Mossbauer Study Of Iron-Based High-Tc Materials

The recent discovery of superconductivity (SC) at Tcs up to55K in iron-based superconductors has triggered enormous interest in this class of superconducting materials. Like the cuprates, SC emerges upon doping carriers into an antiferro-magnetic (AFM) parent compound, while the structural and AFM phase transitions are suppressed. Initial characterization has revealed that magnetism and SC are in-timately coupled with each other; and it is widely believed that the dynamic spin fluctuations associated with the gradual suppression of the AFM order are crucial for the observed SC in these materials. However, consensus regarding the relation-ship between SC and magnetism and even the nature of the AFM phase transition itself is still under highly debate. Detailed studies on the magnetic properties and the phase transitions found in these materials may contribute to address these prob-lems and thus shed lights on the understanding of the pairing mechanism in these materials.In this thesis we have studied various kinds of iron-based superconductors, namely the SrFeAsF and AeFe2As2(Ae=Ca, Sr) parent compounds and the La-doped Ca1-xLaxFe2As2(x=0.2,0.3) and K0.84Fe1.99Se2superconductor, by using Moss-bauer spectroscopy. Following is an enumeration of the detailed results we have obtained.1. SrFeAsF: The relationship between spin, electron, and crystal structure has been one of the foremost issues in understanding the superconducting mech-anism since the discovery of iron-based high temperature superconductors. Here, in chapter4, we present Mossbauer and first-principles calculations studies of the parent compound SrFeAsF, which has the largest tempera-ture gap (-50K) between the structural and antiferromagnetic (AFM) tran-sitions. Our results reveal that the structural transition has little effect on the electronic structure of the compound SrFeAsF while the development of the AFM order induces a redistribution of the charges near the Fermi level.2. AeFe2As2: In chapter5, we present detailed Mossbauer spectroscopy studies of structural and magnetic properties of the undoped parent compounds AeFe2As2(Ae=Ca, Sr) samples. By fitting the temperature dependence of the hyperfine magnetic field we show that the magneto-structural phase tran-sition is clearly first-order in nature for these two compounds. Within the Landau's theory of phase transition, we further argue that the observed phase transition may stem from the strong magneto-structural coupling effect. Tem-perature dependence of the Lamb-Mossbauer factor shows that the paramag-netic phase and the antiferromagnetic phase exhibit similar lattice dynam-ics for CaFe2As2compound in high frequency modes with very close De-bye temperatures.(?)D~270K. Interestingly, for the SrFe2As2compound,(?)D=404K and290K for the segment below and above the phase transition region were found, which we argue might due to different inter-layer cou-plings.3. Ca1-xLaxFe2As2: In chapter6, we report Mossbauer studies of La-doped Ca1-xLaxFe2As2(x=0.2,0.3) superconductors. The spectral line width broadens significantly below~60K, which we attribute to the appearance of short range magnetic fluctuations by analyzing together with the temperature dependence of other hyperfine parameters. We have further interpreted the temperature dependence of the main component of the electric field gradient Vzz using the spin-orbital picture from which the tetragonal splitting of the t2g orbitals are determined to be△=17(2) and19(3)meV. These findings provide information on understanding the nature of the peculiar supercon-ductivity found in these compounds and suggest the importance of the orbital ordering in these iron-based superconductors.4. K0.84Fe1.99Se2: In Chapter7, Mossbauer spectroscopy was used to probe the site specific information of the K0.84Fe1.99Se2superconductor. A spin exci-tation gap,△E≈5.5meV, is observed by analyzing the temperature depen-dence of the hyperfine magnetic field (HMF) at the iron site within the spin wave theory. Using a simple model suggested in the literature, the tempera-ture dependence of the HMF is well reproduced, suggesting that, below room temperature, the alkali metal intercalated iron-selenide superconductors can be regarded as ferromagnetically coupled spin blocks that interact with each other antiferromagnetically to form the observed checkerboard-like magnetic structure.