Imaging emergent correlated phases in the strontium ruthenates

Abstract

In strongly correlated electron materials, charge, spin and orbital degrees of freedom exhibit an intimate relationship, leading to new emergent phases that seemingly break the symmetries of the underlying crystal and are highly sensitive to external stimuli. This is well illustrated in the Ruddlesden-Popper series of the strontium ruthenates, Sr_{n+₁}Ru_nO_{₃n+₁}, where a wide range of properties attributed to such physics can be found, including unconventional superconductivity, quantum criticality, metamagnetic transitions and ferromagnetism. In this thesis, using ultra-low temperature scanning tunneling microscopy, I show a detailed study of the low-energy electronic states at the surface of Sr₂RuO₄, an unconventional superconductor, and Sr₃Ru₂O₇, an itinerant metamagnet associated with quantum criticality. I demonstrate that the increased structural distortions in the surface layer lead to considerable changes in the Fermi surface, allowing the stabilization of new emergent phases beyond those accessible in the bulk.

At the surface of Sr₂RuO₄, we find that the surface reconstruction leads to checkerboard charge order intertwined with nematicity, intimately linked with four van Hove singularities within 5 meV of the Fermi level. Including these orders in a tight-binding model gives excellent agreement with the experiment. By applying a magnetic field, one of the van Hove singularities splits, with one branch extrapolated to reach the Fermi energy at ~32 T, providing a textbook example of tuning towards a Zeeman-driven Lifshitz transition.

Measurements at the surface of Sr₃Ru₂O₇ reveal a magnetic ground state, with substantial anisotropy of the electronic states. With increasing magnetic field, we observe the formation of a stripe order and were able to track a van Hove singularity shift across the Fermi energy. Our measurements establish the surface layer as having a distinct ground state from the bulk, undergoing a magnetic field induced Lifshitz transition at a magnetic field of ~11 T.