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

本文通过热力学实验和静压输运实验研究了RFeAsO(R=La，Ce，Pr，Nd，Sm，Gd)和AFe2As2(A=Ca，Sr，Ba，Eu)型铁锡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，我们将重点研究含稀土元素的铁镍碳化物材料。特别是，我们将研究两种化合物CeFeAsO和EuFe2As2。掺杂系列CeFeAs1-xPxO将CeFePO中存在的近藤物理与铁-磷酸盐超导体的物理联系在一起。用P取代As会导致单位细胞体积的收缩，但会带来无序。我们的压力实验允许研究CeFeAsO在高质量单晶上的压力-温度相图，而不需要额外的无序影响。我们的特别兴趣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EuFe2As2中，Eu2+的局域磁性和超导电性表现出一种特殊的相互作用，类似于在重入超导体中观察到的行为。这种反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，我们将研究这一族材料中磁性和超导电性的相互作用。

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.