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

在各种砷化铁基超导体中，AFe2As2(A=Ba，Sr，Ca)，即所谓的122家族成员，已经成为探索这类新的高温超导体超导电性的模型系统。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、高质量的和均匀的晶体。这些材料的(温度与化学掺杂)相图已被广泛研究，以揭示超导电性与附近磁性和结构有序的相互作用-这一方面已成为这类材料研究的一个主要主题。在这个项目中，我们建议通过流体静压下的热力学测量来研究超导电性与附近磁性和结构有序的相互作用，特别是在流体静压下，特别是Ca(Fe1-xCOX)2As2，BA(Fe1-xCOX)2As2，Ca1-xSrx2Fe2As2和Ba1-xKxFe2As2。这项工作的动机是我们最近的发现，对于x=0.028的Ca(Fe1-xCox)2As2晶体，与砷化铁超导体相关的显著基态，即正交/反铁磁、超导和非磁性塌缩的四方状态，可以在一个样品中以相当小的和真正的流体静压获得。通过各种热力学(磁化率、高分辨率热膨胀、超声波)和输运(电阻率)测量的组合，所有这些都在真正的流体静压(He-气体)压力下进行，除其他外，我们将讨论以下方面：(I)在122系列中，超导电性与结构/磁性秩序共存或不共存的条件。(Ii)对一阶和二阶相变为低温正交/反铁磁(o/AFM)相的系统的结构/磁涨落进行了比较研究。(3)T-P相图中o/AFM和超导相变线相交的三临界点的可能性。(4)详细研究了超导电性与高压侧发生坍塌-四方相的相互关系。(V)研究相图上压力调谐时的弹性行为；与文献中报道的具有不同掺杂水平的材料的反常行为进行比较。

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.