Critical current oscillations in the intrinsic hybrid vortex state of SmFeAs(O,F)

Abstract

In layered superconductors the order parameter may be modulated within the unit cell, leading to non-trivial modifications of the vortex core if the interlayer coherence length $ξ_c(T)$ is comparable to the interlayer distance. In the iron-pnictide SmFeAs(O,F) ($T_c \approx 50$K) this occurs below a cross-over temperature $T^\star \approx 41$K, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around $T^\star$, hybrid vortices between these two characteristics appear. Only in this region around $T^\star$ and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the c-axis critical current $j_c(H)$ periodic in $\frac{1}{\sqrt{H}}$ 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 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 ($γ_ξ\approx 3-5$).