Spinor Transformation and Antiferromagnetism in Iron Based Superconductors

Abstract

This theory is proposed to account for the spinor transformation (ST), antiferromagnetism (AF) and superconductivity (SC) in the direct d-orbital overlap of iron based superconductors. The ST, shows the gradual transition in the pair representation by Nambu to the Fourier's Green function leading up to SC and AF state. The AF dominant in the band regime, is determining from the two component spinor describing its interaction effect on the band electrons, represented by the spin, Coulomb integral and exchange integral. I. Introduction Naturally the quaternary equi-atomic REOFeAs compounds are rather simple structured containing alternating layers of Fe-As and Re-O layers, where FeAs layers are believed to be responsible for superconductivity. The undoped compound of these systems is not superconducting itself but it exhibits both a structural and magnetic phase transition. This structural phase transition changes the crystal symmetry from tetragonal (space group P4/nmm) to orthorhombic (space group Cmma) and leads to an antiferromagnetical (AF) order with a spin structure (1), This is because the Fe magnetic moments along(1,1) direction are aligned (parallel) while the two nearest neighboring Fe are antiferromagnetically (antiparallel) aligned, similar to the well known antiferromagnetic ordering of the cuprates. The cuprates turns superconducting by introducing impurities that create electrons and holes in the parent compound. Unlike the cuprates, the iron oxypnictides are metallic and the antiferromagnetic parent compound is a Mott insulator. This transition is due to direct orbital interaction between Fe atoms at 285 pm, while there is no such direct d-orbital overlap observed in cuprates(1- 2). It is believed that this is caused by instability of the spin density wave (SDW). The electronic states of iron in LaFeAsO and fluoride doped have been extensively studied in detail by 57Fe Mossbauer spectroscopy (2-3). The 57Fe spectra proved spin ordering in LaFeAsO and its suppression upon doping.The isomer shifts of the arsenide oxides are close to the data observed for the phosphate. Below the antiferromagnetic ordering (T=138 K), LaFeAsO shows full magnetic hyperfine field splitting with a hyperfine field of 4.86 T (2-3). The magnetic moment at the iron atoms was estimated to have values between 0.25-0.35μB/Fe atoms (3).In FeAs based superconductors both the structural and magnetic transition can be suppressed by dopant such as fluorine or with the oxygen deficiency (4). The addition of impurity elements affect superconductivity in the doped system and cause pair interaction in the presence of the applied field. This leads to magnetic spin fluctuation thereby