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

Abstract

We present critical fields, thermally activated flux flow (TAFF), and critical current density of tetragonal phase $\ensuremath{\beta}$-FeSe single crystals. The upper critical fields ${H}_{c2}(T)$ for $H\ensuremath{\parallel}$(101) and $H\ensuremath{\perp}$ (101) are nearly isotropic and are likely governed by the Pauli limiting process. The large Ginzburg-Landau parameter $\ensuremath{\kappa}\ensuremath{\sim}72.3(2)$ indicates that $\ensuremath{\beta}$-FeSe is a type-II superconductor with a smaller penetration depth than in Fe(Te, Se). The resistivity below ${T}_{c}$ 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 ${J}_{c}$ from $M$-$H$ loops for $H\ensuremath{\parallel}$(101) is about five times larger than for $H\ensuremath{\perp}$(101), yet much smaller than in other iron-based superconductors.