Doctor of Philosophy (PhD)
PHYSICS AND ASTRONOMY
Transition metal oxides (TMOs) have attracted considerable interest due to the intriguing interactions between spin, charge, and lattice degrees of freedom. The delicate balance between these interactions can be altered by doping, symmetry breaking and other external stimuli such as pressure, electric, and magnetic field. The Ruddlesden-Popper (RP) family of ruthenates, Srn+1RunO3n+1, is a prototype of TMOs with stacking octahedra along the c axis, and the intra- and inter-octahedral structures are known to be crucial to both electronic and magnetic properties. In this thesis, we focus on studying the effect of chemical doping and broken symmetry at the surface of bilayered (n = 2) Sr3(RuxMn1-x)2O7, especially emphasizing the correlation of emergent surface electronic-magnetic properties with the lattice structure.
Bulk Sr3Ru2O7 is paramagnetic and metallic, with octahedra rotating around c-axis. As Ru is partially replaced by Mn, Sr3(Ru1-xMnx)2O7 undergoes a metal-insulator transition (MIT) and an antiferromagnetic (AFM) phase transition coupled with x, leading to an AFM-insulating ground state. Creation of the surface breaks the bulk translational symmetry, leading to further perturbations of the system.
In this paper, we complete the single-crystal X-ray diffraction study of Sr3(Ru1-xMnx)2O7 samples at low temperatures (28 K) and construct a surface phase diagram by using a combination of surface-sensitive characterization methods. Quantitative analysis of low-energy electron diffraction intensity versus voltage (LEED-IV) is used to investigate the variation of the surface lattice. As x increases, the surface octahedron rotation angle decreases and is accompanied by a change in the bond lengths of surface octahedron. The temperature and magnetic field dependence STS spectra of Sr3(Ru1-xMnx)2O7 (x = 0, 0.08, 0.16, and 0.23) indicate that the surface is metallic and nonmagnetic. We also use high-resolution electron energy loss spectroscopy (HREELS) to indirectly verify the surface conductivity by the asymmetric Fano line shape of A1g(2) phonon mode.
Based on our detailed experimental investigations, we found that the surface of Sr3(Ru1- xMnx)2O7 has very different properties from the bulk phase. It allows us to gain insight into the relationship between surface structure and electronic/magnetic properties and provides an ideal platform for exploring the hidden bulk phase.
Yang, Yifan, "Systematic Investigation of Bulk and Surface Properties of Sr3(Ru1-xMnx)2O7" (2021). LSU Doctoral Dissertations. 5556.