Doctor of Philosophy (PhD)
Physics and Astronomy
Correlated Electron Systems have attracted attention because of their complex phase diagrams that often contain several exotic phases that cannot be understood at all with traditional ideas. This is driven by the competition between many low-lying states, competing to be the ground state. That means by tuning parameters like temperature, pressure, magnetic _x000C_field, or doping concentration, one phase would be suppressed and another emerges. These phases are in such a delicate balance that they compete and interact with each other, and experimentally we can always find a way to alter this balance. In this work, two systems have been investigated: Fe-based superconducting compounds (Ba(Fe1-xCox)2As2) and doped strontium ruthenates (Sr3(Ru1-xMnx)2O7). The discovery of Fe-based superconductors in 2008 with the transition temperature higher than 55 K has generated great interest in the materials community in 2008. This superconducting family is based on the conducting layers of iron and pnictides (typically phosphorus or arsenic) and/or chalcogenide (typically selenium or tellurium). In this study, we focus on one compound of this family: Ba(Fe1-xCox)2As2, where giagantic phonon frequency change with temperature with a higher phase transition temperature has been observed at its surface. This anomalous surface lattice dynamics indicates the strong surface-spin-charge-lattice coupling. Another system is the Rudelesden-Popper transition metal ruthenates with Srn+1RunO3n+1. This family is a classical example with strong coupling between charge (orbital), spin, and lattice. The speci_x000C_c compound in this study is Mn doped Sr3(Ru1-xMnx)2O7. In the bulk, Sr3Ru2O7 shows metamagnetic quantum critical point behavior under magnetic field and low temperature. Under pressure it shows ferromagnetic (FM) order with enhanced magnetic moment, indicating the ground state has FM instability . With Mn doping there is a metal-to-insulator transition starting from _x0018_ 5% , whose transition temperature is strongly coupled to the octahedral rotation. Through quantitative surface structure determination we found the surface phase of Sr3Ru2O7 is dramatically diff_x000B_erent from the bulk with the enhanced octahedral rotation and the emergence of tilt. The surface metallicity is also studied through the phonon spectra mediated through the electron-phonon coupling. The asymmetry of the phonon peak is analyzed through the Fano lineshape, which is due to the interaction between the discrete phonon excitation with the electron-hole pairing continuum. With increasing Mn concentration the peak becomes more asymmetric, indicating the surface is more metallic with higher density of states near the Fermi energy. The surface of Sr3Ru2O7 is poorly metallic and presumably antiferromagneticaly ordered while the bulk is metallic. In contrast, the surface of Sr3(Ru0.84Mn0.16)2O7 is metallic while the bulk is insulating. These unusual properties are intimately coupled with the surface structure.
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Chen, Chen, "Structure and Dynamics of Layered Transition Metal Compounds Driven by Broken Symmetry" (2016). LSU Doctoral Dissertations. 4084.