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

在各种铁砷化物基超导体中，AFe 2As 2（A = Ba，Sr，Ca）的成员，即所谓的122族，已成为探索这类新的高Tc超导体中超导性的模型系统。这部分地与这样的事实有关，即对于这些系统，可以生长大的、高质量的和均匀的晶体。的（温度与化学掺杂）相图已经被广泛地探索，以揭示超导性与附近的磁性和结构有序的相互作用-这方面已经成为这类材料研究的一个主要主题。在这个项目中，我们建议研究122家族的选定成员的超导性与附近的磁性和结构有序的相互作用，特别是Ca（Fe 1- xCox）2As 2、Ba（Fe 1-xCox）2As 2、Ca 1-xSrxFe 2As 2和Ba 1-xKxFe 2As 2。这项工作的动机是我们最近的发现，对于适当热处理的Ca（Fe 1-xCox）2As 2（x = 0.028）晶体，与砷化铁超导体相关的显著基态，即，正交/反铁磁、超导和非磁性塌缩四重态可以在一个样品中以合理小的和真正的流体静压全部进入。（磁化率，高分辨率热膨胀，超声波）和运输（电阻率）测量，所有这些都在真正的流体静力学（He-气体）压力下，我们将解决以下方面：（1）超导性与122族中的结构/磁序共存或不共存的条件。(ii)一阶和二阶相变到低温正交/反铁磁（o/afm）相系统的结构/磁涨落的比较研究。(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。(iv)详细研究了高压侧超导电性与超导电性的相互关系以及超导电性四极相的出现。(v)在相图上压力调谐时的弹性行为的研究;与文献中报道的具有不同掺杂水平的材料的异常行为的比较。

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.