The contradictory physical properties and extreme anisotropy of Ca₃Ru₂O₇

The bilayered Ruddlesden-Popper (RP) compound Ca₃Ru₂O₇ displays a wide variety of physical properties and exhibits the signatures of almost every ordered state (except for superconductivity) known in condensed matter physics, including exotic phenomena not found in other materials. For example, Ca₃Ru₂O₇ exhibits conflicting hallmarks of both insulating and metallic states. We present a brief introduction to the layered ruthenates, and then discuss transport and thermodynamic properties of Ca₃Ru₂O₇ with a focus on the following intriguing phenomena: (1) colossal magnetoresistance (CMR) via suppression of a ferromagnetic (FM) state, (2) strong spin valve effect in bulk single crystals, (3) quantum oscillations in a nonmetallic state, and (4) oscillatory magnetoresistance periodic in B (rather than 1/B). We argue that these novel properties derive from the orbital degrees of freedom of the Ru-ions, which drive a complex phase diagram via their couplings to spin (spin-orbit interaction) and the lattice (Jahn-Teller effect).