Resonant Inelastic X-ray Scattering Studies On Transition Metal Oxides

In strongly correlated transition metal oxide systems,the degrees of freedom such as spin,orbit,charge,and lattice are usually coupled together,which will emerge noval properties,such as gaint magnetoresistance,high temperature superconductivity,etc.Studying the correlation of these degrees of freedom can lead to better tuning of parameters to change material properties,enabling them to produce practical application value.This thesis mainly introduces the use of x-ray scattering and resonant inelastic x-ray scattering(RIXS)to study the charge,spin and lattice orders and excitations in copper and nickel oxides.It includes four parts:1.Lattice response to Sr impurities in La1.92Sr0.08CuO4.With x-ray scattering,we observed the Huang diffuse scattering pattern caused by lattice strain induced by randomly distributed Sr impurities.The intensity of the Huang diffuse scattering is comparable to the intensity of thermal diffuse scattering at a temperature of 7 K.At the same time,we quantitatively reconstructed the lattice strain in real space through continuum elastic theory,and find the lattice deformation being of the order of 0.001 A.Although the strain can propagate to the nanoscale,its size is unlikely to be the main reason for the nanoscale electronic property inhomogeneity in copper oxides.2.Plasmon excitations study in La2_xCexCuO4.We measured La2_xCexCuO4 RIXS spectrum with incident polarization,incident energy,and out-of-plane momentum tranfer dependence,and found that the charge excitation is the nature of plasmon.Its energy dispersion is influenced by long-range Coulomb interactions in the form of two-dimensional layered stacking.Doping dependence measurement found that as the doping increases,the energy of the plasmon increases and the lifetime becomes longer.Combined with the calculation of the single-particle spectrum which generates the plasmon,it reflects the change from strongly correlated Mott system to itinerant electronic liquid in the electron-doped cuprates.3.Charge density wave study in La2_xSrxCuO4.We used ultra-high energy resolutionn RIXS measurements and observed the charge density wave in La2_xSrxCu04 and its associated phonon softening.These two phenomena remained to x=0.21,until disappeared at x=0.25,which crosses the Fermi surface topological phase transition at x=0.19.This indicates that the RIXS spectrum contains little or no direct coupling with electronic excitations near the Fermi surface,but they are dominated by the resonant scattering cross section of phonons.We interpret the experimental results as a charge density wave generated by the strong correlation and a phonon response caused by the lattice modulation induced by the charge density wave.4.Magnetic excitations study in R4Ni3O8.We used RIXS to measure the three layers nickel oxide R4Ni3O8(R=La,Pr),and observed magnon excitations with energy of～80 me V.Combined with the spin wave model,we got the nearest neighbor magnetic exchange interaction of J=69(4)meV.With such a large J and the strength of Ni-O hybridization from O K edge x-ray absorption spectrum,we think that the properties of the three layers nickel oxide is in-between infinite layer nickel oxide(Nd0.8Sr0.2NiO2)and copper oxide,which provides a bridge for studying the relationship between these two superconductors.These experimental findings demonstrate the power of RIXS,which is able to couple with various excitations.With the continuous improvement of the energy resolution and the development of theoretical calculations for RIXS process,we believe that this method will play an increasingly important role in the study of condensed matter physics in the future.