Diamagnetic vortex barrier stripes in underdopedBaFe2(As1−xPx)2

Abstract

We report magnetic force microscopy (MFM) measurements on underdoped BaFe2(As1−xPx)2 (x = 0.26) that show enhanced superconductivity along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel superconducting vortices and act as barriers for them to cross. The width of the stripes is hundreds of nanometers, on the scale of the penetration depth, well within the inherent spatial resolution of MFM and implying that the width is set by the interaction of the superconductor with the MFM's magnetic tip. Unlike similar stripes observed previously by scanning SQUID in the electron doped Ba(Fe1−xCox)2As2, the stripes in the isovalently doped BaFe2(As1−xPx)2 disappear gradually when we warm the sample towards the superconducting transition temperature. Moreover, we find that the stripes move well below the reported structural transition temperature in BaFe2(As1−xPx)2 and that they can be much denser than in the Ba(Fe1−xCox)2As2 study. When we cool in finite magnetic field we find that some vortices appear in the middle of stripes, suggesting that the stripes may have an inner structure, which we cannot resolve. Finally, we use both vortex decoration at higher magnetic field and deliberate vortex dragging by the MFM magnetic tip to obtain bounds on the strength of the interaction between the stripes and vortices. We find that this interaction is strong enough to play a significant role in determining the critical current in underdoped BaFe2(As1−xPx)2.

Large-scale modulation in the superconducting properties of thin films due to domains in theSrTiO3substrate

“…Due to the higher superfluid density, vortices avoided pinning on the twin boundaries [32]. Similar behavior was observed in another pnictide, BaFe 2 (As 1−x P x ) 2 [33]. The more typical behavior, observed in other twinned single-crystal superconductors (e.g., YBCO), is pinning of vortices on the boundaries [34][35][36][37][38][39].…”

Visualization of the magnetic flux structure in phosphorus-doped EuFe2As2 single crystals

“…Individual vortices could not be resolved with decoration since the expected magnetic flux density within domains is about 0.9 T (M s = 714 cgs units/cm 3 ) at liquid helium temperatures, 25 whereas the resolution of the decoration method is limited by 0.2 T. 37 The spatial resolution of MFM also cannot identify individual vortices if the local magnetic flux density in domains is much higher than 10 mT. 38,39 At the same time, the fine structure of domains, which is shown in Fig. 3(b), can be explained within the framework of domain branching in ferromagnets.…”

“…38,39 At the same time, the fine structure of domains, which is shown in Fig. 3(b), can be explained within the framework of domain branching in ferromagnets.…”

Non-Gaussian tail in the force distribution: a hallmark of correlated disorder in the host media of elastic objects

“…the particle-particle) interaction force is compensated by the vortex-pinning interaction. Therefore, the spatial distribution of particle-particle interaction force allows for the estimation of local pinning forces at the temperature at which the snapshot was captured [48][49][50][51][52][53][54] . In our experiments we obtain these snapshots by decorating vortex positions with magnetic nanoparticles after following a field-cooling procedure (see "Methods" for technical details).…”