|
It has been long realized that orbital degeneracy in d-electron materials results in a very rich physics due to the interaction between orbital and spin degrees of freedom. In this talk, I will present our theoretical results with regard to two classes of such materials in particular: the iron pnictides and ruthenates.
The neutron scattering experiments on the parent compounds of the 122 family of the iron-based superconductors show a pronounced orthorhombic anisotropy in the spin wave spectra [1,2], also observed in resistivity measurements on detwinned crystals [3,4]. Using a combination of first-principles band theory calculations, phenomenological Landau theory and the insights from the microscopic Kugel-Khomskii model, we show that orbital ordering of the Fe dxz and dyz orbitals provides a likely explanation for the electronic nematic order in the iron pnictides [5]. Our findings are in the quantitative agreement with the recent ARPES measurements [6]. The ruthenates are characterized by the degenerate Ru dxz and dyz orbitals, leading to a rich phase diagram of Sr-substituted Ca2RuO4, incorporating the Mott antiferromagnetic insulator and a series of phase transitions into a paramagnetic metal [7]. I will discuss some theoretical ideas in this regard, as well as implications for the emergent nematic phase in Sr3Ru2O7 under an applied magnetic field [8]. [1] J. Zhao et al., Nature Phys. 5, 555 (2009) [2] L. W. Harriger et al., Phys. Rev. B 84, 054544 (2011). [3] M.A. Tanatar at al., Phys. Rev. B 81, 184508 (2010). [4] J.-H. Chu et al., Science 329, 824 (2010). [5] A. H. Nevidomskyy, preprint arXiv:1104.1747. [6] M. Yi et al., PNAS 108, 6878 (2011). [7] S. Nakatsuji and Y. Maeno, Phys. Rev. Lett. 84, 2666 (2000). [8] R. A. Borzi et al., Science 315, 214 (2007). |
|