Condensed Matter Physics Seminar

Orbital Physics: Unusual Colossal Magnetoresistance and Quantum Oscillations in the Mott System Ca₃Ru₂O₇

Prof. Gang Cao
University of Kentucky
Tuesday, November 8, 2005
Higgins 235, 3pm

The bilayered Ca₃Ru₂O₇ features a Mott transition, colossal magnetoresistance and quantum oscillations. Transport and magnetic studies of Ca₃Ru₂O₇ in high magneticfield show strikingly different behavior when the field is applied along the different crystal axes. A ferromagnetic (FM) state with full spin polarization is achieved for B||aaxis, but colossal magnetoresistance is realized only for B||b-axis by avoiding the ferromagnetic state. For B||c-axis, Shubnikov-de Haas oscillations are observed and followed by a less resistive state than for B||a. Hence, in contrast to standard colossal magnetoresistive materials, the FM phase is the least favorable for electron hopping. In addition, for B rotating within the ac-plane, slow and strong Shubnikov-de Haas (SdH) oscillations periodic in 1/B are observed for T≤1.5 K in the presence of metamagnetism.

These oscillations are highly angular dependent and intimately correlated with the spin polarization of the ferromagnetic state. For B||[110], oscillations are also observed but periodic in B (rather than 1/B) which persist up to 15 K. While the SdH oscillations are a manifestation of the presence of small Fermi surface (FS) pockets in the Mott-like system, the B-periodic oscillations, an exotic quantum phenomenon, may be a result of anomalous coupling of the magnetic field to the t2g-orbitals that makes the extremal cross-section of the FS field-dependent. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are believed to be driven by the orbital degrees of freedom.