First-Principles Study Of 122 Iron-based Superconductors Under Pressure

Superconductors have attracted much attention due to its wide practical application and intrinsic quantum properties. People hope to understand the mechanism of high temperature superconductors through continuous exploration. The iron-based high Tc superconductors discovered in 2008 are the second kind of high Tc superconductors since cuprate superconductors discovered, which gave new vision and idear to investigate the mechanism of high temperature superconductivity. From present research one can see that the superconducting phase appears when the magnetic order is suppressed by application of physical pressure or doping in a general way. In this work, we investigate the effects of physical pressure and doping on the crystal structure, magnetism, charge transfer and metalllicity of 122-type iron-based superconductor by using first-principles calculation. The main experimental results can be concluded as follows:(1) AFe2As2(A=Ca,Sr,Ba) with antiferromagnetic structure undergo tetragonal to Drthorhombic structural phase transition, accompanied with Fe local magnetic moment disappeared at 2.8GPa,8GPa and 13GPa respectively, at the same time, their metallicity increased. Pressure suppresses electrons transfer from Ba and Fe to As and the suppression increased at phase transition point. For AFe2As2(A=Ca,Sr,Ba), Fe local magnetic moment is sensitive to lattice constants a and b, electron transfer is sensitive to lattice constants c.(2) Pressure studies have been performed on BaMn2As2 and BaMn2P2, at pressue of similar to 8GPa and 7.5GPa insulator-to-metal transition are seem to occur in BaMn2As2 and BaMn2P2 respectively. At pressue of similar to 59GPa and 41GPa, BaMn2As2 and BaMn2P2 undergo tetragonal to collapsed-tetragonal structural phase transition. The metallicity of BaMn2As2 and BaMn2P2 increase slowly as pressure increase before structural phase transition, but increase rapidly after structural phase transition. It means that the metallicity is increased when the magnetic moments are suppressed in BaMn2As2 and BaMn2P2 in which the 3d electrons of Mn play an important role.(3) We then find the stable crystal structure of the Mn-doped BaFe2-xMnxAs2 and calculate their formation energies and cohesive energies. The results show that BaFe2-xMnxAs2 are probably thermodynamically stable. There is no interplay between Mn-3d and Fe-3d in BaFeMnAs2 which characterized by a layered Ba-FeAs-Ba-MnAs-Ba systems. Electronic density of states at Fermi level of BaFeMnAs2 is donated by Fe-3d but not Mn-3d, in other words, the metallicity of FeAs-layer is better than MnAs-layer in BaFeMnAs2.(4) Our research shows that the stable magnetic structure of tetragonal CaCr2As2, as well as SrCr2As2, is G-type. We also calculate and analyze the elastic constants, bulk modulus and shear modulus of XCr2As2(X=Ca,Sr,Ba). The results shows that XCr2As2(X=Ca,Sr,Ba) are all mechanically stable. Furthermore, by calculating the ratio of bulk modulus to shear modulus(the value of B/G) we find that XCr2As2(X=Ca,Sr,Ba) shows ductileness and ductileness order is CaCr2As2, SrCr2As2, BaCr2As2. Pressure studies also have been performed on XCr2As2(X=Ca,Sr,Ba), at pressue of similar to 16GPa,32GPa and 24GPa tetragonal to collapsed-tetragonal structural phase transition are seemed to occur in XCr2As2(X=Ca,Sr,Ba) and accompanied with Cr local magnetic moments abruptly change.