Critical fields, thermally activated transport, and critical current density of β -FeSe single crystals

We present critical fields, thermally activated flux flow (TAFF), and critical current density of tetragonal phase β-FeSe single crystals. The upper critical fields Hc2(T) for H∥(101) and H⊥ (101) are nearly isotropic and are likely governed by the Pauli limiting process. The large Ginzburg-Landau parameter κ~72.3(2) indicates that β-FeSe is a type-II superconductor with a smaller penetration depth than in Fe(Te, Se). The resistivity below Tc follows Arrhenius TAFF behavior. For both field directions below 30 kOe, single-vortex pinning is dominant, whereas collective creep becomes important above 30 kOe. The critical current density Jc from M-H loops for H∥(101) is about five times larger than for H⊥(101), yet much smaller than in other iron-based superconductors.