Physics & Astronomy Colloquium

3:30 PM, Friday, November 13, 2009
Room 155, Chem-Phys Building

Dr. Gang Cao
Department of Physics and Astronomy
University of Kentucky

"Orbitally-Driven Physics in Layered Ruthenates and Iridates''

It is commonly expected that 4d-electron based ruthenates and 5d-electron based iridates are more metallic and less magnetic than their 3d and 4f counterparts. The extended nature of the 4d and 5d orbitals leads to a broader d-bandwidth W and a reduced Coulomb interaction U such that Ug(E_F) < 1 (g(E_F) is the density of states); the Stoner criterion therefore anticipates a metallic, paramagnetic state. In marked constrast, many ruthenates and iridates are magnetic insulators with a wide range of exotic properties. In this talk, we present experimental results that address the following three issues: (1) a strong, novel spin-valve effect in bulk single crystals of the ruthenates [1-3]; (2) a giant magneto-electric effects in a J_eff=1/2 Mott insulator Sr₂Ir O₄ [4-6], and (3) a pressure-induced insulating state in BaIr_1-x Gd_x O₃. These novel phenomena highlight the importance of strong spin-orbit coupling that rigorously competes with other energies, thus setting a new balance between various degrees of freedom that characterize the ruthenates and iridates.

References
1. G. Cao et al., Phys. Rev. Lett. 78, 1751 (1997).
2. X.N. Lin et al. Phys. Rev. Lett. 95, 017203 (2005).
3. G. Cao et al., Phys. Rev. Lett. 100, 016604 (2008).
4. B.J. Kim et al. Phys. Rev. Lett. 101, 076402 (2008).
5. S.J. Moon et al. Phys. Rev. Lett. 101, 226401 (2008).
6. S. Chikara et al., Phys. Rev. B 80, 140407 (R) (2009).

Refreshments will be served in CP 179 at 3:15 PM