COEXISTENCE OF SUPER CONDUCTIVITY AND MAGNETISM IN NICKEL SUBSTITUTED CALCIUM POTASSIUM IRON ARSENIDE CaK(Fe1-xNix)4As4 SUPERCONDUCTORS

ABSTRACT Superconductivity and magnetism were previously thought as mutually antagonistic. However, their coexistence had been seen in ternary compounds and some high temperature superconductors. In this thesis the coexistence of superconductivity and magnetism in CaK(Fe1-xNix)4As4 superconductors has been theoretically studied by developing a two band model Hamiltonian on the multiband nature of iron based superconductors. By employing a retarded double time Green function formalism, mathematical expressions for superconducting transition temperature (Tc), magnetic transition temperature (Tm), superconducting order parameter ( ) and magnetic order parameter (M )have been obtained. By using experimental values and considering some plausible approximations in the obtained expression, the phase diagram of superconducting order parameter versus superconducting transition temperature (Tc) in a pure superconducting region, magnetic order parameter (M) versus temperature (T) in a purely magnetic region, superconducting transition temperature (Tc) versus magnetic order parameter (M) and magnetic transition temperature (Tm) versus magnetic order parameter (M) were plotted. The plotted diagram of superconducting transition temperature(Tc ) as a function of magnetic order parameter(M) shows that as the magnetic order parameter increases , superconducting transition temperature decreases. The phase diagram of magnetic transition temperature as a function of magnetic order parameter(M) shows that as magnetic order parameter increases, magnetic transition temperature also increases. The common(intersection) region under the merged phase diagrams of (Tc versus M and Tm versus M) clearly reveals that superconductivity and magnetism coexist in CaK(Fe1- xNix)4As4 superconductors and the obtained findings are in agreement with experimental results.