Investigations in the interplay between magnetism and superconductivity in the iron-pnictides by high-pressure studies

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The superconducting and magnetic properties of the iron-pnictide compounds of RFeAsO (R=La, Ce, Pr, Nd, Sm, Gd) and AFe2As2 (A=Ca, Sr, Ba, Eu) type will be studied by thermodynamic and transport experiments under hydrostatic pressure. Pressure tuning offers the possibility of changing the ground-state properties specifically. In our investigations we will focus on the study of ironpnictide materials containing a rare-earth element. In particular, we will study the two compounds CeFeAsO and EuFe2As2. The doping series CeFeAs1-x PxO connects the Kondo physics present in CeFePO and the physics of the iron-pnictide superconductors. The substitution of As by P leads to a contraction of the unit-cell volume, but introduces disorder. Our pressure experiments allow studying the pressure-temperature phase diagram of CeFeAsO on high quality single crystals without the additional influence of disorder. Our special interest will be on the possible presence of quantum critical behaviour associated with the existence of quantum critical points. In EuFe2As2 the local magnetism of the Eu2+ and superconductivity show a peculiar interplay resembling the behaviour observed in re-entrant superconductors. One reason for this unusual behaviour might be related to the closeness of the magnetic ordering temperature and the superconducting transition temperature. By using different measurement techniques and by combining pressure and doping we will investigate the interaction of magnetism and superconductivity in this family of materials.