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Abstract

The rich multiband physics of the calcium ruthenates gives rise to a complex free-energy landscape shaped by the strong interplay among spin-orbit coupling effects, electronic interactions, lattice distortions, and spin correlations. This competition produces a variety of emergent quantum phenomena with promise for future applications. For instance, beyond its Mott-insulating ground state, the single-layer Ca₂RuO₄ hosts an unconventional current-induced non-equilibrium metallic state, while an incommensurate cycloidal magnetic phase arises in the bilayer Ca₃Ru₂O₇ system. By simultaneously employing transport measurements with angle-resolved photoemission spectroscopy (ARPES), this work reveals how the electronic band structure evolves across the current-induced insulator-to-metal transition in Ca₂RuO₄. A reduction of the insulating band gap and a modification of the Ru bands are observed in the current-induced phase. Historically, transport-ARPES has been confined to the study of materials with clearly defined features in energy; by using core level spectra as the energy reference, not only can transport-ARPES be extended to any ARPES suitable material. Meanwhile, Raman measurements in an external magnetic field reveal an unusually large field-induced energy shift of a phonon mode in Ca₃Ru₂O₇, pointing to exceptionally strong spin–phonon coupling in a regime where the Dzyaloshinskii–Moriya interaction plays the dominant role. While even larger couplings have been observed in some 5d compounds such as the osmates, this is the largest reported value among the ruthenates, underscoring the delicate interplay between the different degrees of freedom in the system, making these materials highly tunable by small perturbations. For instance, applying < 1% compressive strain can shift the insulator-to-metal transition by more than 70K. This thesis therefore integrates external electromagnetic fields with advanced spectroscopic probes in order to study regions of the ruthenate free-energy landscape not previously explored by conventional techniques. A complex landscape is uncovered, illustrating how these materials can be finely tuned with current, field, doping, or strain, offering new insights into the underlying physics of 4d systems and opening opportunities for their incorporation into future quantum devices. Additionally, with the developments in transport-ARPES, this thesis builds upon our ever growing capability to look at strongly correlated electron systems in operando.