Cooperative interplay between spin-orbit coupling and electron correlation and
the realization of novel Jeff=1/2 Mott insulator in layered iridates
T. W. Noh1
1 ReCOE & FPRD, Dept. of Physics and Astronomy, Seoul National Univ., Seoul 151-747, Korea

Mott physics has been successfully adopted to explain physical properties of 3d transition
metal oxides (TMOs). The electronic ground state becomes a Mott insulator when the on-site
Coulomb repulsion surpasses the bandwidth, which is the case of spatially localized 3d
TMOs. On the other hand, 5d TMOs are expected to have much weaker on-site Coulomb
repulsion due to their extended nature of electron wavefunctions, so we expect that most 5d
TMOs should have metallic ground states. However, there exist numerous insulating 5d
TMOs, including Cd2Os2O7, Ba2NaOsO6, and Sr2IrO4 [1-3]. How can such insulating states
appear in 5d TMOs? Can electron correlation also play an important role in spite of its small
value in 5d TMOs?
Interestingly, anomalous insulating ground states of 5d TMOs were reported in layered
iridates, i.e., Sr2IrO4 and Sr3Ir2O7 [3, 4]. For 5d TMOs, the on-site Coulomb repulsion is
expected to be weak, but the spin-orbit (SO) coupling is expected to be large and play a
important role for their properties. However, little is known at present about the role of SO
coupling for their physical properties. In this presentation, we will present how the combined
effects of the SO coupling and the on-site Coulomb repulsion could make the insulating
ground states of the layered iridates.
We investigated the electronic structures of Ruddlesden-Popper series Srn+1IrnO3n+1 (n=1,
2, and ∞) compounds with optical spectroscopy and first-principles calculation. From the
comparison between optical conductivity spectra σ(ω) and the results of first-principles
calculation, we found that the ground states of layered iridates should have different
characteristics to those of a typical S=1/2 Mott insulator. Due to the large value of the SO
coupling, total angular momentum should be a good quantum number instead of the spin. We
demonstrated that the ground states of layered iridates can be represented by Jeff=1/2 Mott
insulator, which can be stabilized by the cooperative interplay between SO coupling and on-
site Coulomb repulsion. This interplay might be the reason why we can observe some
insulators in 5d TMOs.
We also performed a systematic studies on how bandwidth change can affect the
electronic structures of the Jeff=1/2 Mott insulator system. We measured the electronic
structural changes from insulator (n=1 and 2) to metal (n=∞) in Srn+1IrnO3n+1, and found that
they are quite different from those of 3d or 4d S=1/2 systems. These electronic structure
changes provide another evidence that Srn+1IrnO3n+1 should be considered as the Jeff=1/2
weakly correlated narrow band system. We will also discuss how the intriguing electronic
structure of Jeff=1/2 Mott insulator can change with temperature.

[1] D. Mandrus et al., Phys. Rev. B, 63, 195104 (2001).
[2] A. S. Erickson et al., Phys. Rev. Lett. 99, 016404 (2007).
[3] G. Cao et al., Phys. Rev. B 57, 11039 (1998).
[4] G. Cao et al., Phys. Rev. B 66, 214412 (2002).

*This work has been done in collaboration with J. Yu (Seoul Nat’l Univ.), J.-H. Park (POSTECH), C.
Kim (Yonsei Univ.), S.-J. Oh (Seoul Nat’l Univ.), G. Cao (Univ. of Kentucky), and H. Funakubo
(Tokyo Institute of Tech.).

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