Magnetic and thermodynamic properties of iron based superconductors

In this work specific heat and magnetic susceptibility studies on the iron pnictide superconductors RFeAsO1-xFx (R = La, Ce, Pr, Sm, Gd), M(Fe1-xCox)2As2 (M = Ca, Ba) and AFeAs (A = Li, Na) are applied to reveal their electronic and magnetic properties. Except for LiFeAs, all undoped substances show an antiferromagnetic orthorhombic ground state. Studies on the whole doping series of R = La, R = Sm and M = Ca allow constructing the phase diagrams. At low doping x, the onset of the structural and magnetic transitions are slightly suppressed. For higher doping, there is a superconducting ground state while long range antiferromagnetic order disappears. While in LaFeAsO1-xFx the disappearance of magnetic order for the superconducting ground state is in a abrupt discontinuous manner, there is a pronounced coexistence regime in Ca(Fe1-xCox)2As2. For SmFeAsO1-xFx and PrFeAsO1-xFx a possible energy level scheme for the 4f-states in the crystal electric field, respectively, are derived. An essential result of this work is the discovery of an universal behavior of the high temperature susceptibility. If not masked by the rare earth moments, all pnictides under study show a linear increase of the paramagnetic susceptibility. In particular, an universal gradient of the susceptibility independent of doping, ground state or even pnictide group is found. Within a particular doping series the room temperature susceptibility is also doping independent with its value directly correlated to the As-Fe-As-bond angle.