at 27 K. The resistivity of quenched sample exhibits an Arrhenius TAFF behavior for both field directions. Field dependence of thermally activated energy U0(H) implies that the collective flux creep is dominant in high fields and point defects are the main pinning source in this regime. 
Hechang Lei and C. Petrovic
cond-mat.supr-con cond-mat.mtrl-sci