Structure And Superconductivity Of122-type Iron Arsenic Compounds Under Pressure

Following the cuprate superconductors, iron-based superconductor is another type ofunconventional superconductors whose superconducting transition temperature are beyondthe McMillan limit. To investigate the properties of this family of superconductors will bevery helpful to understand the superconducting mechanism of high-Tccupratesuperconductors and to explore new superconducting materials with higher Tc.Till now, many iron-based superconductors have been found. According to the crystalstructure and the components of the compounds, there are mainly six types of iron-basedsuperconductors, such as1111-type LnFeAsO （Ln=La, Sm, Ce, Pr, Nd, etc.）,122-typeAFe₂As₂（A=Ba, Sr, Ca, Eu）,111-type BFeAs （B=Li, Na, K）,11-type FeSe（Te）,21311-typeSr₂ScO₃FeX （X=P, As）, and122*-type C0.8Fe2-ySe2（C=K, Rb, Cs）. Among these families,122-type compound AFe₂As₂（A=Ba, Sr, Ca, Eu） can be easily synthesized to high puritysingle crystal, thus it is an ideal system to study the fundamental physical properties undervarious conditions. Pressure is an effective measure to explore the new superconductingmaterials and to understand the superconducting mechanism. At ambient pressure, the parentcompound BaFe2As2system doesn’t show superconductivity. When pressure reaches4GPa,the transition temperature Tcis approximately29K in the undoped parent compoundBaFe2As2. Application of pressure can also increase Tcin underdoped BaFe_1.92Co_0.08As₂compound. It was reported that the crystal structure of iron-based superconductors is relatedto the superconducting transition temperature Tc, so the structure determination is veryimportant for better understanding superconductivity. But up to now, the structure andsuperconductivity of122-type iron-based superconductors remain controversial. In this paper,we present investigations of pressure effect on structural properties of the parent compoundBaFe2As2and the overdoped compound BaFe_1.8Ni_0.2As₂at room temperature using asynchrotron powder X-ray diffraction technique. And we report the pressure dependence ofthe superconducting transition temperature Tcof the optimally doped compoundBaFe_1.9Ni_0.1As₂by electrical resistance measurement technique. The conclusions are asfollows: （1） At high pressure, no change in the structures of BaFe2As2and BaFe_1.8Ni_0.2As₂takeplace. When the pressure reaches a certain value, the crystal structures undergo a phasetransition from tetragonal to a collapsed tetragonal phase. And we conjecture that thesuperconductivity eventually disappears when the structures completely transform to acollapsed tetragonal phase.（2） We conduct the high-pressure electrical resistance measurement experiment on theoptimally doped compound BaFe_1.9Ni_0.1As₂. The result shows that the superconductingtransition temperature Tcis basically unchanged when pressure is very small. With increasingpressure, the superconducting transition temperature Tcrapidly decreases. Further increasingpressure, the Tcis below10K and cannot be measured accurately restricted to theexperimental condition. Recently scientists found that there are two superconducting region in122*-type superconductors AxFe2-ySe2（A=K, Cs, etc.）. However, we didn’t observe anysuperconductivity in BaFe_1.9Ni_0.1As₂when further increase the pressure. This result indicatesthe FeAs superconductors and FeSe superconductors belong to two different types. Butfurther analysis indicates that spin fluctuation plays an important role in the emergence ofsuperconductivity of BaFe_1.9Ni_0.1As₂.