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

TL;DR

This study reveals oscillations in the critical current of SmFeAs(O,F) superconductors near a specific temperature, caused by hybrid vortex states and lattice interactions, highlighting complex layered superconductor behavior.

Contribution

It uncovers the existence of hybrid vortex states and critical current oscillations in SmFeAs(O,F) within a narrow temperature range, advancing understanding of layered superconductor vortex dynamics.

Findings

Critical current oscillations are observed near T* in SmFeAs(O,F).

Oscillations are periodic in 1/√H and linked to vortex lattice commensurability.

A narrow temperature window indicates suppressed interlayer order parameter.

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 $\xi_c(T)$ 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 ($\gamma_\xi \approx 3-5$).