Study On The Doping Of Co And Ni Elements In Iron-based Superconductor ThFeAsN And Their Superconducting Properties

As an important system of unconventional superconductivity,iron-based superconductivity has become a hot spot in the field of superconductivity research in the past decade.Most of the iron-based superconducting materials discovered so far have a matrix that is not superconducting,but exhibits a certain antiferromagnetic sequence.By doping external elements or high pressure,it suppresses its antiferromagnetic phase transition,which leads to superconductivity.The iron-based superconducting compound ThFeAsN has a superconducting transition temperature as high as 30 K without any treatment.From the current research status,it is a simple and effective method to increase the superconducting transition temperature of the system by electron doping or hole doping.For the ThFeAsN system,the[Fe₂As₂]^2-layer responsible for superconductivity can be directly doped with electrons.Because the atomic radius of Co element and Ni element is close to that ofFeelement and one or two more 3d electrons thanFeelement respectively,they are the first choice elements for direct electron doping.This paper will introduce the influence of the doping of Co and Ni elements in the iron-based superconductor ThFeAsN and the doping of the Nb element in the topological superconductor Nb_xBi₂Se₃ on the superconducting properties.1. In the Co doping of the ThFeAsN precursor,we prepared Th Fe_1-xCo_xAsN polycrystalline samples with different doping ratios.Through testing,it was found that with the increasing amount of Co element doping,the unit cell parameters changed significantly,and the superconducting transition temperature of the parent ThFeAsN gradually decreased.until the superconducting phenomenon disappeared.In addition,we synthesized a new compound ThCoAsN and characterized its basic physical properties.ThCoAsN exhibits a paramagnetic metal state.2. In the Ni element doping of the ThFeAsN precursor,we prepared Th Fe_1-xNi_xAsN polycrystalline samples with different doping ratios.We found that Ni element has a strong suppression effect on the superconductivity of the matrix,and the superconductivity of the matrix ThFeAsN disappears when the doping ratio is 1%.When the doping ratio is below80%,although there is a small amount of ferromagnetic impurities,the sample mainly shows Pauli paramagnetic.At the same time,we prepared a pure phase of Th Ni AsN and found that its superconducting transition temperature is 4.9 K,which is an increase of 0.6 K compared with previous reports.3. With the rise of topological physics research,the relationship between topology and superconductivity has gradually entered people's field of vision,and a new field has emerged:topological superconductivity.The topological superconductor Nb_xBi₂Se₃ is based on the strong-topological insulator Bi₂Se₃ and is superconducted by the doping of the Nb element.Simple element doping can make the topological insulator into a superconductor,but there has been controversy as to whether the doping of Nb atoms into Bi₂Se₃ is substitutional doping or van der Waals interlayer doping.We prepared a high-quality Nb_xBi₂Se₃ single crystal,and determined that the intercalation doping of the Nb atomic van der Waals layer is the cause of Nb_xBi₂Se₃ bulk topological superconductivity.This work is of great significance to further study the superconducting mechanism of Nb_xBi₂Se₃ and the mechanism of topological superconductivity.