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Spin-Orbital Entanglement Emerging from Frustration in the Kugel-Khomskii Model

We investigate the zero-temperature phase diagrams of the bilayer square-lattice Kugel-Khomskii ($d^9$) model involving entangled and singlet phases using mean-field cluster approach. This diagram includes interlayer singlet phase observed in fluoride K$_3$Cu$_2$F$_7$ and exotic entangled spin-orbital phases. For a monolayer case, realized in K$_2$CuF$_4$, we perform similar calculations in finite temperature and show that the alternating-orbital ferromagnet decays first to an entangled uniform ferromagnet and then to a paramagnetic phase. Published in: J. Phys.: Conf. Series 391, 012085 (2012).