| Iron-based superconductors are an important class of superconducting materials.According to BCS theory,the necessary condition for superconductivity in a material is a pairing of two electrons with opposite spins,called a Cooper pair,so superconductivity and ferromagnetism cannot coexist.The discovery of La Fe OP superconductors changed the general view that iron is not conducive to the formation of superconductivity in the past,and it was found that iron played a major role in the emergence of superconductivity.The structure of iron-based superconductors is similar to that of copper oxide superconductors,with quasi-two-dimensional characteristics,and their phase diagrams are very rich and very different from the complex phase diagram evolution mechanism of copper oxides,and the study of iron-based superconductors is helpful to understand the mechanism of high-temperature superconductivity.Fe Se,the simplest compound in iron-based superconductors,has a superconducting transition temperature of 8 K at atmospheric pressure and without chemical doping.As the parent of iron-selenium-based superconducting materials,the discovery of superconducting transitions in Fe Se has made iron-selenium-based superconductors a popular material for superconducting research,and there are countless studies on Fe Se and its derived superconductors.FeTe has similar structure to Fe Se,and its ground state is antiferromagnetic order.Theoretical calculation shows that the overlap degree of hole band and electron band in FeTe is greater than that in Fe Se.It is possible to obtain a superconductor with higher superconducting transition temperature by replacing part of Se atoms in Fe Se with Te atom and bonding with Fe atom.Iron-based superconductors have abundant electronic structures,and the superconducting transition temperature is low but highly adjustable.The study of applying high pressure to Iron-based superconductors plays an irreplaceable role in revealing the regulation law of competitive electron order and the physical mechanism of high temperature superconductivity.We chose FeTe0.6Se0.4 single crystal of ternary system as the research object,and systematically investigated its crystal structure and electrical transport properties by in-situ high pressure experimental measurement technology,and obtained the following innovative results:（1）The superconductivity of FeTe0.6Se0.4 single crystal was obtained under high pressure.It was found that the superconducting transition temperature（Tc）of FeTe0.6Se0.4 crystal under high pressure presented an arch with the change of pressure.In the pressure range of 0-2.8GPa,Tc increased with the increase of pressure,showing a positive pressure effect.In the pressure range of 2.8 GPa to 15 GPa,Tc gradually decreased with the increase of pressure,showing a negative pressure effect.Above 15 GPa,the superconductivity disappeared,and the FeTe0.6Se0.4 single crystal showed insulator-like behavior.（2）The upper critical magnetic field intensity of FeTe0.6Se0.4 crystal under different pressures is obtained.We applied external magnetic fields to the samples at4.98 GPa and 6.39 GPa in the first test and 2.78 GPa in the second test respectively,and obtained the upper critical magnetic fields of the samples at these three pressure points as 58.9 T,41.4 T and 102.6 T,respectively.The results show that the upper critical magnetic field decreases with increasing pressure.（3）It was found that FeTe0.6Se0.4 crystal underwent isostructural phase transition and lattice distortion under high pressure.From 2.4 GPa to 3.4 GPa,the c/a ratio changes with the slope of pressure change,accompanied by the volume collapse of2.2%,indicating that the electronic properties of FeTe0.6Se0.4 change and the crystal has an isostructural phase transition,which is consistent with the Tc change law.When the pressure is higher than 15 GPa,the lattice of FeTe0.6Se0.4 single crystal is distorted,the original symmetry is destroyed,the bond Angle of Fe atom and Se/Te atom and the height of Se/Te ion in the lattice change,resulting in the FeTe0.6Se0.4single crystal in an insulation-like state. |
