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Abstract

In layered superconductors the order parameter may be modulated within the unit cell, leading to nontrivial modifications of the vortex core if the interlayer coherence length ξ_c(T) is comparable to the interlayer spacing. In the iron pnictide SmFeAs(O,F) (T_c≈50  K) this occurs below a crossover temperature T^⋆≈41  K, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around T^⋆, hybrid vortices between these two characteristics appear. Only in this region around T^⋆ and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the c-axis critical current j_c(H) periodic in 1/√H due to a delicate balance of intervortex forces and interaction with the layered potential. j_c(H) shows pronounced maxima when a hexagonal vortex lattice is commensurate with the underlying crystal structure. The narrow temperature window in which oscillations are observed suggests a significant suppression of the order parameter between the superconducting layers in SmFeAs(O,F), despite its low coherence length anisotropy (γ_ξ≈3–5).