Exploration And Physical Property Study Of New Superconductors In Ternary Equiatomic Intermetallic Compounds

Since the discovery of superconductivity in 1911,it has attracted widespread interest of researchers and is still vigorous after a century.The exploration of new superconductors is an important part of superconductivity research,which is crucial for further understanding the superconducting pairing mechanism and realizing the wide application of superconductors.Previous studies have shown that there is a close relationship between superconductivity and crystal structure.Therefore,exploring new superconductors is of great significance for elucidating the relationship between superconductivity and crystal structure,and it also points out the direction for improving the superconducting transition temperature and discovering novel properties.Ternary equiatomic intermetallic compound family contains rich structure types and physical properties,and it is an ideal platform to study the relationship between crystal structure and superconductivity.This thesis is dedicated to the exploration and physical property study of new superconductors in ternary equiatomic intermetallic compounds.A lot of attempts have been made.The main research results include:(1)We have discovered the first Th-based ternary equiatomic intermetallic compound ThIrGe with a TiNiSi-type structure.The compound undergoes a bulk superconducting transition below Tc of 5.25 K,and is a weakly coupled type-II BCS superconductor.Its Ginzburg-Laudau parameter,upper critical field,electronic specific heat coefficient,Debye temperature,and electron-phonon coupling constant are 19.9,2.9 T,11.8 mJ mol-1 K-2,275 K and 0.65,respectively.Theoretical calculations show that Th,Ir and Ge orbitals are significantly hybridized around the Fermi surface and jointly affect the total density of states.The structural difference between ThIrGe and the analogous compound ThIrSi can be attributed to the difference of their Gibbs formation energies.(2)Inspired by the discovery of novel physical properties in the ferromagnetic superconductor URhGe,a similar compound ThRhGe was synthesized through the substitution of U by Th.Compared with the TiNiSi-type structure of URhGe,ThRhGe can form two polymorphic structures of YPdSi-type and TiNiSi-type under ambient pressure.Among them,YPdSi-type α-ThRhGe exhibits a normal metallic behavior down to 1.8 K,while TiNiSi-type β-ThRhGe shows a structural transition at 244 K and a superconducting transition at 3.36 K.These two transitions can be regulated by applying hydrostatic pressure.First-principles calculations qualitatively explain the origin of the differences in physical properties between the two polymorphs.ThRhGe represents the first case of coexistence of polymorphism,structural transition,and superconductivity in ternary equiatomic intermetallic compounds.(3)Based on the novel physical property of coexistence of structural transition and superconducting transition in β-ThRhGe,a series of β-ThRh1-xIrxGe samples were synthesized by implementing isovalent chemical doping of Ir.These compounds all exhibit TiNiSi-type structures at room temperature,and the lattice constants vary linearly.Doping causes the structural transition to be suppressed and superconductivity to be enhanced at the same time.The structural transition disappears at x=0.5,where the superconducting transition temperature Tc reaches a maximum value of 6.88 K and exhibits a dome-like doping dependence.Data analysis shows that the enhancement of superconductivity originates from the appearance of structural quantum critical point around x=0.5 doping concentration.Similarly,a structural quantum critical point can also be induced by pressure in x=0.2 sample.β-ThRh1-xIrxGe provides an ideal platform for studying the interplay between structural quantum criticality and superconductivity.(4)We have discovered the first ternary intermetallic compound in the Th-Ir-P system.Compared with the high-pressure synthesis of LaIrP,ThIrP is synthesized under ambient pressure and exhibits the same crystal structure as LaIrP,that is,a noncentrosymmetric LaPtSi-type tetragonal structure.The compound undergoes a bulk superconducting transition below Tc of 5.07 K,and is a type-II BCS superconductor which may have multiple superconducting gaps.Its GinzburgLaudau parameter,upper critical field,electronic specific heat coefficient,Debye temperature,and electron-phonon coupling constant are 7.5,0.83 T,and 7.5 mJ mol-1 K-2,240 K and 0.68,respectively.Theoretical calculations show that the density of states near the Fermi surface is jointly affected by Th and Ir,and the maximum band splitting induced by spin-orbit coupling reaches 270 meV,implying its strong spin-orbit coupling effect.