Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 34: Correlated Electrons: Metal-Insulator Transition 2

TT 34.11: Talk

Thursday, February 28, 2008, 16:45–17:00, H 2053

The spin state issue in the RBaCo₂O_5.5 cobaltates — •Hua Wu, Zhiwei Hu, Tobias Burnus, Daniel Khomskii, and Liu Hao Tjeng — II. Physikalisches Institut, Universität zu Köln

The double perovskites RBaCo₂O_5+δ (R=rare earth, 0≤δ≤1) display intriguing phenomena such as charge and orbital ordering, as well as antiferromagnetic to ferromagnetic transition, depending on the oxygen concentration. In particular, the δ=0.5 system shows a giant magnetoresistance effect, and its metal-insulator transition has been often interpreted in terms of a spin-state transition [1,2], which, however, is fiercely debated [3,4]. To address the spin-state issue, we performed density-functional theory calculations which include a mean-field correction for the correlation effects caused by the Co 3d electrons. We have investigated various scenarios with different combinations of the low-, intermediate- and high-spin (LS, IS, and HS) states. Our results show that the pyramidally coordinated Co³⁺ ions are exclusively in the HS state since [3], in disagreement with [1,2]. The octahedrally coordinated Co³⁺ can be stabilized into a LS-HS ordered state if we take into account the superstructure recently reported [4]. Our results put limits as to how much spin-state transition could accompany the metal-insulator transition.

[1] C. Frontera et al., Phys. Rev. B 65, 180405(R) (2002).

[2] A. A. Taskin et al., Phys. Rev. Lett. 90, 227201 (2003).

[3] Z. Hu et al., Phys. Rev. Lett. 92, 207402 (2004).

[4] D. D. Khalyavin et al., Phys. Rev. B 75, 134407 (2007).