NP^1,2, A. Magalotti^1,2, A. Alimenti^1,2, V. Braccini³, P. Manfrinetti⁴, K. Torokhtii¹, E. Silva^1,2

¹ Dept. of I.E.M. Engineering, University Roma Tre, Italy
² INFN Sezione Roma Tre, Italy
³ CNR-SPIN, Genova, Italy
⁴ Dept. of Chemistry and Industrial Chemistry, University of Genoa, Italy

In Iron Based Superconductors, strong Pauli limiting of the upper critical field H_c2(T) prevents directly measuring the orbital upper critical field H_c2^orb(T) [1]. Therefore, the anisotropy γ measured in different magnetic field ranges gives varying results, from quasi-isotropic γ~1 to γ > 2. In this communication we exploit the unique features of the field-dependent microwave complex resistivity: the intrinsic flux-flow resistivity ρ_ff can be isolated from vortex pinning effects, that are dominating in dc. We perform high frequency measurements of ρ_ff in epitaxial FeSe_0.5Te_0.5 films in static fields up to 12 T. With the ansatz that ρ_ff depends on a single field scale in sufficiently wide ranges of magnetic fields, we are able to scale ρ_ff(H,T) at low fields, μ₀H<1 T, where Pauli limiting is not effective, with the orbital reduced field: ρ_ff(H,T) = ρ_ff[H/H_c2^orb(T)]. We find that H_c2^orb(T) follows closely the two-band theory for H_c2(T) [2], and γ~2. By contrast, at high fields μ₀H > 4 T scales with the Pauli-limited upper critical field H_c2, ρ_ff(H,T) = ρ_ff[H/H_c2^p(T)].

[1] C. Tarantini et al., Phys. Rev. B 84, 184522 (2011)
[2] A. Magalotti et al., Supercond. Sci. Technol. 39, 035018 (2026)