טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLamhot Yuval
SubjectLocal Characterization of Superconductivity's
Dependence on Doping in the Pnictide
BaFe2(As1-xPx)2
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Dr. Ophir Auslaender
Full Thesis textFull thesis text - English Version


Abstract

Recently, measurements on the iron pnictide BaFe2(AsxP1-x)2 revealed a peak in the superconducting penetration depth (λab) for  T<<TC near optimal doping, where Tc, the superconducting transition temperature, is highest(Matsuda et al., Science 2012). Such a peak suggests that a quantum critical point may be present within the superconducting dome. While these macroscopic measurements were able to characterize sample averaged behavior, superconductivity has not been characterized by a local probe in BaFe2(AsxP1-x)2. In particular the doping dependence of both the local λab and and the local TC remain unclear, as is how they relate to each other on a local level.

In this work we perform local measurements of superconductivity in the compound of BaFe2(AsxP1-x)2 My work focuses on the measurement of λab and TC as well as the nematic behavior. For the measurements I used magnetic force microscopy (MFM) at temperatures down to 4.2 K. An important part of my work was to develop an improved model for the superconductor-microscope interaction and a new data-fitting process. The new model is better suited for the measurement system and takes into account all the relevant length-scales. The new fitting procedure gives more robust and precise results .

In my measurements I used a range of BaFe2(AsxP1-x)2 dopings spanning the entire superconducting dome. By studying the Meissner repulsion of the magnetic MFM tip from the sample I obtained the absolute value of the local λab as well as the local value of TC. Furthermore, I also used the interaction between the tip and superconducting vortices in order to find correlated defects such as domain walls or twin boundaries (TB) that can accompany nematic phases which may exists in the BaFe2(AsxP1-x)2 family. By mapping the phase diagram, I showed the appearance of a peak in the local λab at optimal doping and a divergence of λab at the underdoped edge of the superconducting dome. Finally, I showed that in mildly underdoped samples clear lines of vortices are formed and that verotex pinning is very weak in very underdoped samples. I was able to determine that these lines are in the direction of TB correlated defects if present, thus indicating that these samples are in a mixed spin density wave and superconductive state.