Orbital-driven electronic structure changes and the resulting optical anisotropy
of the quasi-two-dimensional spin gap compound La4Ru2O10
S. J. Moon,1 W. S. Choi,1 S. J. Kim,1 Y. S, Lee,2 P. G. Khalifah,3,4 D. Mandrus,4 T. W.
Noh1
1ReCOE & FPRD, Dept. of Physics and Astronomy, Seoul National Univ., Seoul 151-747, Korea
2Dept. of Physics, Soongsil Univ., Seoul 156-743, Korea
3Dept. of Chemistry, Univ. of Massachusetts, Amherst, Massachusetts 01003, USA
4Materials Science and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennesee
37831, USA

The Peierls-type spin dimerization will result in interesting spin singlet ground states in
one-dimensional (1D) systems. This mechanism does not usually work in most materials with
higher dimension. However, some 3D spinels, such as CuIr2S4 and MgTi2O4, recently
showed intriguing spin-singlet ground states. In order to explain these spin gap formations, it
was proposed that an orbital ordering plays a crucial role for the formation of the Peierls
states [1]. However, due to the rather complex patterns of dimers in 3D, there have been little
direct experimental evidences for the role of orbital ordering and resulting anisotropic
electronic responses in non-1D spin gap materials.
Quasi-2D La4Ru2O10 showed a first-order structural transition accompanying a spin gap
opening at Tc=160 K [2]. Since its structural changes occur only along the crystallographic b-
axis, it would be much easier to investigate electronic structural changes due to the orbital
ordering in La4Ru2O10. Moreover, we recently demonstrated that optical spectroscopy can be
used as a powerful experimental technique for investigating the orbital correlations and
resulting electronic structures of strongly correlated materials [3, 4].
We investigated the temperature- and polarization-dependent optical conductivity
spectra σ(ω) of quasi-two-dimensional bc-plane of La4Ru2O10. σ(ω) along the b- and c-axes
(σb(ω) and σc(ω)) undergo drastic spectral changes across Tc, which is associated with the
spin gap formation and orbital ordering. The change in σb(ω) and σc(ω) revealed the spin and
orbital configuration in the orbital-ordered state, which can settle the debate on the spin and
orbital configurations in the spin gap states [2, 5].
More importantly, we clearly demonstrated that the relative orientation of the orbitals in
the orbital-ordered state induce the anisotropic electronic response. We observed robust
spectral weight (SW) redistribution from σ c(ω) to σ b(ω) across Tc, resulting in the anisotropy
in the electronic structure of the spin-singlet ground state. We successfully explained the
observed anisotropy based on the orbital-dependent hopping model in the orbital-ordered
state. Our studies clearly manifest that the orbital ordering can lead to the peculiar magnetic
states which have lower dimensionality than that of a host material [6].

[1] D. I. Khomskii and T. Mizokawa, Phys. Rev. Lett. 94, 156402 (2005).
[2] P. Khalifah et al., Science. 297, 2237 (2002).
[3] J. S. Lee et al., Phys. Rev. Lett. 96, 057401 (2006).
[4] M. W. Kim et al., Phys. Rev. Lett. 96, 247205 (2006).
[5] H. Wu et al., Phys. Rev. Lett. 96, 256402 (2006).
[6] S. J. Moon et al., Phys. Rev. Lett. accepted.

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