First-principle Study On Doped Superconductivity K₂Cr₃As₃ And Transition Metals XAs₂ ?X=Mo,W?

The superconducting mechanism of superconductors has been paid much at-tention,attracting a large number of researchers into this area.The traditional BCS theory predicts the highest superconducting temperature is no more than 30K.However,in the late 1980s,the appearance of copper oxide superconductors broke this Predicted the upper limit,since then high-temperature superconduct-ing is so hot that more people keep trying to pursue more higher superconducting critical temperature Tc as their goals,but its superconductivity mechanism has al-ways lack perfect theoretical explanations.Also,the superconducting mechanisms of more unconventional superconductors such as iron-based superconductivity and heavy fermion superconductivity can not be explained by conventional BCS the-ory,so one must construct new theoretical models to explain them.The First-principle Calculation method,starting from basic parameters,has show its outstanding advantages in the study of electronic structure and material characteristics of new materials.The support for the experimental and theoret-ical from First-principles Calculation results is more and more important.With the high-speed development of contemporary high-performance computer cluster,which provides effective hardware support for the smooth calculation,combining with the increasingly perfect computing systems and various kinds of computing softwares,the accuracy of the First-principle Method is increasing.The new base chrome-arsenic compounds A₂Cr₃As₃?A = K,Rb,Cs?family were newly discovered chromium-based one-dimensional inorganic superconduc-tors in 2015.The experiment results show these compounds can not be simply classified into conventional superconductor domains.In theory,Prof.Y.Zhou from ZJU,Prof.J.H.Dai from HZNU and other groups applied the Mean-Field Theory,Renormalization Group?RG?and other methods to analysis the band properties and possible superconductivity pairing mechanics of this system.It is very common to study the superconducting mechanism of superconductor by doping method.When the magnetic material is doped into the conventional BCS superconducting material,the critical temperature decreases sharply.While in the unconventional superconductor,the critical temperature will be quickly stifled only when the non-magnetic elements are added,which give us a good guide to determine the superconductivity type of a superconductor.In this paper,we study the superconducting mechanism of zirconium-chromium-arsenic by simulating the doping of different elements in pure potassium-chromium-arsenic materials using First-Principle Calculation.By comparing the difference between magnetic Mn and non-magnetic Zn in the aspect of the energy band structure,Fermi surface,DOS and other changes,which may reflect the different response of single electron state for the magnetic and non-magnetic,may help us study the superconducting mechanism of the material indirectly,combined with other information from calculation.It is found that the direct band gap of ? is increased from 0.19eV to 0.33eV,and the direct band gap of point A is sharply reduced to 0.1eV.The change of the conduction band and valence band is obvious,the orbital component changes,the contribution of the orbit dyz+dxz increases and the quasi-one-dimensional band in the pure state disappear after Zn doped.The addition of the non-magnetic element has a great influence on the electronic structure near the Fermi surface.In contrast,the bands near the Fermi surface were slightly changed after adding the magnetic element Mn under the same conditions,and retain a quasi-one-dimensional band.Mn unchange the main contribution by the three orbitals dxy + dz2 + dx2+y2.And the shape of the fermi surface is less affected than that of Zn.But the orbits contribution to the total density of Mn is larger than that of the Zn.At the same time,it is noted that the change of doping at different equivalent positions is different,especially when doping nonmagnetic elements,the band is different at different positions.The results show that the change caused by the addition of the non-magnetic element zinc is significantly greater than that of the magnetic element manganese,the DOS and the Fermi surface further support this conclusion.Through the comparison and analysis of this calculation in Chapter 4 and Chapter 5,we can find that the effect of Zn is greater than that of the magnetic doping,which is consistent with the general rule of adding non-magnetic elements in unconven-tional superconductors.Therefore,the calculation results support K₂Cr₃As₃ as unconventional superconductor.Secondly,the electron band structure and topological properties of the tran-sition metal compounds of XPn2 series are mainly calculated by using the first principle in my graduate stage.In this paper,we take MoAs₂ and WAs₂ as ex-amples,although they are similar to MoP2 in chemical composition,MoP2 is non-centrosymmetric,while MoAs₂ and WAs₂ have central Inversion Symmetry.From the symmetry analysis,they can not hold Weyl points,but there may be Dirac points.So,we searched the entire BZ,no energy degenerate could be found except the spin degeneracy,excluding the possibility of intrinic Dirac or Weyl points near the Fermi level.The Z2 classification of the two materials indicates that they are topologically strong and the surface of the MoAs₂ surface can be seen using the surface Green function method.