Imaging emergent correlated phases in the strontium ruthenates

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+1Ru_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 talk, I present 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 and nematicity.

Using low-temperature scanning tunnelling microscopy and spectroscopy, I demonstrate that the increased structural distortions in the surface layer of these materials 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 found a checkerboard charge order intertwined with nematicity, and suppression of superconductivity. In Sr₃Ru₂O₇, measurements at the surface reveal a magnetic ground state, with substantial anisotropy of the electronic states. Both systems have van Hove singularities within 5meV of the Fermi level, which we can tune by magnetic field.