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

本文用热力学和输运实验研究了RFeAsO（R=La，Ce，Pr，Nd，Sm，Gd）和AFe_2As_2（A=Ca，Sr，Ba，Eu）型铁磷属化合物的超导和磁性。压力调谐提供了具体改变基态性质的可能性。在我们的研究中，我们将集中在含稀土元素的铁磷属元素化物材料的研究。特别是，我们将研究两种化合物CeFeAsO和EuFe 2As 2。掺杂系列CeFeAs 1-x PxO将CeFePO中存在的Kondo物理学和铁磷族化合物超导体的物理学联系起来。P取代As导致晶胞体积收缩，但引入无序。我们的压力实验允许研究高品质的单晶CeFeAsO的压力-温度相图，而没有额外的无序的影响。我们特别感兴趣的将是与量子临界点的存在相关的量子临界行为的可能存在。在EuFe 2As 2中，Eu 2+的局部磁性和超导性表现出一种特殊的相互作用，类似于在可重入超导体中观察到的行为。这种不寻常行为的一个原因可能与磁有序温度和超导转变温度的接近有关。通过使用不同的测量技术，并结合压力和掺杂，我们将研究磁性和超导性在这个家庭的材料的相互作用。

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.