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(EF) < 1 (g(EF) 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 Jeff=1/2
Mott insulator Sr2Ir O4 [4-6], and
(3) a pressure-induced insulating state in
BaIr1-x Gdx 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.
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