Critical current density, vortex dynamics, and phase diagram of single-crystal FeSe

We present a comprehensive study of the vortex pinning and dynamics in a high-quality FeSe single crystal which is free from doping-introduced inhomogeneities and charged quasiparticle scattering because of its innate superconductivity. The critical current density Jc is found to be almost isotropic and reaches a value of ∼3 ×104 A/cm2 at 2 K (self-field) for both H-c and ab. The normalized magnetic relaxation rate S (=dlnM/dlnt) shows a temperature-insensitive plateau behavior in the intermediate temperature range with a relatively high creep rate (S∼0.02 under zero field), which is interpreted in the framework of the collective creep theory. A crossover from the elastic to plastic creep is observed, while the fishtail effect is absent for both H-c and ab. Based on this observation, the origin of the fishtail effect is also discussed. Combining the results of Jc and S, the vortex motion in the FeSe single crystal is found to be dominated by sparse, strong pointlike pinning from nanometer-sized defects or imperfections. The weak collective pinning is also observed and proved in the form of large bundles. Besides, the vortex phase diagram of FeSe is also constructed and discussed.