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First-principle Study of Pressure Effects on the Structural and Magnetic Phase Transitions and the Electronic Structure of AeFe2As2(Ae=Ba,Sr,Ca) Compounds

Aghajani, Mahdieh | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 51945 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Akhavan, Mohammad; Hadipour, Hanif
  7. Abstract:
  8. The ternary 122-type AeFe2As2 (Ae=Ba,Sr,Ca) compounds are antiferromagnetic metals with the orthorhombic lattice structure at ambient pressure and low temperature. By applying mechanical pressure, structural, magnetic and superconducting transitions occur in these compounds. The isovalent doping of Ba with Sr and Ca elements induces chemical pressure and reduces the onset pressure of these transitions. The main purpose of this thesis is to investigate and compare the effects of mechanical and chemical pressures on the structural and magnetic phase transition in the 122 compounds. To achieve this purpose, we have done first-principle calculations for many properties such as the structural properties, the magnetic moments of Fe atoms, the electron charge density, the total, partial and k-resolved density of states, the band structures, the Fermi surfaces, the onsite Coulomb interaction parameters and the electron correlations of the Fe atoms by using the density functional theory and the constrained random phase approximation. We have found a magnetic transition at the same primitive unit cell volume, around 81 Å3 for the (Ba\Ca)Fe2As2 compounds, which predicts a magnetic transition pressure of 12 GPa for SrFe2As2. The structural parameters of the Fe-As layer and the FeAs4 tetrahedra are obtained after ionic relaxation and compared with the existing experimental results. The change of these internal parameters is ascribed to pressure-induced charge transfer, i.e., as the As-Ae bonds absorb electrons, the Fe-As bonds are depleted. The changes of the density of states, the band structure and the hole- and electron- pockets of Fermi surface by pressure have been discussed. The number of hole carriers increases by pressure in these compounds. The different screening channels, as discussed in four different models (eg, t2g, d and pd models) affect significantly on the Hubbard U while the Hund J remains almost unchanged. The average onsite bare and partially and fully screened Coulomb interactions increase with different rates upon compression. These different rates can be explained by competition between the electronic screening and reduction of bond lengths. Our results show that the 122 compound are classified as moderately correlated materials and the electronic correlations decreases by mechanical pressure
  9. Keywords:
  10. 122-type Iron Based Compounds ; Mechanical, Chemical and Thermal Effects ; Density Functional Theory (DFT) ; Constrained Random Phase Approximation ; Electron Structure ; Coulomb Interaction Parameters ; Structural Properties ; Magnetic Properties

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