Interplay between magnetism and superconductivity in high Tc Superconductors

Abstract

The effect of antiferromagnetic order on superconducting order parameter ( D) in newlinemultilayer cuprate superconductors has been studied by using double time retarded newlineGreen s function technique, and expression for D has been obtained. The newlineexpression is found to be depending on the antiferromagnetic order parameter ( f) newlinealong with temperature (T). It has been found that at low temperatures newlinesuperconducting order parameter decreases linearly with increasing newlineantiferromagnetic order parameter. But at the temperature close to transition temperature (TC), this linearity violated and uperconducting order first decreases newlinesuddenly and then there is gradual decrease with increasing antiferromagnetic order. This shows that at near to transition temperature the area of co-existence of antiferromagnetism and superconductivity decreases. Results also show that for newlinecuprates having TC below 35 k, antiferromagnetic order co-exist with newlinesuperconductivity and could be helpful for superconductivity. The magnetic properties of LnOFeAs system has also been studied using newlineanisotropic Heisenberg model with three in-plane (J1, J2, J3) and one out-of-plane newline(J ^) exchange interactions. Starting with stripe configuration with two newlineinterpenetrating sublattices such that the spins are always oppositely aligned in newlinetwo sublattices, the expression for sublattice magnetization and Néel Temperature newline(TN) has been obtained using Green s function technique within random phase newlineapproximation (RPA). Using the expression for Néel temperature, variation of TN with J ^ has been studied. The effect of J ^ on the sublattice magnetization has been newlinealso studied. For realistic values of exchange coupling constants, Néel newlineTemperature of LnFeAsO system (Ln = La, Pr and Ce) has been evaluated. The theoretical results are compared with experimental results and found in good agreement.