The Mechanical Property And The Microscopic Mechanism Of Its Effect And Regulation On Superconductivity Of Iron-based FeSe

Because of their unique electromagnetic characteristics,superconductors are widely used in strong magnetic field and electric power transport.However,their lower critical temperature limits the wider use.The emergence of high-temperature superconductors greatly increases the critical temperature,and the iron-based superconductors have the advantages of cheap raw materials and higher critical temperature.Because of high magnetic field,variable temperature from the fabrication and operating situation,the iron-based superconductors are susceptible to strain and temperature effects.The mechanical deformation affects not only the crystal structure of the superconductors,but also their superconductivity and even leads to the destruction of the crystal structure.In addition,the hydrostatic pressure and strain effects can enhance the critical temperature of FeSe.Therefore,the research on the influence of mechanical deformation on the properties of FeSe and the mechanical control of its superconducting properties has become a hot topic in the field of superconductivity.This thesis focuses on the influence of mechanical deformation on the crystal structure,electronic properties and superconductivity of FeSe.The changes of its crystal structure,elasticity,electronic property and superconductivity under mechanical deformation by combining microscopic theory and first-principles calculation have been systematically studied.1.Owing to the layered structures of FeSe bulk and thin films,they are prone to structural failure between layers.The crystal structures of FeSe bulk and thin film are constructed by first-principles method,and the crystal structure failure process is simulated under the effect of tensile and shears strain.The tensile stress-strain curve in the [001] direction of FeSe shows that the ideal tensile strength is about 2.3GPa,and its structure is damaged when the tensile strain is 11%.When the tensile strain of FeSe is 26% in the direction [100],the ideal tensile strength is about 26 GPa,indicating that FeSe is prone to have the tensile failure in the direction [001].The tensile stress-strain curve in the [001] direction of FeSe/STO superconducting thin film shows that the stress value reaches the maximum value when the tensile strain is 6%,and the ideal tensile strength is 0.85 GPa.When the tensile strain is 11%,the crystal structure completely breaks down,the ideal shear strength of FeSe/STO superconducting thin film crystal structure is 3.12 GPa,and the stress reaches its maximum value when the shear strain is 17.78%.2.In view of the pressure and temperature effect on the crystal structure and elastic properties of FeSe,the changes of the crystal structure and elastic properties of FeSe are analyzed by combining the first-principles method and quasi-harmonic approximation theory.The three-dimensional diagram of the volume of FeSe under the effect of pressure and temperature is given.The results show that the lattice constants of FeSe decrease with the increase of pressure,and its elastic moduli increase with the increase of pressure.The three-dimensional elastic moduli of FeSe show that the volume modulus has the lowest anisotropic and the Young's modulus has the highest anisotropic.The elastic moduli increase under the pressure effect,but their anisotropy changes a little.The lattice constants of FeSe increase with the temperature,while its elastic moduli decrease under the effect of temperature.The volume of FeSe decreases with the increase of pressure,while its volume increases with the increase of temperature.The three-dimensional volume figure of FeSe exhibits the volume variation under the effect of an arbitrary pressure and temperature.3.The critical temperature of FeSe increases significantly with pressure,and the critical temperature can be adjusted by changing the carrier concentration.Based on the FeSe self-doping effect under pressure,the critical temperature model of FeSe under pressure is proposed by combining the carrier doping theory and first-principles in this paper.Based on the FeSe critical temperature model developed in this paper,the change trend of FeSe/STO superconducting film's critical temperature with pressure effect is analyzed additionally.According to the Luttinger theory,the FeSe electron carrier concentration is estimated,and the predicted values are in good agreement with the experimental results.Based on the self-doping model of critical temperature,the maximum critical temperature of FeSe is obtained when the pressure is 8.6GPa,which is in good agreement with the experimental results.The Luttinger theory is used to estimate the electron carrier concentration of FeSe/STO superconducting thin film crystal structure under different pressures,and it is found that the carrier concentration reached to the optimal value when the pressure is zero.With pressure effect,the critical temperature of FeSe/STO superconducting film decreases with the increase of pressure.4.Due to the lattice constant mismatch between the base material crystal and the of FeSe superconducting thin film material,FeSe is vulnerable to the effect of xy plane strain.The electron-phonon coupling of FeSe under plane tension and pressure is analyzed by the electron-phonon coupling theory,the variation trend of FeSe critical temperature is estimated by the McMillan formula.The role of magnetic effect of FeSe superconductivity is analyzed additionally.The plane compressive strain increases the maximum phonon frequency,while the tensile strain decreases its phonon frequency of FeSe.The magnetic affection plays an important role on superconductivity of FeSe.The electron-phonon coupling coefficient of FeSe increases with the increase of plane compressive strain,and its critical temperature also increases significantly under the compressive strain.The results show that the critical temperature of FeSe can be increased by the compressive strain,while the critical temperature can be decreased by the tensile strain.In this thesis,the mechanical properties and their effects on the superconducting properties of FeSe are numerically studied based on the microscopic theory and first-principles calculation method,and some meaningful results are obtained.The study provides an analytical approach to understand the relationship between the superconducting characteristics and mechanics and the feasibility of strain regulation of superconductivity of FeSe.The relevant results can also provide a guidance for the research and development of superconductors and their engineering applications.