Thermodynamic and Transport Properties of Iron-Pnictide Superconductors under hydrostatic (Hegas) pressure

在各种基于砷化铁的超导体中，AFe2As2 （A = Ba, Sr, Ca）的成员，即所谓的122家族，已经成为探索这种新型高tc超导体超导性的模型系统。这在一定程度上与这样一个事实有关：对于这些系统，可以生长出大的、高质量的、均匀的晶体。这些材料的（温度与化学掺杂）相图已经被广泛探索，以揭示超导性与附近磁和结构序的相互作用，这已经成为这类材料研究的一个主要主题。在本项目中，我们提出通过静水（He-gas）压力下的热力学测量，研究122族中选定的成员，特别是Ca(Fe1- xCox)2As2、Ba(Fe1-xCox)2As2、Ca1-xSrxFe2As2和Ba1-xKxFe2As2的超导性与附近磁序和结构序的相互作用。这项工作的动机是我们最近的发现，对于适当热处理的Ca(Fe1-xCox)2As2晶体（x = 0.028），与砷化铁超导体相关的显著基态，即正交/反铁磁性，超导和非磁性坍缩的四方态，可以在一个样品中以相当小的真正静水压力访问。通过结合各种热力学（磁化率，高分辨率热膨胀，超声波）和输运（电阻率）测量，所有这些都是在真正的静水（He-gas）压力下进行的，我们将解决以下方面的问题：(i)在122族中超导性是否与结构/磁性秩序共存的条件。（ii）对显示一阶和二阶相转变为低温正交/反铁磁（o/afm）相的体系的结构/磁性波动的比较研究。（iii）在T-P相图中o/afm和超导相变线相交处存在三临界点的可能性。（四）详细研究了高压侧超导性与坍缩四方相的相互关系。(v)研究跨相图压力调整后的弹性行为；与文献中不同掺杂水平材料异常行为的比较。

Among the various iron-arsenide-based superconductors, members of the AFe2As2 (A = Ba, Sr, Ca), the so-called 122 family, have become model systems for exploring superconductivity in this new class of high-Tc superconductors. This is partly related to the fact that for these systems large, high-quality, and homogeneous crystals can be grown. The (temperature vs chemical doping) phase diagrams of these materials have been explored extensively for unravelling the interplay of superconductivity with the nearby magnetic and structural orders – an aspect which has become a major theme of research in this class of materials.In this project we propose to study the interplay of superconductivity with the nearby magnetic and structural orders on selected members of the 122 family, especially Ca(Fe1- xCox)2As2, Ba(Fe1-xCox)2As2, Ca1-xSrxFe2As2, and Ba1-xKxFe2As2, via thermodynamic measurements under hydrostatic (He-gas) pressure. The work is motivated by our recent discovery, that for properly heat-treated crystals of Ca(Fe1-xCox)2As2 with x = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic, superconducting, and non-magnetic collapsed tetragonal states can be accessed all in one sample with reasonably small and truly hydrostatic pressure.Through the combination of various thermodynamic (magnetic susceptibility, highresolution thermal expansion, ultrasonic) and transport (resistivity) measurements, all under truly hydrostatic (He-gas) pressure, we will address among others the following aspects: (i) The conditions under which superconductivity does or does not coexist with structural/magnetic order in the 122 family. (ii) Comparative investigations on the structural/magnetic fluctuations on systems showing first- and second-order phase transitions into the low-temperature orthorhombic/antiferromagnetic (o/afm) phase. (iii) The possibility of a tricritical point in the T-P phase diagram where the o/afm and superconducting phase transition lines meet. (iv) Detailed investigations on the interrelation of superconductivity and the occurrence of the collapsed-tetragonal phase on the high-pressure side. (v) Study of the elastic behavior upon pressure tuning across the phase diagram; comparison with the anomalous behaviour reported in the literature for materials with varying doping levels.