טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentYagil Alon
SubjectDiamagnetic Vortex Barrier Stripes in Underdoped
BaFe2(As1-xPx)2
DepartmentDepartment of Physics
Supervisor Dr. Ophir Auslaender
Full Thesis textFull thesis text - English Version


Abstract

We report magnetic force microscopy (MFM) measurements on underdoped BaFe2-(As1-xPx)2 (x=0.26), which 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 it is set by the superconductor interaction with the magnetic tip. Unlike similar resolution limited stripes observed previously by scanning SQUID in the electron-doped compound, Ba(FexCo1-x)2As2, the stripes in the isovalently-doped compound, BaFe2(As1-xPx)2, disappear gradually when we warm the sample towards the superconducting transition temperature. Furthermore, the stripes we report are spatially denser than those in the Ba(FexCo1-x)2As2 study. Surprisingly, we find that the stripes move well below the reported structural transition temperature in BaFe2(As1-xPx)2. When we cool in a 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 a higher magnetic field and deliberate vortex dragging by the MFM magnetic tip to obtain quantitative 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 BaFe2(As1-xPx)2.