Angle-Resolved Photoemission Spectroscopy of Epitaxially Strained Ruthenates
Complex oxides provide a platform for designing new functional material systems and manipulating the properties of the existing ones thanks to a vast array of their tunable and exotic ground states. The interactions between the internal degrees of freedom in such systems often cannot be fully accounted for by modern theory, and it is necessary to directly probe the electronic structure underlying a complex phase to reveal its nature. The emergent properties of ruthenates are particularly tunable, as evidenced by their wide variety of electronic and magnetic instabilities including unconventional superconductivity, metamagnetism and formation of electronic liquid crystalline states, ferro- and antiferromagnetism, and spin-glass behavior. Some of these phases occur in closely related compounds, or are switchable within the same compound by a small external perturbation, such as pressure, strain or magnetic field. This dissertation presents direct measurements of the electronic structure in Ba$_2$RuO$_4$ and BaRuO$_3$, close analogues of the spin-triplet superconductor Sr$_2$RuO$_4$ and ferromagnetic SrRuO$_3$, respectively, using a unique integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy. This approach allows us to access compounds that have no stable bulk form and deliberately manipulate their electronic properties via epitaxial strain and dimensionality confinement. We first discuss the ARPES measurements of Ba$_2$RuO$_4$ and Sr$_2$RuO$_4$. By varying the amount of biaxial in-plane strain through epitaxial stabilization on different substrates, we reveal a systematic evolution of the Fermi surface and quasiparticle mass enhancements. We reveal a topological transition in the circular electron Fermi pocket centered at zone center as a function of strain and cation size. Near the topological Lifshitz transition, we observe clear signatures of quantum criticality. The quasiparticle mass enhancements are found to increase rapidly and monotonically with increasing Ru-O bond distance. By next studying atomically thin films of BaRuO$_3$ we are able to directly see how the dimensional confinement drives the transition from a ferromagnetic ground state to a strongly fluctuating paramagnetic state. Our results provide new insights into the physics of perovskite ruthenates and demonstrate the possibilities for using epitaxial strain and dimensional confinement as disorder-free means of manipulating emergent properties and many-body interactions in correlated quantum materials.
ARPES; Fermiology; ferromagnetism; quantum well states; quasiparticles; superconductivity; Condensed matter physics; Low temperature physics; Materials Science
Shen, Kyle M.
Muller, David Anthony; Kim, Eun-Ah
PHD of Physics
Doctor of Philosophy
Attribution 4.0 International
dissertation or thesis
Except where otherwise noted, this item's license is described as Attribution 4.0 International