Electronic Structure Around A Vortex Core In Iron Pnictide Superconductor

Recently, experiment about vortex cores in hole doping iron pnictide superconductors, Ba.0.6K0.4Fe2As2, were researched by Scanning Tunnel Microscope (STM). A negative-bias conductance peak, which indicate a bound state was discovered in the center of vortex-core. The peak splits into two peaks when away from the core center and eventually merges to the gap edges. In this thesis, we performed exact large-scale calculation of the vortex-core electronic structure in effective lattice models with both inphase s-wave (s++) and antiphase s-wave (s+-) pairing. According to the calculation of tight-binding model, we argue that the asymmetric bound state is due to the band-edge effect. The band-edge effect means, the sign of the peak energy is positive (negative) if the Fermi level is near the bottom (top) of the electron or hole bands.In Chapterâ… , history of the superconductor is introduced. From conventional super-conductors to unconventional superconductors, which are cuprate superconductors and the newly discovered based superconductors were introduced in section 1. The pairing mechanism of iron-based superconductors is introduced briefly in section 2.In Chapterâ…¡, the basic principle of STM, and the measure of vortex state are in-troduced. STM is a skill of microscopy based on quantum tunnel effect. The process of measure on superconductor can be regarded as a N-V-S (Normal Metal-Vacuum-Superconductor) tunnel junction, which is dot-surface. And get a respond current with sample-bias voltage from STM measure. We can get the information such as shape, magnitude, pairing symmetry about superconduction energy gap by the result of dI/dV There are two mainly phenomenon in hole doped iron pnictide superconductors by using STM:it appears a peak below the Fermi-energy at the center of vortex core; the peak splits and merges to the gap edges away from the vortex core.In Chapterâ…¢, we introduce the numerical resolution about the electron structure around a vortex core in iron pnictide superconductors by using a effective lattice models with s++ and s+- wave pairing, and get the result in qualitative agreement with the experiment. And the none-zero bias conductor peak is insensitive to the relative phase of the pairing gaps on the multiple bands. Build a simple basic theoretical model by one-band tight-binding model for the research electronic structure in vortex core, and discover that the peak appears positive/negative when Fermi level close to the band top/bottom. Which called band-edge effect. Then, by more theoretical research of iron-based superconductors, such as 2-band model (A),2-band model (B) which break the A-B lattice symmetry,3-band model and 5-band model which respect the LDA band structure most comprehensively, we found that the band-edge effect still exists. Which means the band-edge effect is not a mistake introduced by the simplified band structure. It's the intrinsic property.In Chapterâ…£, we summarize the thesis and the main results.