Study On The Effect Of Heavy Metal Elements Doping On The Superconducting Properties Of FeSeTe

Fe-based high temperature superconductor is a new type of superconductor after copper oxide high temperature superconductor.Since the discovery in 2008,the related researches on this superconducting family have sprung up.At present,the iron-based superconductors can be divided into six different structural categories according to their chemical composition.These structures have the same layered structure based on the plane layer of iron atoms.The higher superconductor transition temperature(T_c)is based on their common iron atomic layer.Among them,Fe(Te,Se)system has attracted wide attention in the scientific field because of its simplest crystal structure and special magnetic and electrical properties.First,in Fe Se,the internal parameters can be systematically adjusted by the substitution of Se by Te,which provides a platform for further study of possible superconducting mechanisms and topological characteristics.Second,a very stable(no splitting under external magnetic field)zero energy bound state(ZBS)is observed on the surface of superconducting Fe(Te,Se),indicating that a possible topological feature of its electronic structure.It is one of the hot studies in condensed matter physics in recent years.In addition,in the Fe(Te,Se)system with the simplest structure,the superconducting performance can be achieved or improved by changing the ratio of Te/Se,doping of chemical elements,adding high pressure and reducing the excess Fe content.So far,the doping reports on this system are mainly based on the substitution of homophanous elements of ternary compounds,while the studies on the doping of heavy metal elements are relatively few.Due to the larger atomic radius of heavy metal elements,the van der Waals interlayer bonding force is weakened after chemical doping,and the samples are easier to cleave.At the same time,chemical doping is also an effective way to regulate the physical properties of Fe(Te,Se)in iron-based superconductors.Based on the above reasons,we have studied the Fe(Te,Se)superconductors doped with heavy metals as follows:The heavy metal element Pb was doped in the FeTe_0.6Se_0.4 single crystal and its related properties were analyzed.The results show that the single crystal properties of the doped samples are better than that of pure sample,and it is easier to cleavage;the magnetic properties of FeTe_0.6Se_0.4 single crystal are changed after Pb doping.The magnetic curves of doped samples show Ferromagnetism(FM)in a certain temperature range,and the M-H curves at 5 K,10 K and 70 K confirm the existence of ferromagnetic phase,and demonstrate the coexistence of ferromagnetism and superconductivity is intuitively reflected in the experiment.However,previous reports were based on the theoretical research.At the same time,the M-T curve of the doped sample appears magnetic anomaly at about 130 K,and the magnetic moment increases with the decrease of temperature.The origin of the magnetic anomaly may be related to the periodic alignment defects observed by SEM.Combined with the test results of XPS and EDX,it is further studied that the defects may be iron oxides.In order to further explore whether the above results are common in heavy metal doping,a series of heavy metal elements(such as W,Pt,Ir,etc.)were doped in FeTe_0.6Se_0.4 single crystal.XRD results show that the doped samples have good single crystal property and are easy to cleave naturally.SEM results show that obvious defects were observed in both W and Ir doped FeTe_0.6Se_0.4 single crystal samples,but no obvious defects are found in Pt doped samples.In the samples doped with three kinds of elements,there is a small bump in the M-T curve at about 120 K,which indicates that the occurrence of double step transition.Based on the analysis of XPS and EDX results,it is speculated that the defects in the doped samples may be one of the causes of the small bump.Another possible reason is that FeSeTe has a structural transformation around 120 K.In addition,compared with pure FeTe_0.6Se_0.4,the superconducting critical temperature(T_c)of Pt doped sample is increased by 0.9 K.When H?1 T,the superconducting critical density(J_c)of Pt_0.02Fe_0.98Te_0.6Se_0.4sample is three times that of pure sample.When H?1 T,the critical current of Pt_0.02Fe_0.98Te_0.6Se_0.4 sample is lower than that of pure sample.The doping of Pt can enhance the concentration of superconducting carriers at low field,but weaken the pinning of Pt_0.02Fe_0.98Te_0.6Se_0.4.The J_c of the Ir doped sample is about 2 times that of the pure sample,and the J_c decreases monotonically with the increase of magnetic field(H?3 T).