The Study On Superconducting Properties And First-principles Calculation Of Rare-earth Doped Eu_0.5Ln_0.5BiS₂F

Superconductivity is a hot topic in condensed matter physics.For more than 100 years since the discovery of superconductivity,researchers have been searching for new superconductors.The new family of the BiS₂-based superconductors was discovered couple of years ago.As the Cu-based and Fe-based superconductors,the BiS₂-based superconductors are layered structure.In the BiS₂-based superconductor family,the parent compounds are usually insulators,such as La BiS₂O and Sr BiS₂F.However,Eu BiS₂F exhibits different metallic behavior under normal state due to the self-doping effect,which is caused by hybridization of Eu 4f and Bi 6p orbitals.Thus,superconductivity can be achieved without doping.Below 280 K,ρ（T）curve shows a hump which is caused by the possible charge density wave.The superconducting transition temperature T_C of Eu BiS₂F is only 0.3 K.The low T_C of Eu BiS₂F may be caused by three reasons:（i）the charge density wave;（ii）the Pairing breaking caused by the Eu magnetic moment;（iii）the low electron doing level（x～0.2）.The doping can introduce carriers and suppress charge density wave,so doping at the Eu position of Eu BiS₂F is a rational route to improve T_C.The structure,resistivity,susceptibility and specific heat of Eu_0.5Ln_0.5BiS₂F（Ln=La,Ce,Pr,Nd,Sm）were studied in this paper.And the electron structure was calculated by first principles calculation.The main results of the thesis are as follows:1.Eu_0.5Ln_0.5BiS₂F polycrystalline samples were successfully synthesized.The T_C of Eu_0.5La_0.5BiS₂F was 2.82 K,which is 9 times that of the parent compound.After doping,no hump was observed in theρ（T）curve.It means that the charge density wave was suppressed.Theρ（T）curve of Eu_0.5Ln_0.5BiS₂F shows semiconducting-like behavior with negative coefficient of resistivity.The effective magnetic moment（7.96μ_B）of Eu in Eu_0.5La_0.5BiS₂F sample is calculated by means of curie-weiss law,which is in good agreement with the theoretical value of Eu²⁺.After deducting the electron and phonon contribution,the magnetic specific heat shows a relatively rounded peak at 1.3 K,and the parent compound also shows the same behavior at 1.6 K,which could be attributed to the spin glass transition.By integrating the magnetic specific heat,we got the magnetic entropy(9 J·K^-1·mol^-1),which is 50%of the theoretical magnetic entropy of Eu²⁺.According to this,we got the conclusion that the valence states of Eu are basically divalent.2.We optimized the lattice structures of Eu BiS₂F with different magnetic orders and Eu_0.5La_0.5BiS₂F with different doping sites through the first-principles calculation.After comparing the free energy of different models,the ground state is determined.And the electron structure of the ground state is calculated.Before doping,the localized Eu 4f orbitals cross the fermi energy level.Thus,Eu 4f orbitals have filled and empty states.The filled states represent Eu²⁺,and the empty states represent Eu³⁺.The Eu 4f orbitals hybridize with Bi 6p orbitals,and donate electrons to Bi 6p orbitals.So,the conduction band of Eu BiS₂F is filled with the transferred electrons.It is the reason why Eu BiS₂F behaves like metal in normal state and induces SC without doping.After doping,Eu 4f orbitals don’t cross the fermi energy level,and don’t hybridize with Bi 6p orbitals.Therefore,we believe that the Eu is all divalent.The induced electrons is all comes from La.